Date: 23 Apr 91 18:23:41 GMT
From: ssc-vax!bcsaic!hsvaic!eder@beaver.cs.washington.edu  (Dani Eder)
Subject: Re: Gas Guns and Tethers

In article <2753@ke4zv.UUCP> gary@ke4zv.UUCP (Gary Coffman) writes:

>Railgun, coilgun, gasgun, whatever, all are technically feasible for
>small payloads that don't mind thousands of Gs. If you try to scale
>them up to worthwhile payload size, or try to stretch them out for
>reasonable G forces, then either the enviornmental problem of ground
>level hypersonic shock waves, or the sheer physical size of the launcher
>make them impractical.
>
>Tethers need to exceed the theoretical strength of materials limits by
>orders of magnitude to work. Again, a fundamental scientific breakthrough,
>not engineering R&D, needs to occur before tethers can become reality, if
>ever.

The lower bound on accelerator g's can be found by considering the
west slope of the island of Hawaii.  There is a region of nearly
constant slope approximately 20km long extending from about the
1 km altitude to approximately the 4 km altitiude.  If a launcher
has a muzzle velocity of 5000 m/s, then basic physics shows the
acceleration required is a=v^2/2d=625m/s^2, or 63 g's.  While this
is more than people can stand, it is by no means 'thousands' of g's.

As far as wothwhile payload size, I personally believe that I could
do useful things with 10 kg payloads at a time, given some progress
in the direction of 'brilliant pebbles', or very small spacecraft
that have sensors and maneuvering capability.  However, I have designed
a gas gun that delivers 5 ton payloads at a time.

Yes, the noise generated by a big hypersonic launcher is great.  So
is the noise generated by the space shuttle.  Give me a similar
5 km clearance zone and the noise will be tolerable.

On the subject of tethers, it is well known that the minimum mass
tether is an exponential taper in cross section per 'scale length'.
The scale length is the length under 1 g which can support it's own
weight under 1 g.  For Amoco performance products T-1000 graphite
fiber, which has a tensile strength of 6895 MPa and a density of
1845 kg/m^3, the scale length under 1 g is 381 km.

If we limit ourselves to an area taper ratio of 100, then we can\
dangle ln(100)=4.6 scale lengths of cable.  This is 1752 km.  The
Earth's gravity well is 6375 km deep, thus we are 27% of the way there.
The theoretical strength can be found from the work done in
diamond anvil cells that reach extremely high pressures.  The
cells routinely reach pressures of over a megabar, or 1 million
atmospheres.  This is 15 times the strength of the graphite fibers.
Thus there is plenty of room to reach a sufficient strength to
do a simple tether.

This ignores the fact that you do not have to have a tether all
the way from orbit to the ground.  A shorter tether hanging vertically
in orbit will have it's lower end moving sub-orbitally.  A launch
system then only has to reach the bottom of the tether, rather than
orbit.  Anything that makes a launch vehicles' job easier is
beneficial.  The reminder of the ride to orbit (which is at the
center of mass of the tether) can be via elevator.

From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Cannon Launch? (Very cheap access to space)
Date: Jul 19 1996
Newsgroups: sci.space.policy,sci.space.tech

glass@mrbig.rockwell.com (Jim Glass) writes:

[in reference to Gun Launchers]

>Can't launch human beings on it.

Actually, if you have a water immersion seat for the human, you can push
to about 14g's, and if you use the west slope of the island of Hawaii
as your gun site you can get a 20 km barrel length.  Therefore you can
reach a muzzle velocity of 2366 m/s, or about Mach 8.  Tha't only about
one third of orbital velocity, but the onboard rocket to get you the
rest of the way to orbit would have an easier job by one third then an
SSTO.

Assuming a crew of one, with a total payload of 2 tons (waterbed-like
seats could be pretty heavy), and a hardware fraction of 15% (14 g's
requires more structure than an SSTO rocket), and an ISP of 350 sec
(for LOX/hydrocarbon, LOX-H2 takes up too much room in the gun barrel
and in flying through the air), I get a 5% payload fraction, therefore
a 40 ton projectile.

