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From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Airborne Laser: Prelude to Star Wars?
Newsgroups: sci.space.policy,alt.war
Date: Wed, 6 Dec 1995 03:09:35 GMT
In article <49vvs9$68b@news.us.net> softwa19@us.net (Charles R. Smith) writes:
>>>Not to mention the spinoff possibilities of a reliable "field"
>>>high-power laser, such as a workable laser launch system.
>>No, I don't think that any of the technical issues that ABL will resolve
>>[if they are resolved] will address this matter.
>
>"Laser" launch system? You would need an nuclear reactor to push a
>few grams around...
Nope, not correct. To achieve the sort of accelerations needed for
Earth-to-orbit, you need about a megawatt per kilogram. So long as you
restrain yourself to small cargo launches, say 50kg and under, that kind
of laser power is no big deal. (Note that a laser launcher does not use
photon thrust -- it uses beamed power to heat rocket propellant.)
However, I agree that this is largely irrelevant to the ABL stuff. A
laser launcher doesn't *want* to be mobile -- the whole point of the
exercise is that the heavy hardware gets to stay on the ground where power
is cheap and weight is unimportant. And the laser characteristics that
are good for a laser launcher are not good for weapons.
>...biggest problem with ABL is being in position to defend and to
>discriminate what is a target. Range is also an issue. I am certain
>that stopping the javilin on the ground (ie... SCUD hunting) will be
>more effective than shooting it down in the air.
ABL is being talked about primarily as a boost-phase interception method,
where decoys and fragmentation are not an issue and discrimination is
fairly easy. (If it's got a big long bright rocket plume under it,
shoot it down.) The whole point of ABL *is* greater range -- reaching
the missile during boost phase is very difficult with projectiles.
As for stopping the thing on the ground, the one experience with that
approach so far -- against an unsophisticated opponent in a situation
where friendly forces had total control of the air -- was spectacularly
unsuccessful. What miracles are you assuming that will make it actually
work next time?
--
Look, look, see Windows 95. Buy, lemmings, buy! | Henry Spencer
Pay no attention to that cliff ahead... | henry@zoo.toronto.edu
Newsgroups: sci.space.policy
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Laser Launch (was Re: Airborne Laser: Prelude to Star Wars?)
Date: Tue, 12 Dec 1995 16:15:41 GMT
In article <4ahq84$kr5@optima.cs.arizona.edu> nicko@GAS.UUG.Arizona.EDU (Nickolai B. Ogurtsov) writes:
>: I wouldn't say a 50-megawatt laser that can operate continously
>: for several minutes is "no big deal". I'm certainly not aware of anything
>: that's in that range...
>: There are also beam control issues for keeping the
>: spot small enough to be useful...I'd say it's still a big deal.
>
> I think Mr. Spencer was talking about the power requirements, not
>the technical difficulties involved in utilizing that power in laser
>launches. 50-megawatt power supply is no big deal on the ground.
Power supply is mostly what I had in mind, but the laser itself is not
a formidable problem either. Don't confuse what *has* been built with
what *can* be built. A 50MW CO2 laser would be a big project, yes, but
it is not so far beyond current experience that it would become an R&D
effort. There are folks who would fairly confidently bid on it as a
big custom-engineering job. (They'd probably overrun their budget, but
not by a terribly large factor.)
One reason why such lasers haven't been built already is that the weapons
people have problems with the long wavelength. Laser launchers work just
fine at long wavelengths.
The beam director is a bigger headache, but that technology is more or
less in hand, especially at long wavelengths where optics are easier.
Again, not a trivial project, but not a requirement for major R&D either.
> Btw, there is no reason not to use a pulsed laser. As long the
>pulses are frequent enough it's just as good as a continuous beam.
Exactly what type of laser you can put up with depends on the exact type
of laser launcher, but the easiest near-term solution -- the heat-exchanger
rocket -- will work with almost anything. More advanced methods like
detonation-wave thrusters need pulses, and specific pulse characteristics
at that.
--
Look, look, see Windows 95. Buy, lemmings, buy! | Henry Spencer
Pay no attention to that cliff ahead... | henry@zoo.toronto.edu
Newsgroups: sci.space.policy
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Laser Launch (was Re: Airborne Laser: Prelude to Star Wars?)
Date: Thu, 14 Dec 1995 06:06:49 GMT
In article <4alikp$ng3@news1.deltanet.com> Bruce Lewis
<bchan@deltanet.com> writes:
>To my mind the best solution for laser launch is distributed beam
>technology. Instead of one honking big launch laser, you build an array
>of lower-powered lasers, which are focused on the vehicle using
>adaptive optics.
The big lasers themselves would be built as phase-locked arrays of smaller
ones, and any laser-launcher beam director setup is going to use adaptive
optics, so this is a difference in details only.
Do remember that if you're doing something like detonation thrusters, the
smaller lasers would have to be synchronized extremely precisely. Some
things are easier with one big one.
--
Look, look, see Windows 95. Buy, lemmings, buy! | Henry Spencer
Pay no attention to that cliff ahead... | henry@zoo.toronto.edu
From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Laser Launch demo with MIRACL?
Date: Fri, 29 Dec 1995 02:41:47 GMT
In article <30DE8531.28A2@mail.GANet.NET> "William H. Mook, Jr." <wm0@s1.GANet.NET> writes:
>What could you do with $2e6? What would be the use of funds?
I'm not Jordin... but based on things he's said in past talks...
You start by testing some heat exchangers with non-laser light sources.
One nice thing about heat-exchanger rockets is that they don't much care
what kind of light hits them; experimenting with ablative thrusters, by
comparison, is painfully difficult.
Then you build the "laser bottle rocket", a one-dimensional captive
demonstrator that runs up a pair of wires and uses an inert propellant but
does incorporate a two-part heat exchanger so you can demonstrate thrust
vectoring by beam steering (hit one part with more light and the other
with less).
Around this time you do things like wind-tunnel testing on a model of your
sounding rocket, exploring aerodynamics and jet interactions. You also
figure out how you want to launch the thing -- it is probably better to
start the laser propulsion after it's already in the air rather than
trying to launch it from the ground. For early tests, probably you just
drop it from an aircraft. There's lots of other detailed engineering
issues, like pump vs. pressure feed (pressure feed is simpler, but it
does make the tanks heavier, and there are some very nice miniature
pumps that LLNL invented a while back...).
Then you build your sounding rocket, and gradually expand its flight
envelope.
>Furthermore, how does use of MIRACL compare to non-standard
>methods like solar pumped diode lasers?
Mostly, you can buy MIRACL time right now. Jordin is talking about
things he could actually do in the next year or two, given funding,
not about hypothetical new technology that would need to be developed
at length first.
>...mirror of the type described by Forward would weigh 1/8 gram. You'd
>even give the disc a dihedral so it'd self center in the beam. You'd
>just turn on the beam and fire the mirror to escape velocity! At 10
>gees it'd take 112 seconds or so. You could have the same PR impact
>as #2 and #3. A lot less engineering it seems...
No, a lot more engineering. Featherweight diamond mirrors are nowhere
near reality today. Again, Jordin is talking about things that are.
>If you built an OPTICAL RAM-ROCKET that took air as a working fluid
>during the first part of its flight and then used hydrogen in the
>later part of its flight, you might overcome lifting a lot of
>propellant...
Perhaps, but why bother? Propellant is cheap, almost certainly cheaper
than the hardware you'd need for using air... especially since you can
buy propellant today, while hypersonic airbreathing hardware has to be
developed (and that's not easy). Even in the longer term, with fully
reusable vehicles, it's not clear that it's worth major complications
to the hardware just to save some cheap propellant.
--
Look, look, see Windows 95. Buy, lemmings, buy! | Henry Spencer
Pay no attention to that cliff ahead... | henry@zoo.toronto.edu
From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Alternative means of achieving orbit
Date: Mon, 25 Mar 1996 16:35:19 GMT
In article <4iia3p$4bb@nuntius.u-net.net> tim@flexagon.u-net.com (Tim Oddy) writes:
>>A very wide range of non-traditional launch concepts has been examined,
>>although lack of funding has prevented serious development work on most.
>
>Because most of them are VERY VERY expensive...
Actually, no, quite a few of them would need only modest development
funding. Trouble is, NASA's budget for serious work (building hardware,
not writing papers) on such things has been *zero* for many years.
>One idea that I saw in a SF (Pournelle or Niven possibly) novel was to
>place capsules into orbit using a rocket that was "powered" by
>pointing a very powerful ground based laser into the mouth of the
>engine...
That's from Pournelle's SF, almost certainly. It's a real concept.
