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From: Henry Spencer <>
Subject: Re: Shuttle Solid Boosters
Date: Mon, 7 Apr 1997 18:40:36 GMT

In article <5i4gk2$dg7$>,
Jerry Aguirre <> wrote:
>The SSME takes the approach that the engines are kept and the empty
>fuel tank is permanently discarded.  The SRBs take the approach of
>dumping the entire thing and trying to recover and recycle later.
>Why is one a good choice for the SSME and the other for the SRBs?

Basically, the SRBs are tough, robust hardware jettisoned at relatively
low speed.  They were also originally supposed to be fairly simple,
although that, alas, got lost along the way.  So recovering them looked
easy enough to make sense.  It is not clear, actually, that recovery
really saves NASA much money, when you consider economies of scale in
production and the performance increase that could be had by deleting the
recovery hardware.  It is good PR, though.

Recovering anything from near-orbital velocity is difficult.  The original
intent was to put the tanks inside the orbiter, and bring the whole thing
back, permanently discarding nothing.  Unfortunately, this runs up the
size and cost of the orbiter.  Drop tanks are cheap enough that trying to
recover them is arguably not worthwhile, and once this notion was
accepted, the tankage rapidly migrated out of the orbiter entirely.  The
engines stayed in the orbiter because they are expensive (the SSMEs are
especially expensive) and reusing them is much more important than reusing
the tanks.

>In other words why doesn't the shuttle have bigger/more SSMEs and
>bigger/more external tanks?  Is the SRB thrust to weight ratio that
>much better?

There were shuttle designs with no SRBs, just drop tanks.  But for a short
while after takeoff you need a lot of thrust, which needs a lot of engine,
and life becomes considerably easier for the designer if he doesn't have
to haul all that hardware all the way to orbit.  It is also somewhat
easier to get high thrusts with non-hydrogen engines, but hydrogen engines
are so fuel-efficient that they look very attractive for the later part of
the ascent.  Using two different kinds of engine for the two parts of the
ascent, and shedding the takeoff engines partway up, is attractive.

>Or why doesn't the shuttle dump the useless dead weight
>SSMEs with the tank?

Not impossible -- Buran, the Shuttle's Russian counterpart, does -- but
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: Shuttle Solid Boosters
Date: Mon, 7 Apr 1997 18:26:58 GMT

In article <>,
Michael P. Walsh <> wrote:
>> ...lower costs (provided they
>> were recovered, which would require somewhat different methods than those
>> used for the SRBs)...
>Somewhat debatable to say the least.  This depends a lot on the design
>of the replacement.  The design and development would be a high cost
>item and I just don't believe that either the current SSME or an
>attempted resurrection of a F-1 engine would be an economical
>replacement.  I believe this is probably the worst place for a
>LH2-LO2 stage (You are Henry Spencer of the "evil hydrogen" persuasion?).

Yup, that's me -- down with that evil hydrogen! :-) I mention SSMEs mostly
because NASA has mentioned them in this context in the past.  I agree that
this is a really poor place for LOX/LH2 (even if one stipulates that there
are good ones :-)), because Isp is not critical, the existing hydrogen
engines mostly don't really have enough thrust, and all the extra hassles
of hydrogen don't pay for themselves.

I rather favor the F-1 or F-1A for this job myself.  A good reliable
engine, plenty of thrust, and LOX/RP-1 so propellants are dense.  The one
open question is its operating lifetime.  It was specced for 20 starts and
2250s of operation, and the demonstration of this involved racking up
5000s+ on each of six test engines.  This is probably adequate, but could
stand improving, and there are perhaps some legitimate doubts about extra
stresses imposed by flight conditions, especially depending on how you do

>Parachute-in-the-ocean recovery of a liquid propellant booster
>stage might result in higher refurbishment costs than those which
>occur with the current SRBs.

That's why I alluded to different recovery methods.  I think you'd have
to do flyback recovery.  It does run up the development costs, alas, but
operationally I think reusing F-1s, or whatever, after dunking them in
seawater is just not going to work.

>Having said all this, I have wondered if this isn't one of the
>places where a "Big Dumb Booster" approach makes sense...

That would be an interesting alternative, agreed.

>I think its a little late for big changes to the Shuttle, but
>if SSTO loses out and NASA wants to come up with a big
>Shuttle refurbishment plan for long term operation then a
>liquid, recoverable booster would be an excellent candidate.