To accelerate at 140 m/s^2 requires a force of 5.6 MN (about 1 million 
pounds).  If the barrel diameter is 2 meters, this is spread over 3.14
square meters, hence the pressure required is 1.78 MPa (about 258 psi).
This is not too challenging a pressure.  Since the muzzle velocity is
below the characteristic velocity of a LOX-Hydrocarbon engine, you
could use a rocket engine at the bottom of the barrel to fill the
barrel with hot gas.  Something about the size of an F-1 engine would
do nicely, but since it doesn't have to fly, it could be water-cooled
and built heavier than a flying rocket engine.

Dani Eder

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Mass Drivers and All That
Date: Sun, 24 Dec 1995 04:18:06 GMT
Organization: SP Systems, Toronto
Lines: 49

(Prefatory note:  I actually agree with some of the things Keith is
saying, but he's going a little overboard in a few places...)

In article <4bevhi$3qj@chip.klic.rain.com> keithl@chip.klic.rain.com
(Keith Lofstrom) writes:
>Our 100Km mass driver produces accelerations of 50 G's (and much larger
>deccelerations when the payload leaves vacuum and hits atmosphere) .
>For reference, a long range naval cannon produces about 150 G's.  Not
>too many things are capable of being delivered by artillery.  I'm not.

Actually, a remarkably large number of things are capable of being
delivered that way, especially if one is allowed to modify the design
slightly for greater robustness.  The electronics in a smart artillery
shell look surprisingly ordinary in terms of physical construction; if
you keep the circuit boards small, anchor them solidly, and make sure all
parts are firmly secured to the boards, your electronic device has a high
probability of surviving an artillery launch.

The most fundamental limitation of such launch systems is not the high
acceleration, but the small payloads.  The launcher hardware scales almost
entirely with the size of a single payload, and only minimally with the
launch frequency, so it's obvious at first glance that the way to maximize
mass throughput for a given capital investment is lots of small payloads,
as small as practical.  (The lower limit for a gun system is probably set
by the requirement of surviving passage through the atmosphere.)

>We don't need raw materials in space - there aren't any refineries,
>metal shops, semiconductor fabs, or assembly lines up there...

Raw materials don't necessarily have to be inputs to sophisticated
manufacturing processes.  For example, about half the mass currently
lifted into LEO is rocket fuel.

>Even
>liquids and gasses will probably have to be frozen solid to survive
>the trip...

Uh, why?  Liquid-filled artillery shells were made by the millions
80 years ago.  No particularly remarkable problems were involved (apart
from those caused by the nasty nature of most of those liquids).

>...and require heavy tankage...

If half the mass goes for tankage, and the launch cost per kilogram is
half that of a rocket, you're still ahead of the game (even if the only
use for the empty tanks is ballast mass to reduce the frequency of reboost
burns for LEO stations).
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu

From: higgins@mecheng.mcgill.ca (Andrew Higgins)
Newsgroups: sci.space.tech
Subject: Re: Maglev Launchers
Date: Tue, 07 Apr 1998 01:44:29 +0200

In article <35294AFF.BAE3EEA8@ASmit.edu>, Jason Goodman
<goodmanj@ASmit.edu> wrote:

> Andrew Higgins wrote:
>
>>Calculations, please.
>>Some very clever people have thought about this.  By using techniques such
>>as aerobreaking, the delta V required to circularize the orbit of a
>>gun-launched satellite can be keep to under 1 km/s.
>
> Calculations, please.

See the reference cited below.

> How on Earth is this possible?  aerobraking
> always reduces the kinetic energy of a projectile (in a reference
> frame fixed WRT the rotating Earth); you need to increase the kinetic
> energy by a factor of about 100 to circularize the orbit.

The problem (or perceived problem) with gun launch to space is the
*direction* of the velocity vector, not it's magnitude.  Aerobraking does
not save kinetic energy; rather, it is a more efficient means of
re-directing the momentum vector than rockets.

For example, with muzzle velocities of 8 to 10 km/s, it is quite easy to
launch a payload to an altitude of several thousand kilometers.  However,
regardless of what apogee you reach (less than escape), your gun-launched
projectile will be in an orbit that returns to intersect the Earth's
surface.  Of course, some delta V is required to circularize the orbit.