What's more, it can be done on a much smaller scale than Pournelle showed:
a system costing a few hundred million dollars, capable of putting more
cargo into orbit per year than the shuttle and Titan fleets put together,
(and at much lower cost per kilogram) could be built without any major R&D
effort. The trick is to launch the stuff in relatively small lumps, tens
of kilograms, for collection and assembly in orbit; the cost and technical
challenge of a laser launcher is driven almost entirely by the size of a
single payload, not by how many you launch. You wouldn't go straight to
this system -- there is *some* R&D needed -- but the technology for it is
not formidable, and two or three years of modest funding should suffice.
In particular, a laser big enough for tens-of-kilos payloads would be a
large custom-engineering project but is within the state of the art,
unlike the multi-gigawatt monsters needed to launch tons at a time.
For a few years, there was actually a tiny dribble of funding for serious
work on this concept, from the military (in particular, SDIO) rather than
NASA. Some good work was done at LLNL. The money ran out and the project
stopped. This was the biggest (!) effort in advanced propulsion that the
US had mounted since the 1960s.
--
Americans proved to be more bureaucratic | Henry Spencer
than I ever thought. --Valery Ryumin, RKK Energia | henry@zoo.toronto.edu
Newsgroups: sci.space.tech
Subject: Re: Laser propulsion (was:coal powered spaceship)
From: kare@sirius.com (Jordin Kare)
Date: 13 Mar 1997 03:58:23 GMT
In article <5g1qof$pml@ptdcs5.ra.intel.com>, greason@ptdcs2.intel.com
(Jeff Greason) wrote:
> In article <3318EB24.4E0E@erols.com>, Bill McGovern <mcgover@erols.com>
writes:
.. What if a power plant on
> |> the ground generate electricity, then beam it to a space ship in the
> |> form of a high intensity mazer? The space ship could then convert the
> |> mazer back into electricity and use it to power plasma rockets.
>
> This scheme has been proposed, many times, under many names. The general
> class of "externally powered" rockets is a "beamrider" -- since most
> schemes propose laser power beaming, this is also a "laser launcher".
The semiofficial name for the concept is "Beamed Energy Transport" or BET.
A workshop on the subject was held at NASA Marshall last fall.
> Whether you beam the power in optical or microwave wavelengths, I think
> it's a promising scheme. LLNL has done a tiny bit of work in this area.
About $5 million total, in the period from 1986 to 1991. I ran the program.
> Unfortunately, it's got the kind of huge up-front capital cost problems
> that doom most good ideas. You either need one f*****g high power laser
> and power direction system, which is expensive, or you need one f*****g
> huge area microwave sending system ...
This is only partly true. It should be possible to build a laser launch
system for under $1 billion, with an annual launch capacity of several
hundred tons. The problem is that that launch capacity is in the form
of a large number (>10,000) of small (20 - 50 kg) payloads. Thus, laser
(and other beamed energy) launch systems are capital intensive for
large payloads, but potentially fairly cheap, even in capital cost,
for large total mass that can be packaged in small pieces. In particular,
the LLNL project was supported by SDIO as a potential means of launching
small space based interceptors (what later became known as "brilliant
pebbles") and decoys.>
> However, if someone walked up and said "Here, take $20B in up-front
> financing which you won't have to pay back, and lower launch costs as far
> as possible" -- then I think a beamrider ...is one of your best bets.
As I once noted after a talk, if anyone offered me several $B in up-front
financing, I'd probably run off to Brazil with it...
> At least for optical
> wavelengths, hitting a small target isn't too tough, and the capital costs
> scale closely with the laser array power output. So a system that launches
> small payloads is cheaper and easier than one that launches big payloads.
> The economics here are complicated, however, since you generally can only
> have one ship "in the beam" at once.
Rarely a significant limitation; each launch takes only 5 - 10 minutes.
> These systems also make more economic sense (like *any* launch system other
> than an ELV) when your flight rate is high enough to keep it busy all the
> time. Even a minimum sized beamrider launcher probably is only "busy" when
> you're launching 50-100 million lbs. to orbit /year. That's a market big
> enough to support 10+ RLV's flying every day, for example.
As noted, the numbers are bit high; a laser system would be economical
relative to expendables at less than 100,000 lbs/year.
> A beamrider can easily bring you down to the
> $10-$20/lb to LEO range and may, in the limit, be able to approach $3-$5/lb.
> (The energy cost eventually becomes limiting. The orbital energy of 1lb
> of mass is on the order of 5KW-hr, which at $0.05/KW-hr is $0.25/lb in
> final energy cost. And your beamrider is likely to be *at best* 10%
> efficient, which is $2.50/lb -- and that's after so many years have elapsed,
> and you're at such a high flight rate, that capital costs are negligible.
> Not soon, in other words).
A little on the optimistic side, barring radical breakthroughs or some
really enormous-scale applications -- unless you cheat and ignore
amortization and system maintenance costs. The incremental cost of
launching one more vehicle can be brought down to the $20/lb payload
range fairly easily, but the true system cost tended to start around
$100/lb and be tough to get below $30/lb even for a fully-utilized system.>
"Wallplug" efficiencies (payload kinetic energy / input energy) hover
around 1-2% even with optimistic system estimates. (I've gotten
a little more conservative as I've gotten older -- not much, but a little :)
Jordin Kare
Jordin T. Kare kare@sirius.com
Date: 24 May 89 04:10:36 GMT
From: jtk@mordor.s1.gov (Jordan Kare)
Subject: Re: Launch noise
In article <8905221828.AA22915@cmr.icst.nbs.gov> roberts@CMR.ICST.NBS.GOV (John Roberts) writes:
>>From: concertina!fiddler@sun.com (Steve Hix)
>
>>In article <166@ixi.UUCP>, clive@ixi.UUCP (Clive Feather) writes:
>>> The *BIG* cannon in Jules Verne's "From Earth to the Moon" was called
>>> the "Columbiad". Close enough ?
>
>>... Probably exceeds local noise limits, though.
>
>This is a legitimate concern for any earth-based ballistic launcher (explosive,
>electromagnetic, etc.) Even if the noise of the initial impulse can somehow
>be controlled, a projectile of the size generally mentioned would create a
>tremendous sonic boom, which I suspect would be painfully loud even many miles
>away. This would place constraints on a suitable location for such a launcher.
>
>Have any studies been conducted on the magnitude of the noise problem?
> John Roberts
> roberts@cmr.icst.nbs.gov
It's been a significant concern for laser launching -- enough to
generate a couple of calculations. Typical numbers are that a
100 MW launch system generates 80-90 db noise levels --
10 km from the launcher! I've occasionally been known to suggest
modulating the laser rep rate (nominally ~100 Hz) to play
a really impressive bass line for a rock concert :-)
Along the same lines, someplace I have a nice PR mailing from
a small company promoting the electromagnetic launcher concept
that shows an artist's conception of a launcher seen from the
exit end. The view is of a cliff face with the launcher end
embedded in it, with a line of something (power poles?) stretching
off along the far side of the ridge to show how long the thing is.
In the foreground is a nice cigar-shaped projectile flying out of
the launcher mouth. And at the top of the cliff, perhaps 50 feet
from the launcher mouth, is a nice modern-looking control building....
with big plate glass windows!
Jordin (Big Noise) Kare
Date: 20 Apr 87 23:33:23 GMT
From: jtk@mordor.s1.gov (Jordan Kare)
Subject: Re: Near Term Laser Launcher Prospects
I have been watching the discussion here on Laser Propulsion for some
time. Jim Kempf's article has finally compelled me to write a reply.
You see, I am currently in charge of the SDIO Laser Propulsion project,
a completely unclassified program with a budget of order $1M for 1987.
So when someone says "forget laser propulsion", I just can't sit still.
**********************************************************************
The material presented here represents my own opinions, and is in no way
representative of the official position of the Lawrence Livermore
National Laboratory, the Strategic Defense Initiative Organization or
its Program in Laser Propulsion, or any other government agency.
**********************************************************************
In article <1628@hplabsc.UUCP> kempf@hplabsc.UUCP (Jim Kempf) writes:
>
>INTRODUCTION
>In response to the frustration of watching the Russki's cavorting about
>over our heads doing the real thing while we sit around with our noses
>buried in "Space Wars" video games, I decided to do some investigation
>into the near term feasibility of laser launched vehicles for cheap
>single stage to orbit lift...
>My initial interest was inspired by a net discussion last fall, in
>which Myrabo and Ing's book "The Future of Flight" was mentioned.
>Myrabo has been doing paper studies of laser launch systems for years
Myrabo's work is very good, but he does tend (in some part at the
request of his supporting agencies) to design second- and third-
generation systems rather than things we can build right away. The jump
from Ing's part on ultralights to Myrabo's on ultra-lasers is quite
amazing.