Yes, alas, just about everyone who has looked at Shuttle changes has
identified recoverable liquid boosters as a desirable long-term change,
but short-term considerations have always dominated, either because of
budget crunches or because of uncertainty about the Shuttle's remaining
operating life.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

Subject: Re: Shuttle Solid Boosters (historical question)
From: Henry Spencer <>
Date: Wed, 9 Apr 1997 15:13:23 GMT

In article <5ienqm$>,
Brynn Rogers <> wrote:
>At some point aren't we going to use wire-wound SRB casings?  I thought
>that they could be made one piece then [no O-rings, no O-ring problems].
>(wire-wound should not require the same kind of machining)

No, this is confused.  For a while there was work underway on filament-
wound SRB casings, in which most of the casing would be a non-metallic
composite material.  (I forget exactly what they were using.)  However,
the filament-wound casings were segmented, just like the metal ones.  The
major purpose was lighter weight, for extra performance, notably on USAF
missions.  (Officially the filament-wound casings were reusable like
the metal ones; I have heard doubts expressed about that privately.)
I believe the first Vandenberg shuttle launch was slated to use them.
This effort quietly died in the post-Challenger hiatus, partly because
the USAF had lost interest.

The problem with one-piece SRBs is not the casings, but the handling
difficulties of solid rockets 150 feet long weighing over a million pounds
each.  There are also some serious concerns about uniformity of fuel when
casting such a huge motor in one piece.  There have been proposals for
one-piece designs, but none of them ever got very far.

The now-defunct ASRM project was intended to produce an improved SRB,
but it was still using segments, although fewer of them with a better
joint design.

>In fact isn't the biggest reason they are in four? sections so they
>can be shipped from Utah [getting Utah's votes] instead of being
>made somewhere close by in one piece? [but no pork for Utah]

Remember, there was quite extensive experience with big segmented solids
on the Titan III program, and very little experience with big one-piece
solids.  There's no reason to invoke pork to explain the segmented design;
it was a perfectly reasonable technical decision.  (This is not to say
that there was no pork involved, but there is no need to invoke that to
explain it.)

Remember, also, that NASA is required by law to use competitive bidding
wherever possible, and that means writing requirements that more than one
contractor can satisfy.  I know Aerojet has suitable facilities for doing
one-piece SRBs -- their losing ASRM bid specified that -- but I'm not sure
anyone else does/did.  Certainly Thiokol couldn't bid on such a contract,
not from Utah.  In the absence of compelling technical need for a
one-piece design, which would be pretty hard to establish given the Titan
experience, it would have been politically very difficult to write a spec
that barred segmented designs.

>Another Pork Barrel move with the shuttle was Rockwell as prime contracter
>instead of Boeing [who I hear had the lowest bid]...

NASA (quite properly) is supposed to take the *best* bid, not the lowest,
although cost is supposed to figure into the assessment of "best".

Note also that Boeing was involved only as a junior partner to Grumman.
The Grumman/Boeing bid was the runner-up.
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |

From: Henry Spencer <>
Subject: Re: throttled SRBs (was Re: Apollo 13 and LEM AGS)
Date: Mon, 22 Dec 1997 18:51:33 GMT

In article <>,
Alex R. Blackwell <> wrote:
>Henry I have a couple of questions.  What is the shape of the central
>hole on the shuttle's SRBs?

Its cross-section is mostly circular, but in the forwardmost segment it's
an 11-pointed star instead, and it also tapers somewhat toward the igniter.
The star pattern in that segment gives greater surface area at the start,
giving extra thrust.  As the points of the star burn down and smooth out
to a more or less cylindrical hole, thrust drops off a bit, reducing
acceleration a bit to ease the period of maximum dynamic pressure.  Later,
the outward burning throughout the motor slowly enlarges the hole, giving
more surface area and somewhat increasing the thrust again.

Presumably the ASRM design used a more pronounced version of this, but
I don't think I ever saw details.

>Also, would the solid propellant grain size
>have any effect on throttling?

The physical properties of the propellant certainly influence burn rate,
but varying them would require doing something tricky like casting each
segment in two steps (to give two layers of propellant with different
compositions).  I don't see any good way to get a change in propellant
properties within a single cast.