The original poster was performing calculations where, as the projectile
reached the desired orbital altitude, a single delta V burn was performed
to circularize the orbit.  His conclusion was that a delta V of several
km/s was required.

Fortunately, there are more judicious techniques available for
circularizing the orbit.  For example, if you gun-launch to an altitude of
several thousand kilometers, then perform a small delta V burn to raise
the perigee to 50 or 70 km, you can aerobrake to lower the apogee for as
many passes as needed.  When the apogee is lowered to the desired orbital
altitude (say, 500 km), a final small delta V burn circularizes the
orbit.  The *total* delta V required for this technique can be keep to
under 500 m/s.

The details of the calculations can be found in:

Ref:  Kaloupis, P., Bruckner, A.P., "The Ram Accelerator:  A Chemically
      Driven Mass Launcher," AIAA 88-2968, AIAA/ASME/SAE/ASEE 24th Joint
      Propulsion Conference, July 11-13, Boston, MA, 1988.

..available in any good engineering library or can be ordered directly
from the AIAA.
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec

From: higgins@mecheng.mcgill.ca (Andrew Higgins)
Newsgroups: sci.space.science
Subject: Re: Gun lanched space craft
Date: Fri, 20 Nov 1998 20:11:47 +0200

In article <3651DABF.73500F71@burningbridges.com>,
burningb@burningbridges.com wrote:

> bjaques@ea.com wrote:
> >
> > Bull also had built a very big gun at North Hatfield, Quebec, for
> > ballistics testing, using three welded-together US Navy WW I naval > cannons
>
> I'd think that such a system could be useful for delivering bulk raw
> material to orbit, but because of the G forces involved in accelerating
> down the barrel probably not applicable to launching entire operational
> spacecraft.

You might tell that to the U.S. Navy.  They just introduced a new
gun-launched rocket (ERGM--Extended Range Guided Munition) which will be
fired from standard naval 5-inch guns.  Each shell carries a GPS receiver
and a fiber-optic laser gyroscope.  The laser gyros used are going to be
commercially available gyros developed for the automobile industry and
cost about $25 a piece.  Apparently, no special modifications are required
to withstand the 10,000 g's of gun-launch.

>
> Specifically, are the engineering requirements for the
> projectile reasonably attainable-- eg., can the thermal stresses
> inherent, even for a short period of time, in firing a shell through
> standard atmospheric density at 8 km/sec be handled by known active
> ablatives?
>

In a word, "yes."  Quite a few people have looked at this for a variety of
gun-launch-to-orbit concepts (railgun, coilgun, gas gun, ram accelerator,
etc.) and the answer is that standard ablatives are good enough for the
job.  Of course, you might not need ablatives at all.  Take a look at:

     http://asm.arc.nasa.gov/projects/sharp/sharp.shtml

>
> And just how much projectile kinetic energy will be lost to
> atmospheric drag for a typical shell geometry-- small enough to still
> make the method economically advantageous over rockets?
>

For a muzzle velocity of 8-10 km/s, the vehicle looses 1-2 km/s due to
drag during atmospheric transit.

As for "economically advantageous", sounding rocket packages gun-launched
by Project HARP in the 1960's were at least an order of magnitude cheaper
than competing sounding rockets.
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec

From: Andrew Higgins <higgins@mecheng.mcgill.ca>
Newsgroups: sci.space.science
Subject: Re: Gun lanched space craft
Date: Tue, 01 Dec 1998 00:35:36 -0500

Richard Kaiser wrote:
>
> In article <3661FE16.6608@mecheng.mcgill.ca>, higgins@mecheng.mcgill.ca wrote:
> >
> >Richard Kaiser wrote:
> >>
> >> A gun launch is only 20% of the solution.
> >>
> >> The gun would place the playload into an orbit that would include the gun if
> >> the earth were not rotating.  In other words, the payload would return to
> >> earth.  Turning this ballistic trajectory into an orbit requires a delta-V
> >> burn in orbit that is four times bigger than the delta-V imparted by the
> >> gun.
> >
> >Calculations, please.
> >
> >Some gun launch concepts have muzzle velocities of 7 to 9 km/s.  The
> >burn required to circularize the orbit once above the atmosphere can
> >be as small at 0.5 km/s.
>
> I don't have time to redo these calculations now.  If you are really
> interested get youself a copy of "Fundamentals of Astrodynamics"
> by Bate, Mueller and White.  Its ISBN0-486-60061-0 and its only
> about $11.  If you want to be taken serious in astro know this book.
>

Yes, and with Bate, Mueller, and White, you get what you pay for.