>In addition, the AIAA series on recent advances in astronautics has a
>volume from several years back discussing more technical details
>(sorry, exact reference not at hand)
Vol. 89, Orbit Raising and Maneuvering Propulsion, L.H. Caveny, Ed.
>Myrabo and Ing claim that a laser launch system would allow single
>stage to orbit transfer of large amounts of material for a fraction of
>the current cost... the savings come from the inherent simplicity of
>the system. All the power generating equipment is on the ground, so the
>launch vehicle itself can be made very light weight (modulo aerodynamic
>forces) and need only carry a fraction of the amount of fuel needed for
>a conventional rocket, since thrust generation is not occuring due to
>combustion but from the externally beamed laser power. Additionally,
>the driving fluid ... could just as easily be plain old water.
A very good summary. In fact, the _main_ advantage of laser propulsion
is probably not the increased performance (high specific impulse), but
the ability to operate as a "pipeline", with very high throughput for a
minimum of manpower.
>PHYSICS OF LASER LAUNCHING
>
>The point of the following calculations is to determine how much power
>is needed to put a body of a certain mass into orbit (as a function of
>the mass)...
I refer readers back to the original article for calculations, except
for a small note that in eq. 1
> v = u * ln( m(0) / m ) - g * t (1)
>
>where:
>
> v = final velocity
> u = velocity of the exhaust
> m(0) = initial mass of rocket
> m = final mass of rocket
> g = acceleration due to gravity (9.82 m/sec^2)
> t = time of burnout
>
g is not a constant, but represents the component of gravity along the
direction you are accelerating; this makes things slightly better.
>Assuming a specific impulse of 1000, we can calculate [exhaust
>velocity] as 9.82 x 10^3 m/sec.
Isp = 800 to 1000 is about right
>This gives the mass ratio as a function of the time to orbit. Assuming
>a 15 minute flight time, the mass ratio is 5.5, a 30 minute flight time
>gives a mass ratio of 13.6.
For fairly straightforward reasons (you can't accelerate arbitrarily
slowly, you can't stay in range of one laser site forever) realistic
trajectories have time to orbit of 5 to 15 minutes. Mass ratios range
from as low as 3 to as high as 10, depending on things like initial
vehicle mass, drag coefficient, etc.
>Assume, for the sake of argument, a mass ratio of 5.5, and thus
>a flight time to orbit of 15 minutes. What amount of laser power
>would be needed to achieve this?
>
> E = 3.2 x 10^7 * m Joules ( Newton-meters)
>
>Now comes the interesting part. We assume this amount of energy is
>deposited in the rocket during the 15 minute ascent (a more
>sophisticated calculation is possible). The resulting power required
>is:
> P = 3.5 x 10^4 * m watts (Joules/sec)
>
>Taking m at orders of magnitude in kg. gives the following table
>
> m=10^2 kg ------------------> 3.5 x 10^6 watts
> m=10^3 kg ------------------> 3.5 x 10^7 watts
> m=10^4 kg ------------------> 3.5 x 10^8 watts
This is optimistic. The "real" answers (my current best number, based
on a fairly detailed trajectory simulation) is that you get about 1.5 kg
into orbit per megawatt of laser power. The biggest unknown is the
efficiency of the thruster at converting laser energy to kinetic energy
of exhaust -- I think we can get about 40%.
>Thus, to launch a 0.1 metric ton payload would require 3.5 Megawatts, a
>1.0 metric ton payload would require 35 Megawatts, etc. The initial
>mass of the vehicles, including driving fluid, would be 5.5 times the
>mass at orbit.
I have a viewgraph which cites two cases: a "small" laser launcher is a
100 MW laser launching about 150 kg; the "large" laser is 1 GW launching
1.5 tons (that's a Mercury capsule, by the way). Keep in mind that,
running flat out (say, 4 launches an hour, 80 per day, 28,000 launches
per year) even 150 kg at a time gives you over 4000 tons per year in
orbit (that's 200 shuttle flights worth), and even at a 10% duty cycle
(one launch an hour, one shift a day) you can launch more mass with the
small system than the whole shuttle fleet can launch on NASA's best
schedules.
>ENGINEERING A LASER LAUNCH SYSTEM
>
>...After all, the NOVA Nd:Yg inertial confinement fusion laser at
>Livermore, the ASTERIX iodine laser in Germany, and others have
>demonstrated powers upwards of 10 *terawatt*.
These are all peak powers, and completely irrelevant. You need average
power.
>Free electron lasers (FEL's) have theoretical conversion efficiencies
>of near 30%, compared to about 10% for the CO(2) laser which forms the
>backbone of current industrial lasers.
The "wall plug" efficiency (power line to light) of projected FEL's is,
I believe, 20-25%. FEL's of certain types (induction linac driven) also
tend to produce a very convenient size and shape of pulse.
>If we want to catch those Russki's (and, in the process, maybe make
>some bucks) we've got to start NOW and we can't wait for exotic new
>technologies. A laser launch system will require high power lasers
>which have long duty cycles, good stability, are well characterized in
>terms of power and gas consumption, and are priced competitively.
Competitively compared to what?? Actually, CO2 lasers are indeed the
logical thing to do experiments with, and that's what we're doing them
with...
>Unfortunately, the power range of current commercially available CO(2)
>lasers is too low, by almost 2 orders of magnitude. The largest current
>commercial laser manufactured in the US is a 15 kilowatt model built by
>United Technologies in Conn. (source: Laser Focus, March 1986). ...
>While one could design a system made of clustered smaller modules, the
>3.5 Megawatt figure for only 100 kg. payload (not really useful)
Nonsense! _Provided_ you have some on-orbit assembly capability, 100 kg
is big enough to launch almost anything except a man. It is more than
sufficient for fuel, oxygen, water, shielding mass, and consumables of
all sorts. Even without on-orbit assembly, there are a wide variety of
"single purpose" satellites that can be designed to weigh under 100 kg.
Freeman Dyson has advocated building a launcher for _1 kg_ payloads --
sufficient to carry a microminiaturized scientific experiment, at least.
>would require 235 such units. At $1/2 Million a module (I'm guessing,
>but that's only 2 houses in Palo Alto) that's $120 Million for the
>laser system alone, provided you could engineer the optics to collect
>and focus 235 high powered beams.
"Commercial" is a misleading term. No, you can't buy a 100 KW laser off
the shelf, but there are several companies that will build you one on
fairly short notice and at a not-unreasonable price. They will quote
you on megawatt-scale lasers, but the quotes are subject to considerable
uncertainty. However, your price estimate is considerably too high.
While I do not have formal quotes (and could not release them if I did),
"guesstimates" for megawatt scale systems are in the $5 to $20 per watt
range, so $120 Million will probably buy something like 10 MW of laser.
There are two competing philosophies for design -- one BIG laser or
stacked small lasers -- if you try to build a CO2-based system. FEL's,
by their nature, lead to one BIG laser. Beam combining optics for
arrays are complex, but have been designed.
I generally estimate the overall cost of a 100 MW "test" launch system
at $2 billion (less than 1 shuttle orbiter, and well within the reach
of, say, Boeing), and the cost of a 1GW system, built for continuous
flat out use, at $20 billion. Someone at a talk once asked me "You
mean, if I gave you $2 billion today, you would build this?", and I
said, "If you gave me $2 billion today, I'd head for Brazil, quick :-)
:-) -- but yes, I think it could be done".
>There are some commercial laser technologies which could achieve higher
>power (~100 Kw), transverse axial flow being one, but they are limited
>to continuous wave (CW) lasers. Due to the tendency of shock waves to
>propagate down the beam, any laser used in a launch system will
>probably have to be pulsed.
Again, my personal favorite designs involve a pulsed laser,
because CW laser thrusters seem to me to be more complicated, and thus
heavier and more expensive. With a pulsed system, you can hope to build
a "Four-P" thruster -- everything stays on the ground but Payload,
Propellant, and Photons, Period (A. Kantrowitz, 1986) -- which is
basically a block of "ice" (water may not be the right thing to use;
much of our current research is on what propellant to pick) with a
payload on top -- all the guidance, etc. is done from the ground.
The technology for very high average power transverse flow
pulsed CO2 lasers does exist, however, and has existed since the mid
1970's.
>CONCLUSIONS
>
>Initially I had hoped to come up with evidence that a venture funded
>startup on the scale of $100 Million or so could build a system in
>three years which would achieve a couple orders of magnitude reduction
>in to-orbit costs over Ariane and the Shuttle.