(Side note on this:  possible *unintentional* variation between casting
runs is a sufficiently serious worry that there's a rule that
corresponding segments of the two SRBs must come from the same casting
run.  The two SRBs have to deliver almost exactly the same thrust curve,
including burning out at almost exactly the same time, to within about 1%
or the shuttle goes out of control and breaks up.)
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        |

Date: 23 Aug 87 20:02:21 GMT
From: ssc-vax!  (Dani Eder)
Subject: Re: Welded SRBs

In article <8450@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes:
> starter.  The only way to make it safe would be to weld the casings
> before the propellant was cast, in which case there's no point: the
> joints with problems are the field joints (joined after casting), not
> the factory joints (joined before casting).
> > ... Why can't we weld the
> > suckers together and throw away the parachutes and LAUNCH!
> Despite the above, the suggestion of throwing away the parachutes
> deserves serious attention.  The "Spacefaring Nation" report
> specifically suggested abandoning booster recovery, on the grounds
> that it is mostly a PR stunt that saves very little money.

The reason for joints in the first place: When Thiokol was awarded the
contract for the Solid Rocket Motors, there had to be a way to get them
from Utah to Florida.  Rail was selected.  A combination of diameter
limits (12 feet) and railroad track limits (320,000 lb on one car) set
the size of a segment.  It turned out to be 4 segments.  The steel case
for each segment comes in 2 pieces, 12 ft long each.  The alloy used for
the case is a very high yield strength one.  There is a proprietary
processing step used in making the one-piece case section, which is
limited by the size of a treatment room the company that does the
process has.  Hence the case is made from 8 total pieces of steel.
There is no place to do post-weld heat treatment.

It is slightly cheaper to reuse the solids, about $18 million per solid
per use, vs $50 million for a brand new solid.  Two things that would
help their life a lot would be: (a) retro rockets to reduce the impact
speed in the ocean (currently 60 mph), and (b) a recovery barge that
could haul the solids out of the water for retrieval.  The alloy used in
the case is not resistant to seawater.

Dani Eder/Boeing/Advanced Space Transportation

From: "Dennis R. Jenkins" <>
Subject: Re: Why are the SRB's segmented?
Date: Wed, 06 Sep 2000 11:15:26 GMT

Henry Spencer wrote:

> In article <n%bt5.22860$>,
> Jeroen <> wrote:
> >Someone in this group mentioned recently that "Segmented SRB's are disasters
> >waiting to happen". Surely the Challenger accident demonstrated this to be
> >true at least to some degree. So the question arises: Why are the SRB's
> >segmented at all?
> Mostly because, at the time when the decision was being made, there was
> plenty of operational experience with large segmented solid rockets -- on
> Titan III -- and none with large one-piece solids.  Segmented solids were
> known to work well, so they were the obvious choice.
> There were lesser issues like problems with handling one-piece solids
> (which would be extremely heavy), but the experience issue pretty much
> settled the question.  The whole reason NASA was opting for solid boosters
> was to minimize development costs, so pioneering a new design approach was
> exactly what *wasn't* wanted.
> >Are the boosters of other launchers also segmented?
> All current large solid boosters are segmented, including those on Titan IV,
> Ariane 5, and H-II (although the H-IIA SRBs will be one-piece).

I'll throw in a few statistics that may or may not really be valid. At the time
of the award for the SRB contract, Aerojet (who proposed monolithic SRBs instead
of segmented versions) pointed out that over 800,107 non-segmented solid rockets
had been fired in the United States (this includes the Scout launch vehicle,
various ICBMs, SLBMs, and JATO/RATO bottles, but ignored tactical missiles such
as Sidewinder, etc.). It also pointed out that fewer than 100 segmented solid
rockets had been fired - including 25 pairs of UTC 120-inch 5-segment strap-ons
for the Titan IIIC and a collection 120- and 156-inch test rockets from Aerojet,
Lockheed, Thiokol, and UTC - hardly a large experience base.

So the experience base was solidly with the monolithic devices.

Aerojet had successfully built and fired three 260-inch diameter solids (each
producing over 5 million pounds of thrust) in monolithic cases. Pouring and
curing did not appear to be a problem.