>
> The big problem with guns is they can never get the velocity direction
> correctly.  In a simplified launch, 20% of the delta V is converted to
> potential energy to get into space.  Once in space the remaining
> delta V is required to reach orbital velocity.
>

This is simply incorrect.  No one who has seriously studied gun launch to
orbit using the advanced high-velocity launcher concepts available today
suggests launching straight up.  While determining the optimal launch
angle is not a straightforward calculation, most design studies for gun
launch to orbit have selected a launch angle of about 20 deg.  With such
a launch angle, and a muzzle velocity of 5 km/s or greater, it is
possible to get above the atmosphere with a significant velocity
component in the correct direction (perpendicular to the position
vector).

With some clever techniques, like apogee burns and aerobreaking, it is
possible to circularize the orbit with a modest expenditure of
propellant.  For example, if you gun-launch to an altitude of several
thousand kilometers, then perform a small delta V burn to raise the
perigee to 50 or 70 km, you can aerobrake to lower the apogee as needed.
When the apogee is lowered to the desired orbital altitude (say, 500 km),
a final small delta V burn circularizes the orbit.  The *total* delta V
required for this technique can be keep to under 500 m/s.

For the details, look at a design study produced by my old research group
at the University of Washington, Seattle:

      Kaloupis, P., Bruckner, A.P., "The Ram Accelerator:  A Chemically
      Driven Mass Launcher," AIAA 88-2968, AIAA/ASME/SAE/ASEE 24th Joint
      Propulsion Conference, July 11-13, Boston, MA, 1988.

..available in any good engineering library
--
     Andrew J. Higgins            Department of Mechanical Eng.
     Shock Wave Physics Group     McGill University
     higgins@mecheng.mcgill.ca    Montreal, Quebec

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Easy way of getting something into orbit
Date: Sat, 12 Feb 2000 15:55:42 GMT

In article <881b6g$o5v$1@pegasus.csx.cam.ac.uk>,
Ziggi <ziggi@mindpower.swinternet.co.uk> wrote:
>...Would it not make sense to build
>a horizontal linear accelerator, a cut down version of Maglev, over a few
>kms, accelerate the spacecraft (or whatever) this way till it's a couple
>times the escape velocity, then put a bend at the end of the track,
>launching it vertically?

I'm afraid not.  As Doug has already pointed out, the engineering problems
of that bend are formidable.

More significantly yet, typically you don't *want* to launch things
vertically.  Yes, rockets go up vertically... but notice how quickly they
tip over and start angling off horizontally.  Horizontal velocity, not
altitude, is the hard part of reaching orbit.  Rockets take off vertically
only because they have to get out of the atmosphere before they start
serious (horizontal) acceleration.

Gun/catapult launch schemes typically end up working best if launched only
moderately above horizontal.  Some angle up is desirable, to minimize the
mass of atmosphere the projectile has to punch through.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sat, 12 Feb 2000 16:21:09 GMT

In article <8819uv$hjf$1@crib.corepower.com>,
Nathan Urban <nurban@vt.edu> wrote:
>> One of Gerald Bull's projects was an attempt to use a cannon to fire a payload
>> into orbit. The cost was miniscule compared to conventional launches...
>
>There are still people who talk about electromagnetic launchers (coilguns,
>railguns).  I remember reading about one of the better known research
>facilities on Usenet, but I can't remember what or where it was.  I don't
>know of anyone pursuing a traditional chemical propellant based design.

Nobody's really pursuing a classical "powder" gun, because there are some
fundamental limits on its muzzle velocity, which are too low to make it
very attractive.

(Bull's projectile needed *four* solid-rocket upper stages to make it into
orbit after gun launch.  Which is ridiculous; four solid stages can get
things into orbit by themselves.  The idea that such a gun launch would
somehow be enormously cheaper than rockets is a peculiar fantasy, usually
based on stacking the deck, e.g. assuming very cheap upper-stage rocket
technology but comparing the results against conventional high-cost
rockets.)