Alas, I do not know of any approach, including catapults (which are
cheaper than laser propulsion provided your payload will take 10,000
g's) with development + capital costs of under $100 million. Even the
"low budget" expendable booster operations, some of which are being
funded by venture capital, tend to project losses well over this figure
before they achieve profitability. Laser propulsion is likely to be
very inexpensive compared to, say, the Orient Express, or a new heavy
lift booster. A "proof of principle" demonstration, however, can be
conducted relatively cheaply if a suitable laser exists.
>However, it looks like we'll be dependent on ol' "Deep Pockets" Uncle
>Sugar to pull this one off. The technology simply isn't there, and it
>isn't likely to develop except in the context of military applications,
>since the current power plateau in industrial laser technology is about
>as high as most applications need. Since anything the Uncle develops
>for military purposes is likely to remain a deep dark secret (unless
>some Marine guards get a hold of it), it is unlikely we'll see a laser
>launch system in the near future.
The SDIO Laser Propulsion program is _not_ classified. It is an open
program, and participation from industry and universities is encouraged.
Our goal is to do the research and development needed for laser
propulsion _exclusive_ of laser technology itself (which is supported by
other programs) so that when large lasers (probably FEL's) are
available, we will be able to use them to launch payloads into space.
Our schedule calls for us to be prepared for high power, long range
tests -- essentially launching a grapefruit into orbit :-) -- in the
early 1990's, laser resources permitting. Of course, I have personal
hopes that a real laser launch facility will be built, possibly well
before the year 2000, but I cannot predict the future....
If you are interested in more information, please write to me (regular
mail, not E-mail) at Mail Stop L-278, Lawrence Livermore National
Laboratory, P.O. Box 808, Livermore, CA 94550, and request the
Proceedings of the SDIO/DARPA Workshop on Laser Propulsion, Volume I,
CONF-860778. I will answer questions on a time-available basis -- I've
spent far too long writing this posting already :-{
Dr. Jordin Kare jtk@mordor.UUCP
Date: 26 Apr 91 17:16:09 GMT
From: usc!rpi!news-server.csri.toronto.edu!utzoo!henry@ucsd.edu (Henry Spencer)
Subject: Re: Laser launchers
In article <908@puck.mrcu> paj@uk.co.gec-mrc (Paul Johnson) writes:
>Errm, 'scuse me. Dumb question time. How does a laser launcher actually
>work?
From an old posting of mine:
There are a variety of schemes using lasers to transmit power from big
fixed power plants to spacecraft (and aircraft). One problem that does
appear is that rocket engines are ferociously powerful, and really
enormous lasers are needed to transmit enough power to run a big one.
(The Saturn V first stage power output at launch was circa 35 gigawatts.)
Another difficulty is building a "combustion" chamber with a very highly
transparent window in it. (It is possible to transmit the beam up the
nozzle, but this has its own problems.)
Both of these problems can be minimized if you are using the system for
satellite maneuvering rather than boost to orbit, since you can then use
a small, low-thrust engine which doesn't put massive demands on chamber
materials. This is a very promising idea.
However, Earth-to-orbit is what we'd really like to do. And it looks
viable, if you change some of the assumptions. The power needs can be
brought within reason simply by scaling down the size of the payload,
on the assumption that higher launch rates will make up for small size.
You can do a useful launcher with a few megawatts per kilogram, if I've
remembered the numbers correctly.
As for the chamber problem... one scheme for a solid-fueled laser rocket
was basically just a stick of propellant with a nozzle around one end,
with the nozzle sliding up the stick as the laser wore away the end.
You can go one better, if your laser can generate a pair of closely-spaced
short pulses. The first pulse vaporizes a thin layer of the surface. The
second starts a "laser-supported detonation wave" in the vapor, heating it
to very high temperatures. Put the two pulses close together, and the
vapor is still a thin layer on the surface of the propellant when the
second pulse arrives. Now you don't need a nozzle, because the vapor
expands mostly at right angles to the surface. So your spacecraft is
very close to the ideal: a block of propellant with a payload glued on top.
There are several other advantages to this approach. For one thing, the
thrust is perpendicular to the surface, not the beam, so the beam can be
coming in at almost any angle. This also means you can steer the thing
with the beam, varying the power distribution across the beam to rotate
the spacecraft. It is probably possible to literally have nothing but
"propellant, payload, and photons", although in fact a bit of cooperation
from the spacecraft makes things like beam pointing rather easier. If
you are concerned about effects on the ozone layer, or whatever, you
can just turn off the beam and let the spacecraft coast while it passes
through sensitive regions. Range safety is easy, because the spacecraft
has no independent maneuvering ability and its trajectory is very
predictable. The accelerations are a few gees, low enough that a big
system could probably be man-rated.
Could you use air as propellant? Maybe. A reflective plate with parabolic
hollows carved in it will focus an incoming pulse (provided it's pretty much
perpendicular) to a set of hot spots near the plate, where the air will
break down and absorb the beam, producing miniature thermal explosions
that will push on the plate. It's been tried in the lab; it works.
Could you build one today? Maybe a small one. Both lasers and optics are
beyond the off-the-shelf range, but there are contractors who could build
the laser as a routine custom engineering job. It helps that this system
works fine, in fact better, at relatively long wavelengths, where almost
everything is easier. The right thing to do would be to build one with
a payload of, say, one kilogram, as a test system. There are still a lot
of unknowns in the detailed engineering. Once the test system proved
feasibility, a few hundred million dollars could build one with a yearly
payload to orbit equalling the (theoretical!) payload of the entire
shuttle fleet. Costs depend on how intensively you use it, because the
capital costs of construction tend to dominate the power bill. A man-rated
system would be really huge because of the sheer mass needed; better stick
to sending up cargo in small pieces for now. You can do an awful lot with
20kg pieces delivered cheaply to orbit in large numbers.
Even the test system would be useful, actually, since it could vaporize
small pieces of space debris and de-orbit larger ones, if it were equipped
to deal with completely uncooperative targets.
Would it be useful as a weapon? Against satellites, maybe, depending
on how much cooperation it needs from the target. Against smaller and
faster targets, not very. The long wavelength works against it in a
weapons application, where doing maximum damage quickly is essential.
Is it being worked on? Yes, in a minor way. There is a small group at
Lawrence Livermore that is looking into things like the laser-propellant
interactions. They are assuming that SDI will push laser and optics
technology far enough, and are working on the non-weapon-relevant aspects.
What's the holdup? Money.
--
And the bean-counter replied, | Henry Spencer @ U of Toronto Zoology
"beans are more important". | henry@zoo.toronto.edu utzoo!henry
Date: 25 Mar 93 01:21:10 GMT
From: Jordin Kare <jtk@s1.gov>
Subject: Mach 25
Newsgroups: sci.space
In article <821@rins.ryukoku.ac.jp> will@rins.ryukoku.ac.jp (William Reiken) writes:
> I was reading in the Popular Science "March 93" 'Science Newsfront'
>on page 35 about the Mach 25 Transporter. Some questions:
> 1). Power is lasers or microwaves. What kind of lasers would
> these be?
Extremely large (1 - 10 GWatt) free electron lasers
> 2). How much energy would be required to operate such lasers
> and how much loss would there be?
FEL's are 10 - 20% efficient, so the power consumed would be of order
10-100 GW. Leik Myrabo generally assumed orbiting lasers with their own
solar power satellites.
> 3). Lasers on the craft for power. Again what kind of lasers
> would these be?
The vehicles do not carry lasers.
> 4). The lasers for driving the craft heat a small area of air
> to 30,000 degree K. How much energy does it take to do
> this?
Lots. Typical fluxes to do this are >10^8 watts/cm^2 (albeit for times
measured in nanoseconds. Typical energy densities in the focal region
are 10's of Joules/cm^3 (10's of kJ per gram of air)
> 5). Laser to electric power for MHD propulsion in space. What
> kind of equipment is nessasary for this kind of thing? What
> is the efficiency of such equipment?
Well, Leik claims he can do it with a fairly simple design using hydrogen
heated by a laser-supported plasma, seeded with something like potassium
for conductivity, and flowed out thru an MHD channel. Nothing like this
has been demonstrated, but it's allowed by the laws of physics; the rest
is "mere engineering" :-)
> 6). Rensselar Polytechnic Institute in Troy NY.. Anyone know any
> of these people so that I may contact them direct for more
> information?
Prof. Leik Myrabo
Dept. of Mechanical and Aeronautical Engineering
Rensselaer Polytechnic Institute
Troy NY 12180
> Will...
>
Leik designs some pretty fancy vehicles, and has done a good deal of
nice mechanical and aerodynamic design and testing, but he's a _very_ long
way from having something that will fly.