The segmented choice appears to have been largely based on cost and politics,
not technology. Everybody I have talked to believes Aerojet could have produced
the monolithic boosters without significant difficulty. Their production
facility was located in Florida, so transporting the boosters by barge would
have represented no particular problem, even to Vandenberg,

To quote the Aerojet proposal (hardly an unbiased source however): "The
segmented solid rocket configuration represents a design compromise created
solely to meet a special transportation requirement [ Titan III from UTC ] that
is not a Space Shuttle operational or program requirement. The decreased
reliability and increased cost inherent in the segmented SRM result from
compromises that are totally unnecessary in the Space Shuttle SRM."

Nevertheless, technically the Aerojet proposal was non-compliant with NASA
requirements since the RFP specifically required segmented SRMs. Aerojet,
however, was not disqualified on that basis.

Interestingly, during the hurry-up evaluations of SRBs conducted very late in
the airframe competition, McDonnell Douglas did a failure analysis on solid
boosters.  In its February 1972 final briefing, McDonnell Douglas summarized
2,128 solid-rocket motor firings that covered Delta and Titan strap-on boosters,
Minuteman ICBMs, and the small four-stage Scout light launch vehicle. Of this
number, 13 boosters had failed in ways that mattered to shuttle, and McDonnell
Douglas carefully detailed the failure mode and what could be done to prevent
each failure in the future. The report noted that for most of these failures it
would be possible to safely abort a shuttle launch.

Most ? an important word. McDonnell Douglas noted there was one case that
appeared to offer no reasonable abort scenario. Large solid rocket motors, such
as the Titan III strap-on, were manufactured in segments that were bolted
together. This was the only way the rockets could be transported to their launch
sites, and also eased manufacturing. There was a concern, however, that the
propellant could `burn through' one of the joints. McDonnell Douglas noted that
if this occurred near the external tanks, particularly the hydrogen tank,
"sensing may not be feasible and abort not possible."

Food for thought.

Anyway, I will be covering more of this in the third edition of the shuttle
book.  Hope this helps.


From: (Henry Spencer)
Subject: Re: Why are the SRB's segmented?
Date: Wed, 6 Sep 2000 13:18:42 GMT

In article <F7qt5.814$>,
Christopher M. Jones <> wrote:
>Hmmm, here's an interesting question: When will solid rocket motors
>be completely gone?  i.e. When will we see no operational launch
>vehicles that use solid rockets?

Probably not until after the transition to reusable launchers.  NASA is
unlikely to use large solids again -- assuming NASA ever builds another
launch system, which is unclear -- and Titan IV is being phased out, but
Ariane 5 has a long working life planned, the Japanese have similar hopes
for the H-IIA, and smaller one-piece solid strap-ons are everywhere
(including some of the EELV configurations).
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: (Henry Spencer)
Subject: Re: Why are the SRB's segmented?
Date: Wed, 6 Sep 2000 13:14:44 GMT

In article <>,
Dennis R. Jenkins <> wrote:
>> Mostly because, at the time when the decision was being made, there was
>> plenty of operational experience with large segmented solid rockets -- on
>> Titan III -- and none with large one-piece solids.  Segmented solids were
>> known to work well, so they were the obvious choice.
>I'll throw in a few statistics that may or may not really be valid. At the time
>of the award for the SRB contract, Aerojet (who proposed monolithic SRBs
>instead of segmented versions) pointed out that over 800,107
>non-segmented solid rockets had been fired in the United States...

Essentially none of them anywhere near the size of the shuttle SRBs.  The
*operational* experience with *large* solids was all segmented.  (There
had been a handful of *experimental* firings of large one-piece solids.)
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: "Dennis R. Jenkins" <>
Subject: Re: Why are the SRB's segmented?
Date: Thu, 07 Sep 2000 11:00:14 GMT

Justin Wigg wrote:

> "Steve Vernon" <> wrote in message
> news:01c01889$93460680$450c8ad1@beaner-s...
> > "third edition" expected on the shelves when?
> We have to wait until at least after the 100th shuttle flight (STS-92)
> first...
> --

Actually, I will not make my self-imposed date of the 100th mission! I am
shooting to get the new edition out for the 20th anniversary of STS-1, so
say next April sometime. For those that asked, it is ISBN 0-9633974-5-1,
will be 500 pages (including 20 color). The third edition will include a
lot of not-seen-before early concepts, a much better explanation of the
final evolution in 1971-1972 from the large two-stage system to what we
have today, updated technical details (covering all the latest mods), and
an expanded section with more coverage of each mission.