(Many novel launch schemes need some amount of help from rockets.  What
kills a lot of them is doing a tradeoff study of just enlarging the rocket
part and getting rid of the non-rocket part.  Surprisingly often, that
works out to be better and cheaper.  Rockets are a very good way to reach
space; the problem is *not* the basic physics of rockets, but rather the
way current rockets are developed and operated.)

There is some work in progress on the ram accelerator, which is a fairly
unorthodox chemical-fuel gun:  a tube filled with fuel/air mixture and a
carefully-shaped projectile accelerating along it, with the gap between
the projectile and the wall forming a ramjet (more or less).  This work
is being funded partly because the Army thinks it might lead to better
tank guns.

Light-gas guns, which use chemical (or other) means to heat and compress
hydrogen and then use *that* to push a projectile out of a barrel, have
had some work, e.g. Livermore's SHARP gun.  Their limits are rather higher
than those of powder guns.  Funding is pretty minimal at the moment,
though.

Various forms of electromagnetic catapult are being investigated on a
modest scale, over a wide range of performance, everything from a maglev
sled which just gives a rocket a running start (perhaps enough to
eliminate the first rocket stage), to more ambitious schemes which would
achieve most of orbital velocity.  At the moment, the less ambitious ideas
have better funding.

Many gun/catapult schemes run into a fundamental difficulty:  it's hard to
fire something out of the atmosphere with enough horizontal velocity that
it only needs one rocket kick stage, especially since that rocket stage
has to be pretty tough to survive the launch.  (Needing no kick stage
would be still better, but is effectively impossible.)  And if you start
needing more than one kick stage, the improvement over an all-rocket
solution is no longer very impressive.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: Doug Jones <random@qnet.com>
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sat, 12 Feb 2000 19:32:40 -0800

Bill Bonde wrote:
>
> Balloon floats up to 135,000 feet or as close to the edge of the
> sensible atmosphere as possible. Attached cannon fires projectile up a
> bit and horizontal. Shaped charge on projectile explodes sending
> sub-projectile payload into orbit. If you don't think this quite gets
> the velocity there, than a sub-sub-projectile is used.
>
> Obviously your payload needs to be able to take a hit good but if you
> can get past that, talk about a cheap way to put a microsat in low earth
> orbit.

That's not a microsat, that's *vapor*.  Incandescent vapor, at that.
Have you ever seen ultraslowmo footage of a shaped charge firing?  Even
with microsecond exposure times, the image of the jet was overexposed,
since it was glowing at several thousand Kelvin.

Self forging fragment projectiles achieve lower velocities (only about 3
km/s instead of 5 or so) but even they are, as the name says, forged.
The original copper plate flows plastically into a peanut shaped
projectile in the time it takes for the shock wave to traverse the
plate- and it is heated to near melting.  Even if some part of it does
reach orbit you've successfully launched- debris.

--
Doug Jones
Rocket Plumber, XCOR Aerospace
http://www.xcor-aerospace.com

From: "Andrew Higgins" <higgins@mecheng.mcgill.ca>
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sat, 12 Feb 2000 23:51:29 GMT

Henry Spencer <henry@spsystems.net> wrote in message
news:Fpts39.BoC@spsystems.net...
>
> In article <8819uv$hjf$1@crib.corepower.com>,
> Nathan Urban <nurban@vt.edu> wrote:
> >> One of Gerald Bull's projects was an attempt to use a cannon to fire
> >> a payload into orbit. The cost was miniscule compared to conventional
> >> launches...
> >
> >There are still people who talk about electromagnetic launchers
> >(coilguns, railguns).  I remember reading about one of the better known
> >research facilities on Usenet, but I can't remember what or where it
> >was.  I don't know of anyone pursuing a traditional chemical propellant
> >based design.
>
> Light-gas guns, which use chemical (or other) means to heat and compress
> hydrogen and then use *that* to push a projectile out of a barrel, have
> had some work, e.g. Livermore's SHARP gun.  Their limits are rather
> higher than those of powder guns.  Funding is pretty minimal at the
> moment, though.
>