Jordin Kare
Date: 10 Jun 1993 18:45:29 GMT
From: Jordin Kare <jtk@s1.gov>
Subject: Never-ending launch cost fantasies
Newsgroups: sci.space
I find myself unable to resist comment...
In article <C8EGJ3.GF2@agora.rain.com> jhart@agora.rain.com (Jim Hart) writes:
>gnb@baby.bby.com.au (Gregory N. Bond) writes:
>
>>It could be done with mongo sea-launched
>>expendibles built by shipyards,
>
>I'm sorry, but rocket control is a bit trickier then putting
>down a board in the water to steer the boat.
True, but no more complex than controlling an airplane (bearing in mind that
the Wright brothers' major breakthrough was not building something that
flew, but building something that was controllable in flight) or a
submarine in close quarters... and submarines are built by shipyards.
>Rocket mass
>scaling is just a bit more harsh than making sure a boat floats.
>For those who haven't worked the numbers I'm being more
>than just a bit facetious! We're talking orders of magnitude
>difference in the engineering constraints.
For an SSTO this is true. For Big Dumb Boosters with 2 or 3 stages,
the constraints are only marginally tighter than they are for
high-performance aircraft-- the rocket, after all, has a much shorter
lifetime and doesn't need to do 9-G maneuvers :-) The Long March
really is built in a refrigerator factory...
>> by scaled up SSTO launchers (which get
>>cheaper and less technically challenging as payload increases),
>
>But first you have to get the small ones flying. We are now
>learning that will cost on the same order of magnitude as it
>cost to get the Shuttle flying, and then there is no more
>insurance that we will reduce the "standing army" then there
>was with the Shuttle.
This is central to Max Hunter's (and many other folks') arguments
about SSTO. If NASA or a comparable bureaucracy runs an SSTO, it
may well have a Shuttle-sized standing army -- but there is no
_engineering_ reason why such a standing army is necessary for an
SSTO. Because of corner-cutting in development, Shuttle operations
_do_ require a fair-sized standing army, which is multiplied in
size by the bureaucratic and political nature of NASA
> Like the Shuttle, DC-1's miraculous
>launch cost savings come from miraculous assumptions about
>market volume, not from improvement in technology or
>organization or up-front R&D costs. SSTO operates on a razor-thin
>design margin: Once you factor in the normal design slop, not
>to mention the normal MacDac and other NASA contractor
>"incredibly shrinking functionality", it is doubtful that SSTO
>will be able to orbit any payload at all.
THis has been the fundamental _technical_ objection to SSTO at all times.
What is remarkable is that in the last year or so, the consensus of
engineering opinion seems to have shifted from skepticism that an SSTO
could achieve positive payload to acceptance. Razor-thin margins have
a way of widening with time and technology; the current margins are
thin, but apparently within the realm of reasonable engineering practice.
>> laser
>>launchers,
>
>SDI's "battle lasers" may have fooled the Soviets for a little
>while, but the don't fool anybody now. There aren't any lasers
>that one can affordably build that will launch even the
>silly 20 kg payloads that have been mentioned.
Ah, my home turf... A 20 kg payload requires roughly a 20 MW laser.
For various technical reasons, laser propulsion actually _prefers_ a
somewhat longer wavelength laser than the SDIO ground-based laser
concepts, and longer wavelength lasers are consistently easier to
build. MIRACL is a megawatt-class laser that operates now, albeit with
high operating cost and short (of order 1 minute) run times.
Thumper, a CO2 laser of the same order of size, was operated back in
the 70's. Scaling laws for these lasers are well understood.
Newer lasers (FEL and diode-pumped solid state lasers in particular)
prospectively have lower capital and operating costs and shorter
operating wavelengths.
"Affordably build"?? Depends what you consider affordable. Avco
Research Labs, which built Thumper, routinely quoted $10/watt for
very large CO2 lasers, and delivered many-kW class lasers for
$30-100/watt. Lasers other than CO2 currently cost in the range of
$100 - $1000/watt. Even with pretty conservative assumptions, a 20
MW laser would cost less than $2 billion using CO2 and less than
$20 billion using other technologies. That is pretty steep, but not
unaffordable if we had a national need. I personally estimate the
true cost of a CO2 laser today at <$600 million ($30/watt) and
expect that either FEL's or diode-pumped lasers could be built
for <$100/watt -- cheap enough, if there were a market for a
reasonable fraction of the 600 ton/year launch capacity of a 20 MW
launcher. Right now, however, there's no such market.
>Practically of that payload has to be a rocket engine so
>it won't fall back right where it came from.
Sorry, no. One of the key advantages of laser propulsion over
gas guns, etc., is that the thrust vector is not necessarily along the
beam vector; as a result, you can inject payloads directly into
circular orbits without needing a circularization engine.
(Actually, there is a way to get to a circular orbit even if the
thrust _is_ directly along the beam, but it requires overshooting
the desired altitude and isn't very efficient)
>Similar problems
>with guns, assuming we can lick the atmospheric
>scaling laws that start biting big-time at 1/2 orbital velocity
>and get much worse very quickly.
Guns do require circularizations motors, but atmospheric drag
is not difficult to overcome; you just need a reasonably high-Beta,
low drag projectile.
>> Skyhooks,
>
>These would weigh millions of tons, so you can't afford to launch
>them in the first place.
There are more modest skyhook designs that weigh anywhere from
tens of pounds (the Small Expendable Deployment System, SEDS, which
can be used to raise and circularize the orbit of few-hundred-pound
payloads) to hundreds of tons. They don't reach all the way to the surface,
but they greatly reduce the delta-V needed to get to orbit.
>>Mcelwaine's free space drive,
>
>Ah, yes, this will do the trick. :-)
Yeah, I've always been fond of Cavorite myself :-)
>>If the market is big enough, at least
>>one of these ought to be able to do it.
>
>Anything is cheap if "the market is big enough", but it's not,
>because it's not cheap, by orders of magnitude. There is
>no path from here to there via obsessing on launch costs.
Regrettably, this seems to be true, but the "activation energy"
seems to be in the few $billion range (to build one or more new
launch systems and drop the costs enough to allow industrial
activities to start growing). Unfortunately, there doesn't seem to
be any government or group with that class of $$ willing to invest
in a program of no direct short-term benefit. So folks like me keep
looking for ways to reduce the activation energy. A few years ago it
looked as though if we could reduce it to a few $100 million, we might
be able to get support, e.g., from SDIO. Alas, at the moment, the
available "free energy" is at most a few $10 million, too low for any
technology I'm aware of, and likely to remain there until there are
substantial changes in the US and/or world economy.
>There's very good reason why launch costs are the way they
>are, why $10's of billions of launcher R&D during the last
>decade has produced not even reasonably modest cost reductions,
>much less the silly orders-of-magnitude fantasies blithely tossed
>around in these parts. Rockets are massive flying gas tank
>eggshells where one small perturbation and it's
>hasta la vista, baby. By contrast airliners and boats have
>a huge safety margin when it comes to control (only during
>airline landings, where most of the crashes happen, do airliners
>approach the on-the-edge flight control regime of rockets).
This has been true of most past rocket designs, but is by no means
inherent in rockets. This is the basis for, e.g., insisting on
"engine out" capability in an SSTO. No rocket has had such capability
in the past. As Max Hunter points out, a 747 carries far more
energy in its fuel tanks than an SSTO, and in a particularly dangerous
form -- a volatile liquid. If a 747 hits a mountain, it might as well
be a flying eggshell. The difference is that the 747 doesn't automatically
crash if it has an engine failure or a minor system failure -- and there's
no reason an SSTO should, either.
>No
>surprise, airplanes are hundreds of times more reliable than
>rockets.
>
>Point of fact, both the cost of airliners and the cost of ocean shipping
>has dropped faster than the cost of rocketry, which issn't much of a
>feat since the cost of rocketry has dropped hardly at all.
As far as I know, the cost of ocean shipping hasn't dropped all that much
since diesel engines became common. Cargo handling has become cheaper
with the introduction of containerized freight. No reason at all
we can't have the equivalent -- standardized payload shrouds and
interfaces -- on rockets or SSTO's.
>It's very
>easy to write science fiction stories where launch costs are as low as
>you like, but in the real world it doesn't seem to work out quite
>so well, does it, hmmm?
>
>Jim Hart
>jhart@agora.rain.com
Reality is usually harder to deal with than fiction -- but
not by any means intractable. And who says writing SF stories is easy :-)
Jordin Kare
Date: 14 Jun 1993 21:38:56 GMT
From: Jordin Kare <jtk@s1.gov>
Subject: Never-ending launch cost fantasies
Newsgroups: sci.space
In article <C8Jvw8.54w@agora.rain.com> jhart@agora.rain.com (Jim Hart) writes:
>jtk@s1.gov (Jordin Kare) writes:
>
>>>I'm sorry, but rocket control is a bit trickier then putting
>>>down a board in the water to steer the boat.