In response to Henry's comment on the 800,000 solid number. He is most
certainly correct (and I meant to say that in the original post),
depending what you call 'large.' There was a tremendous experience base
with 90-100-inch diameter monolithic solids since that is what the ICBM
designs used (Minuteman, etc.). But when you got to 120-inch or greater,
the base was roughly 100 Titan SRMs (about 50 used operationally at that
point, and the rest tests) plus about 20 'experimental' ones ranging
between 120 and 260-inch diameter.

I think the point of my quoting those numbers was that there was
essentially no experience base with large motors at all, and even less
with segmented motors of any type. There had been maybe ten test motors
that were segmented (often monolithic motors cut in half and reattached),
plus the Titan SRMs. And the failure modes of segmented motors were well
known and scared even the guys that built the motors.

Interestingly, although I thought it had never been completed, in their
Shuttle proposal UTC claims that both the UA1205 and UA1207 motors (Titan
IIC and Titan IIIM, respectively) had completed their man-rating tests a
few years earlier. I have no independent verification of this claim.

And somebody asked about the abort solids on shuttle. I believe (but have
not checked yet) that all were to be monolithic designs. I doubt anybody
had done any final engineering on them.


From: Josh Hopkins <>
Subject: Re: shuttle SRBs (was Re: MirCorp signs...)
Date: Fri, 30 Jun 2000 09:17:36 -0700

Henry Spencer wrote:
> In article <>,
> Reed Snellenberger  <> wrote:
> >> But the truly disastrous decision--to use segmented solids--was made
> >> for the raw political reason that Fletcher wanted Thiokol to get the
> >> work, and if the motors were to be manufactured in Utah, instead of
> >> poured at the Cape, they had to be built in pieces that were
> >> truckable...
> >
> >I don't think they could build solids that large solids without
> >segments, could they?
> Big one-piece solids had been done, experimentally.  However, all the
> *flight experience* with big solids was with segmented ones, and that
> carried considerable weight.
> In any case, there's an important point being missed here.  There was no
> reason *not* to use segmented solids.  They had a good flight record on
> Titan III, and there was nothing *then* to suggest that they were any more
> hazardous than one-piece designs.  So it was quite reasonable to make the
> choice on secondary issues, like fewer changes needed to handling gear.

It's interesting to note that the two largest solid motors outside the
US (the Ariane 5 MPS motor and the PSLV PS1 motor) are cast at their
respective launch sites, and yet they're segmented anyway.  This despite
the fact that they are both significantly smaller than the Space Shuttle
motors, they were both designed after the Challenger/Titan 34D-9
failures, and they both have new manufacturing and launch facilities (so
they didn't need to be compatible with existing infrastructure).

So, after observing American experience, both good and bad, Europe and
India still chose segmented solids rather than very large monolithic
ones.  Presumably the greater operational experience and the simplified
handling and manufacturing tipped the balance in favor of segments.  It
makes me wonder if the segmented Shuttle SRB decision may have been the
right one, even if it was politically motivated.  I suppose NASA could
have installed a 1000 ton crane in the VAB if they'd had to, but I don't
blame them for not wanting to.

Josh Hopkins

From: (Henry Spencer)
Subject: Re: Yet another SRB question.....
Date: Wed, 3 May 2000 15:01:59 GMT

In article <>,
Wabash Slim <> wrote:
>From the photos I've seen it appears that at the field joints, the
>sections of propellant do not contact one another but remain separated by
>a foot or more, is this correct?

Mostly.  The fuel isn't in contact, but the insulation between the fuel
and the casing is -- in fact, the insulation on the adjacent segments is
glued together, to prevent hot gases from reaching the casing joint.
(This replaces the pre-Challenger putty, which was for the same purpose
but didn't work well.)

>And if so, do these exposed 'ends' of propellent become burning
>surfaces and therefore add considerably to the overall thrust?

No, the ends are coated with inhibitor specifically to prevent that.  Not
only would it increase the thrust undesirably, it would generate hot gases
very near the joints.  An inhibitor failure was one possibility that the
Rogers Commission examined (and rejected) as a possible cause of the
Challenger accident.

>Does this mean also that the insides of the field joints remain directly
>exposed to the full heat & pressure of the burning propellant?

Nope, see above.
"Be careful not to step                 |  Henry Spencer
in the Microsoft."  -- John Denker      |      (aka

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