If interested, a readable distillation of the latest design study of a
light-gas-gun-based launcher system performed by Lawrence Livermore and
the Applied Physics Lab of Johns Hopkins is available in pdf format from
the site listed below.

    http://www.jhuapl.edu/digest/td2003/index.htm

    Gun-launched satellites

    Gilreath HE, Driesman AS, Kroshl WM, White ME, Cartland HE, Hunter JW
    JOHNS HOPKINS APL TECHNICAL DIGEST
    20: (3) 305-319 JUL-SEP 1999

    Abstract:
    This article summarizes and updates a study conducted for the
    Defense Advanced Research Projects Agency concerning the
    technical and economic feasibility of using a distributed-
    injection, light-gas gun to launch small satellites. In
    principle, a distributed-injection launcher can produce high
    muzzle velocities at relatively low acceleration levels. The
    technical feasibility of such a launcher depends on the
    successful development of fast injector valves and high-
    temperature, high-pressure hydrogen heat exchangers. Thermal
    protection, aerodynamic stability, and packaging are
    significant, but not insurmountable, challenges in designing
    a launch vehicle that can withstand high-acceleration loads
    and survive hypersonic flight across the atmosphere Although
    many spacecraft components are readily adapted to high g loads,
    the mass budget of a gun-launched spacecraft is affected
    substantially by large mass fraction allocations for structure
    and power subsystems. The results of a financial analysis
    suggest that a low-volume launch business might provide an
    attractive total mission cost relative to current systems.
    Author Keywords:
    light-gas gun, small satellites, space access

    Publisher:
    JOHNS HOPKINS UNIV, LAUREL

    ISSN: 0270-5214
--
   Andrew J. Higgins            Department of Mechanical Eng.
   Assistant Professor          McGill University
   Shock Wave Physics Group     Montreal, Quebec CANADA
   higgins@mecheng.mcgill.ca

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sun, 13 Feb 2000 19:08:02 GMT

In article <Linp4.9689$jb7.292125@carnaval.risq.qc.ca>,
Andrew Higgins <higgins@mecheng.mcgill.ca> wrote:
>> (Bull's projectile needed *four* solid-rocket upper stages to make it
>> into orbit after gun launch.  Which is ridiculous...
>
>No.  *One* of Bull's designs (the Martlet 2G-1) would have used a
>4-stage launcher.  This was an interim design...

The projectile for the Babylon gun was also reportedly four-stage.
(Ref:  Lowther's book.)

>The Martlet 4 would have been the true orbital launcher, a full bore
>rocket (16.6" in diameter, compared to the Martlet 2G-1's 11.3"
>diameter).  It was a 3-stage launcher.

I'm still underwhelmed.  Three solid stages should have been enough to
reach orbit without gun launch, even in the 60s, although I'm not aware of
any specific launcher along those lines in those days.  Certainly it's
routine now; Athena I reaches orbit with two solid stages.  And if you're
willing to use liquid fuels, two stages has been plenty since the
beginning of the space age.

The advantages of (powder) gun launch still seem slight.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sun, 13 Feb 2000 19:10:33 GMT

In article <ttnp4.9690$jb7.292516@carnaval.risq.qc.ca>,
Andrew Higgins <higgins@mecheng.mcgill.ca> wrote:
>>Many gun/catapult schemes run into a fundamental difficulty:  it's hard
>>to fire something out of the atmosphere with enough horizontal velocity
>>that it only needs one rocket kick stage...
>
>If you permit a two burn maneuver, however, it is possible to accomplish
>gun launch to orbit with a relatively modest delta-V (1 km/s or less)
>once above the atmosphere, provided you are not constrained on muzzle
>velocity...

Agreed, but that's a big proviso.  In fact, I was thinking mostly of the
difficulty in achieving such high muzzle velocities.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Mon, 14 Feb 2000 14:42:58 GMT

In article <38A62112.8BE7B631@peterlynnkites.co.nz>,
Robert Lynn  <robert@peterlynnkites.co.nz> wrote:
>> Many gun/catapult schemes run into a fundamental difficulty:  it's hard to
>> fire something out of the atmosphere with enough horizontal velocity that
>> it only needs one rocket kick stage...
>
>If you can fire the a satellite straight up with sufficient velocity to
>get it to the moon then the moon's gravity can be used to accomplish
>most of the redirecting required to put the satellite into a high
>orbit...