>
>>True, but no more complex than controlling an airplane
>
>Very wrong! I described why later in my post, but I'll repeat
>myself. A rocket can fail in a fraction of a second, and this
>regime lasts for minutes. An airplane in normal flight typically
>has numerous chances, minutes to recover from control mistakes.
People fly fighters in "nap of the earth" modes where the recovery
time is extremely short. Modern fighters are unstable; a control system
failure can immediately take the fighter out of control leading to
a crash. These things are made safe by redundant control systems, safety
systems (e.g., ground-avoidance warnings), and operating procedures and
training. The same engineering practices can be applied to rockets.
However, the original statement was that the _control_ problem is worse
for rockets -- this has a relatively specific meaning, i.e., that the
actual control system of sensors, actuators, and feedback loops needed
to control a rocket is somehow more complex or requires higher performance
than that required for an airplane, that the rocket is inherently
farther from stability, etc.; this is not particularly true. Whether
the consequences of a control system _failure_ are more severe for a
rocket than for an airplane is a separate question.
>The only exceptions are brief periods during landing and takeoff.
>Overall, rockets are orders of magnitude harder to control. The
>launcher failure rate (c. 5% of flights ) vs. the airplane failure
>rate (<<1%) bears this out. We've been flying liquid fuel rockets
>since WWII, and solids since long before airplanes, so you can't
>lay the problems on "we haven't climbed the learning curve".
>We might improve things a little bit, at some added expense,
>by throwing on dozens of engines and having "engine out" capability,
>but that doesn't give anything close to the safety provided by wings.
>Rockets are inherently unstable and unreliable.
Oh, nonsense. Rockets per se are neutrally stable (which took people
quite a while to realize; Goddard's early rockets had the engine at the
front because it seemed so obvious that that would make the rocket
more stable -- but it doesn't.....) Rockets in atmosphere are
subject to exactly the same stability rules as airplanes, and
are if anything easier to stabilize. And rocket engines are by nature much
simpler and more reliable than jet engines -- for a given level of
engineering margin. Alas, rocket engines have never been produced in
the quantity or with the margins of jet engines -- although they could
be, even for an SSTO -- with the exception of
small solid rockets for everything from model rocketry to missiles, which
actually have a pretty good reliability record for something that can't
be tested in operation.
>>What is remarkable is that in the last year or so, the consensus of
>>engineering opinion seems to have shifted from skepticism that an SSTO
>>could achieve positive payload to acceptance.
>
>Not true; there's still plenty of Shuttle fans, NASP fans,
>Big Dumb Booster fans, etc. arguing against SSTO. Besides,
>there was largely consensus on the Shuttle until the late
>1970's; there was almost universal expectation of dramatically
>lower launch costs, due to reusability. The reverse occured.
You seem to be mistaking advocacy for engineering judgement.
Of course there are many people who think their way is better than
an SSTO, and probably even more who think that SSTO advocates are
overly optimistic in their cost estimates. But the proportion of
engineers who believe an SSTO is an engineering impossibility seems
to be steadily dropping.
>Engineers go through fads just like other disciplines; this
>is even more true when a government monopoly holds the purse
>strings.
>
>>Even with pretty conservative assumptions, a 20
>>MW laser would cost less than $2 billion using CO2 and less than
>>$20 billion using other technologies.
>
>Same old story -- launch cost reduction assumes huge launch
>volume. Problem is your market is near zero -- nobody is launching
>20 kg payloads, not even Iridium (by an order of magnitude).
Well, yes, as I noted, that is a basic problem with a laser launch
system; it is primarily a volume launcher. When I began work on the
concept, we had both a potential massive subsidy and a potential customer:
SDIO was expecting to build a laser for its own use that would be
easily large enough to use as a launcher, so the incremental cost to
equip it for such use would (we hoped) be small, and SDIO was considering
various space-based hardware that would have been well-matched to laser
launch -- Brilliant Pebbles and associated decoys in particular, and
fuel and other consumables for space hardware in general. I'm _not_
working on laser propulsion actively right now precisely because there
is neither a market sufficient to pay for a commercial system, nor
a national need (for either a laser or a launcher) sufficient to
get one built at government expense. It is a feature of a laser launch
system, however, that once built, it is likely to have very large
excess capacity and low operating costs, and thus substantially change
the market economics. One example is that NASA is considering (albeit
at a low level) building 10 MW-class lasers to beam power to a lunar base;
such a system would be comparable in costs to other options (reactor,
solar/fuel-cell) for powering a lunar base, but once built could do
many other things, including launching small payloads. NASA does not
consider the technology sufficiently well established to make it the
first choice for lunar base power, though, and more important, the U.S>
seems to have decided not to spend the money to build such a base.
>Even if you could get the entire commercial space user industry
>switched over to these "nanosats" in ten years you'd never amortize the
>thing.
This is a classic chicken and egg problem, and applies to all commercial
space activities, quite correctly. The existing (and linearly projected)
market is too small to support _any_ advanced development costing more
than a few $10 million (e.g., Pegasus @ $50M). Most of us who advocate
space development believe that at some launch cost ($100/lb is my guess)
and ease of access to space, there will be a class of space industries
that will generate large revenues, but since such revenues are neither
immediate nor accurately predictable, no sizeable investment can be
justified to the capital markets of the world.
Given that situation, however, one can either cite it as proof that there's
no way low cost access to space can ever be achieved, or one can
continue to advocate R&D work, promote interest in space, and
hunt for engineering or political loopholes that will either lower the
capital needed or raise the capital available for space flight -- and
wait for the next turn of the wheel.
Jordin Kare
From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: beamed power (was Re: a question to an expert...)
Date: Thu, 12 Mar 1998 23:08:07 GMT
In article <3506B850.D3@mit.edu>, Jason C Goodman <AS_goodmanj@mit.edu> wrote:
>> If we're using lasers at 1 um wavelength ...
>
>Yeah, but two things. First, we're nowhere near able to build
>transmitters and receivers which operate efficiently at that
>wavelength...
Sorry, wrong. Oh, we can't do 90%, but we can do 50% (at each end) at
wavelengths in that vicinity. Not great, but workable, at least for
modest distances (interplanetary distances will be problematic for a while
yet, I fear). Laser power beaming has been seriously proposed for both
geostationary comsats (to avoid spending a lot of mass on batteries) and
for night power at a lunar base (where the storage problem is greatly
compounded by the very long nights).
>The specs on a typical high-power IR LED
>(http://support.tandy.com/support_supplies/doc9/9734.htm)
>suggest that it's maybe 1% efficient (forward voltage drop 1.5 volts;
>forward current 100 mA, radiant power output 16 mW); a solid-state
>laser should have similar efficiency...
Nope, false analogy. Diode lasers already get up into the 50% range; they
are by far the most efficient light sources known. They don't scale up
easily, and there are some headaches in combining the output from large
numbers of them, and the available range of wavelengths isn't as wide as
one would like... but they're getting better rapidly. If you insist on
having something that comes in bigger lumps, even CO2 lasers get up into
the 10-20% range (although receiving is more of a problem at their long
wavelength).
>IR photodetectors are even less efficient.
Solar cells actually do quite well -- again, in the vicinity of 50% --
when the incoming wavelength is well matched to the type of cell. The
only reason they are so inefficient with sunlight is that sunlight is such
a wide spread of wavelengths. (A photon which is not energetic enough to
kick an electron across the cell's bandgap is completely wasted; one which
is more energetic than necessary wastes the extra energy. I oversimplify,
but that's the right general picture.) Silicon cells, in particular, work
quite well with near IR.
--
Being the last man on the Moon | Henry Spencer
is a very dubious honor. -- Gene Cernan | henry@zoo.toronto.edu
From: kare@sirius.com (Jordin Kare)
Newsgroups: sci.space.tech
Subject: Re: Laser propulsion method in space?
Date: Fri, 22 May 1998 14:28:57 -0700
In article <35615540.2C45@Biogate.com>, JBordas@Biogate.com wrote:
>... I
> attended a panel at a science fiction convention in Chicago 1991 that
> covered the use of lasers as a means of propulsion. The researchers
> refered to the test vehicles as MIRV's for Multiple re-entry
> vegetables. The researchers used pototoes pumpkin etc as the ablative
> material.
Well, no, we didn't. But we thought about it. Specifically, we thought
about using cucumber slices as a convenient way of getting nearly-pure
water in "solid" form, but it turned out not to be a useful approach. But
it did lead to some after-hours speculation about Reentry Vegetables,
Watermelon boosters with zucchini strap-ons, etc.