True, and it doesn't have to be vertical.  But that means reaching
essentially escape velocity (the difference between a lunar trajectory and
an escape trajectory is very small), which is an even bigger problem than
reaching near-orbital velocity.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech,
	rec.arts.sf.science
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Mon, 14 Feb 2000 14:52:18 GMT

In article <885f63$h0f@news-central.tiac.net>,
Harry H Conover <conover@tiac.net> wrote:
>: (Bull's projectile needed *four* solid-rocket upper stages to make it into
>: orbit after gun launch.  Which is ridiculous...
>
>I followed Bull's work for a number of years, including HAARP, but never
>ran across any mention of other than pure ballistics being involved for
>other than the final correction to place the projectile into orbit.

You could probably get the projectile to orbital altitude, which would
mean needing only that "final correction"... but that's not a small
correction.  Bull's highest demonstrated muzzle velocity, as I recall, was
a bit over 2km/s.  Even ignoring gravity and drag losses, that's only
about 1/4 of orbital velocity.

>Please provide a citation for your rather remarkable statement.

Lowther's biography of Bull.  Andrew Higgins has already noted that in
Bull's earlier work, the interim orbital-demo design (Martlet 2G-1) had
four rocket stages and the intended definitive launcher (Martlet 4) had
three.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: Bruce Dunn <bdunn@genastro.bc.ca>
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Tue, 22 Feb 2000 18:59:17 GMT

Henry Spencer wrote:

  The problem for gun launch is probably
> going to be the propellant tanks, which *will* end up having to be beefed
> up quite substantially.

I am inclined to disagree - I think that only minor changes will be
needed for tanks.  Bull's problems with rockets were that he was using
solid fuels cast in a grain with a central hole. The propellant tended
to slump into the hole during acceleration, causing large shear forces
in the solid grain which tended to tear the propellant loose from the
walls.  Liquid propellant rockets do not suffer from this particular
problem.

Assume that a gun launched rocket uses storeable liquid propellants.
Under severe acceleration, the structure at the top of the rocket
(payload, electronics etc.) must be accelerated by forces transmitted
from the bottom of the rocket.  If the propellant tanks are full, the
bulk of the force is transmitted by hydaulic forces, rather than by
compression in the tank walls.  As a thought experiment, successively
make the propellant tank walls thinner and thinner.  Even when the tank
walls are disappearingly thin, the head end of the rocket is still
accelerated by the slug of propellant trapped beneath it.

While the liquid propellant is being accelerated, it generates very high
hydraulic pressures.  The tanks however do not have to withstand the
hydraulic pressure in a radial direction, as the walls of the cannon
provide support.  In a longitudinal direction, there needs to be some
structure to cleanly transmit gas pressure to the liquid in the tanks.
Most or all of this structure can however be in the form of a pusher
plate which isn't physically attached to the rocket, and which is
discarded after the barrel is exited.  One solutions which comes to mind
is to have a pusher plug which seals the barrel, which is topped by a
liquid plug of fluid which in turn without any air spaces contacts the
engine, nozzle, and lower dome of the propellant tank. The rocket thus
becomes something like the driven piston in a hydraulic system.  Direct
mechanical support for delicate parts such as the rocket nozzle could be
provided by shaped structures on the top of the pusher plug.

As an extreme form of this, build a liquid propellant rocket without any
ullage space in the tanks, and completely encase the rocket in a
supporting discarding sabot, similar to the sabots used by Bull to
launch sub-calibre projectiles.  This would relieve the tank walls from
having to be designed to withstand sliding contact with the walls of the
cannon.  All parts of the rocket would then be accelerated by direct
mechanical contact with a shaped supportive structure, or would be
accelerated by hydraulic pressure from propellants.



Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
http://www.genastro.com/
Reliable, low-cost transportation to low Earth orbit and beyond

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Sun, 20 Feb 2000 04:06:02 GMT

In article <38AC7AF3.EA1B579E@genastro.bc.ca>,
Bruce Dunn  <bdunn@genastro.bc.ca> wrote:
>Accounting for all the benefits, a gun launched rocket has a much
>simpler time of it than a hypothetical ground launched SSTO.  Since the
>latter is generally thought to be technical possible, it should be quite
>feasible to make simple single stage rockets which can carry a
>substantial payload fraction to orbit when launched from a gun...

The gun-launched rocket does benefit from being, essentially, an upper
stage, with fewer worries about gravity losses and operation only in
vacuum.  Unfortunately, it also pays the penalty of being an upper stage,
i.e. it has to take everything the lower stage dishes out, fully loaded.
And in this case, that's one humdinger of a lower stage. :-)  If memory
serves, Bull's own papers commented about the design constraints imposed
on the rocket stages by the severe gun-launch environment, and the poor
mass ratio that resulted.  I fear that building a gun-launched SSTO is
probably as hard as building a self-launched one, although for different
reasons.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: gherbert@crl3.crl.com (George Herbert)
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: 23 Feb 2000 17:47:43 -0800

Jason Goodman  <goodmanj@ASmit.edu> wrote:
>I don't like using the cannon walls to support the radial component of
>hydrostatic pressure force.  First, the fuel tank skin will be pressed
>very strongly against the walls, causing extreme frictional
>heating and possible damage.  Second, as the fuel tank leaves the muzzle,
>there will be a short period where the rear of the tank is still being
>accelerated by the cannon, but the sidewalls are not constrained by
>the cannon walls.  I'm worried the tank will rupture at this time.

You can cheat by not having the tank in direct wall contact and having
a slightly denser than the propellant liquid fill the tank/wall gap.
It imposes a slight compressive force during most of the accelleration
and then cushions the release hammer you correctly identified.

Also, using better tank materials than stainless steel helps.
Kevlar wrapping should do quite well.

An advanced system might resemble this:
The projectile is carried in a multisegment sabot/bucket
which is then filled with a slightly denser-than-propellants
filler liquid.  The sabot itself has a slight gap between its
cylinder wall and the barrel wall, also liquid filled,
though there is a fullbore pusher plate at the bottom.

Near the end of the barrel are a series of vent holes forming
a muzzle brake aparatus.  This serves to vent gas pressure
prior to projectile exiting the muzzle and lower the release
hammer transient.  As the sabot passes these holes some of its
outer gap liquid is lost and the sabot will start to fail,
but will not do so fast enough to rapidly hammer the main
projectile with pressure waves.  As the assembly exits the
muzzle, the sabot disintegrates and the buffer liquid disperses,
leaving the main projectile and its tanks intact.

An alternate configuration is to avoid any voidspaces in the
propellant and to use a puller sabot with the tanks exposed directly
to the driving gun gases.  That will expose them (and the rocket motors
and such) directly to the propellant chamber pressures and temps,
but only briefly and in a compressive rather than tensile or shear mode.


-george william herbert
gherbert@crl.com

From: Bruce Dunn <bdunn@genastro.bc.ca>
Newsgroups: sci.physics,sci.physics.electromag,sci.space.tech
Subject: Re: Orbital cannons (was: Re: Gerald Bull)
Date: Fri, 25 Feb 2000 16:50:06 GMT

Gregory L. Hansen" wrote:

> How about using a discarding sabot to support the fuel tanks?  The sabot
> can be as rigid as you like.

There are a number of possible approaches to supporting the fuel tanks
without having them directly pressing on the walls of the barrel.
George Herbert posted a couple of interesting ideas about using fluids.
The essential thing is that:

1) most probably, at least one of the relatively low-tech solutions can
be made to work

2) the needed structures or fluids are not part of the rocket, therefore
don't impose a mass penalty on it

It has gone unsaid in the discussion so far, but the obvious payload for
a gun launch system is storeable rocket propellant.  The propellant
could well be the same as that used in the ascent rocket itself.  In
this case, the rocket has no "payload compartment", but simply
transports itself to a rendevous point where it is captured and its
tanks drained of unburned propellant.  This would include both a nominal
"payload", and any propellant from unused performance margins.

--
Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
http://www.genastro.com/
Reliable, low-cost transportation to low Earth orbit and beyond