> There are several draw backs to laser launching. ... Another is the high
accelleration that is required.
No again. Laser launch systems run at accelerations very similar to
chemical rockets, and can easily be "throttled" to limit the maximum
acceleration.
Jordin Kare
Original developer of the Multiple Independent Reentry Vegetable concept
From: jtkare@ibm.net (Jordin Kare)
Newsgroups: sci.space.tech
Subject: Re: Laser launch
Date: Fri, 01 Jan 1999 16:03:04 -0800
In article <F4p0L3.66J@T-FCN.Net>, maury@remove_this.istar.ca (Maury
Markowitz) wrote:
> In <19981227191138.08217.00002472@ng-fb1.aol.com> Beanstalkr wrote:
> > Not too accurate. Optical focus is dependendant on the geometric
> > precision of the mirror shape. If you send a light beam from a perfectly
> > parabolic mirror, you get a perfectly parallel beam.
>
> Sorry, but this simply isn't true, Bruce's message is right on the money.
> All optics have specific limits, even in theory, that are the result of
> diffraction.
I regret missing the beginning of this thread. I assume the proposal had
to do with a non-laser source for "laser thermal" (i.e., beamed thermal
energy) propulsion.
The limit for a non-laser source is rarely diffraction, and is independent
of how perfect the mirror is.
>The problem with a typical "lamp" is actually twofold, one is
> that the light comes from a region that is macroscopically large, on the
> order of inches
This is correct. The key parameter for the light source is "radiance",
which is power per unit area of the source per unit solid angle. The
"brightness" of a source (power per unit solid angle) can be increased by
using an optical system, such as a parabolic mirror, to increase the
effective area of the source (i.e., replace the small area of, say, an
incandescent filament with the much larger area of the mirror) but no
geometric optical system (mirrors, lenses, prisms, etc.) can increase the
radiance of a source. For thermal sources (incandescent lamps, arc lamps,
etc.) the radiance has an upper limit set by the temperature of the
source; the maximum possible radiance is equal to the radiance of a black
body at that temperature. The highest broadband radiance numbers are
achieved by high-pressure arc lamps, which can produce a few hundred
watts/cm^2-steradian. (There's some neat technology to do this at up to
>100 kW in a single lamp!)
Sunlight in space near Earth is around 2000 W/cm^2-sr (0.13 W/cm^2 from a
little under 1/2000 steradian)
The best flux you can put on a target is (source radiance) * (source
aperture) / (4*pi*range^2). So as has been noted, you can use a really
large source aperture to make up for low source radiance, but if you want
useful laser-launch fluxes (in the ballpark of 1 kW/cm^2, perhaps 1/10 of
that on the Moon) your mirrors have a useful range of not much more than
their diameter -- no matter what shape they are.
Noncoherent arrays of laser diodes can do better by a factor of 1000 in
radiance, and thus a factor of 30 in range. If all you need is
sunlight-intensity 0.1 W/cm^2 (e.g., to run solar panels) instead of 1
kW/cm^2, that's another factor of 100 in range. So noncoherent laser
diode arrays can beam useful power over 10's of kilometers from
meter-sized "searchlight" apertures, but not do laser launch without
really unreasonable (km-sized) mirrors.
> One solution though is to use "unreasonably" sized mirrors and simply
> accept optical lossage by increasing the power input.
Sorry, no matter how much power you feed in, you don't get any more flux
on target -- you can't stuff a second lightbulb into the space already
occupied by one bulb. You just illuminate more area at the target plane.
> Another issue that Bruce didn't touch on is that conventional bulbs put
> out their energy in a very wide spectrum. The issue here is atmospheric
> diffraction which is different for different wavelengths, and you'd thus
> need a spectal-aware mirror on the spacecraft end that changes the focus
> points for various distances from the laser.
You can correct for such things with a suitable chromatic corrector
(basically a prism somewhere in the optical system) but it's not going to
do much good.
> Finally it's possible to reduce all of this into a single number - the
> amaount of energy being received is simply the "brightness" of the source.
As noted, the key number is radiance, rather than brightness.
> Lasers are, by far, the brightest sources known to man. In the x-ray
> region for instance, modern FEL's outshine the sun by a wide margin.
X-ray FELs? Are you sure you don't mean X-ray synchrotron sources? To my
knowledge, no one has achieved FEL operation in the X-ray regime.
> Even
> in the visible lasers are the brightest known light sources - FOR SPECIFIC
> FREQUENCIES! The later point is the issue because of the last paragraph
> though.
Even broadband in the visible, laser sources have orders of magnitude
higher radiance than the Sun -- just not as big an aperture :-) :-)
> > It is only atmospheric scattering that diverge the beam. In space,
> > this would not be a problem over millions of miles.
>
> Even for an earth bound launcher system this problem can be largely fixed
> with a feedback loop between a retroflector (what do they call the
> time-reversing ones?)
Phase conjugate reflectors
> on the tail of the spacecraft,
You don't need phase conjugation on the vehicle, just an ordinary corner
cube retroreflector. Phase conjugation is one option for correcting the
beam at the laser; it's currently used in some laser systems to correct
for distortions in the lasing medium.
> and active mirrors on
> the launch laser.
This is not a trivial or solved problem; there are other factors involved
such as coupling between "thermal blooming" (the beam heating the air it
goes through) and turbulence. However, it appears to be solvable for
laser launching, especially if one uses infrared rather than visible
sources.
> One thing that I don't seem being mentioned much is the use of laser
> propulsion for deep space exploration. Laser _launching_ is a little hard,
> but using the same system as a propulsion system for vehicles already in
> space is a no-brainer. Has anyone done any analysis on what such a system
> would do for missions like man-on-Mars? It seems to me the power-weight
> for a ablator disk on the ship plus an even _moderately_ powerful laser on
> earth far exceeds the Mars Direct systems or nuclear-ion systems.
The difficulty is that for deep space missions, the laser only boosts you
for a tiny fraction of the distance unless you have truly enormous
transmitting and receiving optics, whereas a nuclear or solar-electric
system can provide thrust over as much of the trajectory as you want. (I
took part in a NASA Horizon Mission study of a fast (1 month transit time)
mission to Jupiter; my proposal used laser thermal propulsion with, as I
recall, a 100 kilometer transmitter aperture. The laser system was
capable of directly lighting a cigarette on Io, assuming anyone on Io
smokes, and uses *very long* cigarette holders :-)
Also, the main beauty of laser systems is that they reuse the expensive
part (the laser and transmitting optic) for many missions -- up to tens of
thousands per year, for Earth-to-orbit launch. By contrast, even the most
optimistic plans call for only a handful of Mars missions per year, so the
capital cost of a useful-scale laser is exorbitant.
> > Why be negative when you don't know the subject?
Because if you're negative, you attract positive people??
> Seems pretty clear to me that Bruce knows the subject pretty well, as one
> would expect given his address, bmac@igpp.llnl.gov - LLNL is working on
> laser propulsion, or WAS anyway.
Was. But may be again.
Jordin (fully coherent, but not monochromatic) Kare
Formerly jtk@llnl.gov; former Technical Manager, SDIO Laser Propulsion Program
From: jtkare@ibm.net (Jordin Kare)
Newsgroups: sci.space.tech
Subject: Re: Microwave Lightcraft -any info?
Date: Thu, 28 Oct 1999 18:27:19 -0700
[[in response to a question about Leik Myrabo's work]]
Last time I ran into Leik, a few weeks ago, he mentioned that he'd gone
back to look at some of the work done in the SDIO Laser Propulsion Program
in 1990, where Dennis Reilly of Avco Everett Labs got very high "coupling
coefficients" (impulse per unit laser energy) by ablating Delrin, a light
plastic. Leik tried one of his vehicles with a Delrin ring placed where
the laser focuses (the vehicle geometry causes the laser to focus in a
circle inside the "skirt") and got several times higher thrust than he'd
been able to get with his prior vehicles that used air heating alone.
Apparently he banged the vehicle against the lab ceiling hard enough to
damage something -- vehicle or ceiling, I forget.
Unfortunately, it wasn't clear in 1990, and it still isn't clear today,
why Delrin gives such high coupling, but it's not likely to be achievable
in a launch system. If I recall (it *has* been almost a decade) the
measured Isp (impluse per unit mass ablated) was also quite high, so it
wasn't simply that the Delrin was ablating at low velocity (that gives
high coupling but low Isp). My hypothesis at the time was that the
ablated Delrin was burning in the air, releasing substantially more energy
than the laser pulse delivered (but obviously not in a fashion that would
work in vacuum, or even very well in air at high velocities). There were
also possible systematic errors in mass and impulse measurements. Alas,
the program was shut down before we could find out exactly what was going
on.
Still, Leik was pretty excited; I bet his Delrinned vehicles are neat to
watch...
..
> Suppose they gave him the lasers from the National Ignition Facility
> (NIF is big waste of time IMHO, the the HO of quite a few fusion
> researches), would that sort of power be able to launch small
> satellites into LEO.
>
> Barry Adams
As Henry notes in another reply, NIF has an extremely high peak power and
substantial pulse energy, but can fire only a few pulses per day, so its
average power is very low. (NIF's predecessor, NOVA, could produce 100 kJ,
10 TW (yes, terawatt) pulses, but took a minimum of an hour or so between
shots, and normally operated at 1 - 2 shots per day, or about 1-2 watts
average).
Similar technology -- diode-pumped solid-state lasers -- has been used for
multi-kilowatt average power lasers, and has been proposed in slightly
modified form for lasers producing multiple megawatts of average power for
a few seconds at a time.
Jordin Kare
Newsgroups: sci.space.science
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Theoretical ISP of ground based laser powered "rocket looking
thingy"
Date: Thu, 29 Jun 2000 16:31:32 GMT
In article <39581ebe.264076964@24.92.1.45>,
James Lerch <Jlerch1@tampabay.rr.nospam.com> wrote:
>"Launch" vehicles couple hundred feet into the air. My first thought is "How
>the hell is he going to stabilize the thing" As during the launch the vehicle
>kept falling out of the beam.
His current little test vehicles are more or less self-stabilizing, but
larger ones would need active control. No big deal.
>#1 There is no AIR in space!
So take along propellant. And there's air when you start out, and you
might as well use it if you can. (Admittedly, Myrabo has some awfully
optimistic ideas about complex airbreathing cycles for use on the way up.)
>#2 Your gonna need one hell of a power source AND a laser capable enough to
>handle it AND a way to accurately aim it!
Power sources are not a serious problem until you get up into the
multi-gigawatt range. Aiming technology is more or less in hand, from SDI
work. Big lasers are less developed, but adequate ones for at least small
payloads can be built fairly straightforwardly today.
>#1 How much thrust do you get and is it scalable?
Depends on how much energy you put in and how much propellant you use;
there are a number of different schemes. Accelerations of several G are
almost certainly feasible. It scales up better than it scales down.
>#2 How powerful a laser do you need, and can we build it?
For orbital launches using on-board propellant, a megawatt per kilogram of
payload is a good planning number. Even small payloads are interesting,
because you can launch one every few minutes, and most things you'd want
to launch can be subdivided. (People are about the only exception.)
>#3 Can we accurately aim a laser this powerful?
Yes. Basically a solved problem. Not cheap, but understood.
>#4 How important is the difference in angle between the vehicle/flight path and
>the vehicle/ground based laser?
Depends on the vehicle design. For many of them, it makes no difference
at all, except that you need to finish accelerating before the thing goes
over the horizon.
>At this point I'm thinking "ain't no way..."
Lots of ways. For the cost of one shuttle or Titan IV launch, you could
build a system capable of maybe 50kg to orbit (every few minutes -- the
total annual payload throughput, if the thing is used intensively, equals
the total annual payload of the shuttle and Titan IV systems). It
wouldn't be one of Myrabo's fancy multi-cycle airbreathers, but a simple,
conservative pure-rocket system using liquid hydrogen heated in a heat
exchanger.
>Then I get to thinking. "Could" we build a laser powered 1st stage booster to
>boost a vehicle straight up to 100,000 ft or so at a 3G acceleration rate then
>have a conventional second stage provide the tanginal velocity to achieve
>orbit?
This is not a good use of the technology, as it turns out. The cost of
the laser and optics are very sensitive to the mass they have to lift, so
you would prefer to avoid including a heavy upper stage. They are rather
less sensitive to the velocity achieved, at least up to orbital ranges,
so you want to do SSTO rather than messing around with kick stages. The
system has enough performance advantage that SSTO ought to be easy.
>Would you actually gain any advantage over conventional Launches?
Lots. Once you've built the laser, *using* it is cheap.
--
Microsoft shouldn't be broken up. | Henry Spencer henry@spsystems.net
It should be shut down. -- Phil Agre | (aka henry@zoo.toronto.edu)
Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: laser propulsion (was: Re: femto, atto, ecto, and yacto seconds)
Date: Sun, 27 Aug 2000 05:58:15 GMT
In article <8o800n$lar$1@mach.thp.univie.ac.at>,
Jonathan Thornburg <jthorn@galileo.thp.univie.ac.at> wrote:
>To the best of my knowledge, no payload has yet been launched to orbit
>via a laser-based launcher.
Correct. It could probably be done with a modest development effort and a
big chunk of cash to build the laser and the optics. There are people
trying to find funding for this, so far without success.
>>very powerful, efficient, quiet, and safe.
> ==========================
>
>Lasers tend to be rather inefficient -- 10% conversion of input power
>into output light is considered rather good.
Not any more. The best semiconductor lasers are 40-50% efficient -- they
are much the most efficient light sources available! (Provided that you
want infrared and aren't fussy about how coherent it is.) Many other
types of lasers are now pumped using them instead of flashlamps etc.
>Has anyone seriously studied noise levels from laser launchers?
Depends on the type. Heat-exchanger systems would be no more noisy than
any other small rocket. Pulsed-ablation systems would be LOUD.
>As to safety... well, any laser launcher would also make a great
>antisatellite or antiaircraft weapon.
Not as good as you might think; launch lasers and weapon lasers optimize
quite differently -- for example, weapons want short wavelengths while
launch systems prefer long ones -- and launch lasers would probably rely
on help from the vehicle for beam tracking etc.
--
Microsoft shouldn't be broken up. | Henry Spencer henry@spsystems.net
It should be shut down. -- Phil Agre | (aka henry@zoo.toronto.edu)
Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Laser propulsion... push or pull?
Date: Wed, 6 Sep 2000 18:59:31 GMT
In article <39AE1C62.177B@ix.netcom.com>,
Scott Lowther <lexcorp@ix.netcom.com> wrote:
>1)Push: the laser is essentially stationary with respect to the craft,
>and the trajectory can be tailored so that the laser is always due aft.
Actually, that tailoring is difficult, it's actively undesirable, and many
laser-launch schemes don't require it.
It's difficult because at least the final moments of thrust must be
tangential to the destination orbit, unless you do something tricky and
wasteful.
It's undesirable because you end up firing through the whole exhaust
plume. The ability to bring the beam in at an angle simplifies things
immensely.
And it's unnecessary because many of the designs either can compensate for
an off-center beam, or just plain don't care. (For example, the laser-
detonation-wave thruster thrusts at right angles to the propellant
surface, regardless of which direction the laser beam arrives from.)
>1) Push: limited horizon, fires through increasignly more atmosphere and
>fires through engine exhaust.
As noted above, if the beam comes in at an angle, it doesn't have to
penetrate much exhaust. What little there is, basically doesn't matter:
the remaining travel of the beam is too short for defocusing etc. to have
any significant effect.
The atmosphere is a constraint but not a severe one. It does make you
prefer a relatively high acceleration -- a few G -- to get the thing up to
orbital velocity while still within line of sight.
>So given equal lasers, which would be better?
The overwhelmingly dominant issues for near-term systems are that a
ground-based laser and its power supply can be arbitrarily heavy and can
be fixed when they break.
--
Microsoft shouldn't be broken up. | Henry Spencer henry@spsystems.net
It should be shut down. -- Phil Agre | (aka henry@zoo.toronto.edu)
Newsgroups: sci.space.tech
From: henry@spsystems.net (Henry Spencer)
Subject: Re: laser propulsion (was: Re: femto, atto, ecto, and yacto seconds)
Date: Wed, 6 Sep 2000 20:23:28 GMT
In article <8ofp0t$1cb$1@mach.thp.univie.ac.at>,
Jonathan Thornburg <jthorn@galileo.thp.univie.ac.at> wrote:
>>The best semiconductor lasers are 40-50% efficient -- they
>>are much the most efficient light sources available! ...
>
>What's the highest average power that's been demonstrated at that
>sort of efficiency level? The last I heard it was 500 milliwatts
>or so, but I haven't followed recent developments closely.
Off-the-shelf laser arrays with that sort of efficiency have been up in
the multiple kilowatts for several years now. Note, "arrays". These are
not single laser diodes, and as I noted, some of their properties are less
than ideal.
--
Microsoft shouldn't be broken up. | Henry Spencer henry@spsystems.net
It should be shut down. -- Phil Agre | (aka henry@zoo.toronto.edu)
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