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starship-design: FW: SSRT: Space Access Update no. 71
From: Chris W. Johnson [SMTP:email@example.com]
Sent: Monday, May 05, 1997 5:49 PM
To: Single Stage Rocket Technology News
Subject: SSRT: Space Access Update no. 71
From: hvanderbilt@BIX.com (hvanderbilt on BIX)
Subject: Space Access Update #71, part 1 5/4/97
Date: 5 May 97 01:08:17 GMT
Space Access Update #71, part 1 - 5/4/97
Copyright 1997 by Space Access Society
Yes, it's been six months since we put out an Update. We've delayed for
a variety of reasons - we didn't want to get into detail on NASA X-33's
problems while the coming year's funding was still vulnerable, for one.
We were not, for that matter, completely agreed among ourselves on the
nature and severity of the problems until quite recently. And frankly,
we were more than a little burnt out after spending much of the last
nine years working to bring SSTO to respectability. The urge was strong
to tell ourselves everything was fine, we'd succeeded, we could pass the
torch on to a new generation and go back to tending our own gardens.
Alas, it looks as if what we started is now, left alone, as likely to
discredit the whole idea of cheap access via fast-turnaround reusable
rockets as to prove it. And the younger generation seems not yet
totally cognizant of the nuances. So we're baaa-ack! And as long as we
can't retire after all, we intend to have some fun.
We're going to start with a two-part Update dedicated to what's been
happening with X-33 this last year. This part 1 begins with a summary
of our views, then covers X-33 configuration and technical issues. SAU
#71 Part 2 will cover political and organizational aspects of X-33.
It's fifty years since the Cold War started, forty since Eisenhower
warned of a "military industrial complex" threatening to become the tail
wagging the dog, and well over thirty since NASA was founded and given
responsibility for US civilian space exploration. Both NASA and the US
government-aerospace complex have grown large, powerful, and inflexible
in the decades since. Both have accumulated a lot of bad habits.
It's not our job to reform NASA in all its widely distributed diversity.
Nor is it our job to reform the US government-aerospace sector in
general, nor Lockheed-Martin Corporation's particular collection of
dubious practices. Nor for that matter is it our job to fix any of the
other major government aerospace contractors' various failings. Life is
It is our job to promote radically cheaper, more reliable, widely
available access to space, ASAP. Period. Any reforms we end up
pushing, explicitly or by implication, are purely a means to that end.
Anyone who doesn't want us pointing out places where their pursuit of
organizational self-interest conflicts with the overall public interest
can stop us easily: Clean up your act with regard to government reusable
launch vehicle (RLV) R&D. We don't care what you do elsewhere.
We do not claim to have a great deal of instant clout. What we have is
a fundamentally sound idea, good information, better advisers, and a lot
of persistance. The Administrator of NASA last fall accused us of
"nipping at his heels" after we'd buttonholed him for several minutes
about some of our concerns. Yes, sir, and proud of it - that's what we
do. Occasionally, of course, we convince someone who does have clout to
support us on one point or another... But mostly we persist.
X-33: One Year Later
The best way to describe where we are is to go back over how we got
here. It's all considerably clearer in hindsight... What follows is
our analysis of the last year or so of the ongoing NASA X-33 process,
based on information ranging from official published statements through
reliable sources down to plausible rumor. We've spent a good bit of
time working on this - we think we have a pretty accurate fit to the
data. Your mileage may, of course, vary.
X-33 has serious problems. We think those problems mainly come from:
- Inclusion of too many new technologies in what should have been a
fast-turnaround lean operations demonstrator using mostly ready-to-go
technology. Much of the new tech is having teething troubles.
- Possible lack of commitment to project success (as opposed to bidding
success) by Lockheed-Martin's top-level management, with consequent
imposition of inappropriate project organization and inattention to
adequate project support. There's also a certain amount of unsolicited
inappropriate "help" (and occasional outright sabotage) from various
other parts of NASA.
We think X-33 can still end up being a useful proof-of-technology
X-vehicle. The NASA people involved show some signs of learning from
their experience. The key, in our opinion, is concentrating minds in
Lockheed-Martin's top management on doing what it takes to significantly
improve the odds of project success, while continuing to fend off
extraneous "help" from elsewhere in NASA.
We recommend a two-track policy toward this end. One, NASA HQ should
maintain axe-poised oversight on X-33 cost, schedule, and technical
milestones. The contractor has to be made to understand that they are
in genuine danger of losing the project if they mess up too badly. The
threatened cancellation of the "Clark" science satellite for exceeding
Dan Goldin's new 15% cost overrun limit could help in this regard.
Two, there should be credible and vigorous competition for the project,
in DOD, NASA, or (preferably) both, to ensure that contractor top
management understands that even if they get away with failing
protractedly, they will not buy much extra time for their existing space
launch cash cows. They must understand that their main option for
remaining competitive in space launch past 2000 is to do what it takes
to make X-33 succeed.
Lest anyone take this as mindless attack rather than constructive
criticism, we do support continued funding for X-33, pending the results
of this spring's scheduled Critical Design Reviews, the final step
before freezing the design and committing to construction. (We have
however just heard that the CDR's have been postponed to allow more work
on reducing the current X-33 design's 35% over-weight problem. We await
the eventual CDR schedule and results with considerable interest.)
X-33 Technical Description And Current Status
Last year we described Lockheed-Martin's winning X-33 design as the most
"elegant" one submitted, the one that packs the most sophisticated
components into the smallest most closely integrated package. This
sounds wonderful - until you have to either compensate for components
not turning out quite as well as you'd hoped (closely integrated means
lots of interaction between the pieces; lots of other components are
affected) or, once it's assembled, until you have to go back in to fix
something. Small closely integrated packages are a royal pain in the
butt to service.
But NASA's Old Boy Net, bless their ivory-tower souls, think maximum new
technology and "elegant" complexity are just peachy. (Increased
operating complexity? No problem, we'll just pile on more guys with
clipboards and checklists. They're on the payroll already anyway...)
And NASA's Old Boy Net has, we've discovered repeatedly over the last
year, a lock on the NASA source selection process. (More on that some
other time - suffice it to say for now that NASA needs to take a serious
look at how they might find truly impartial people to serve on selection
boards.) (The White House, by the way, also had a hand in skewing the
selection criteria toward excessive new tech, as part of the deal they
made to allow project go-ahead - but it's unclear how much of those
provisions originated there, and how much was whispered in their ear by
the Old Boy Net.)
But, we do have to admit, L-M's "VentureStar" X-33 design is indeed
downright elegant. More important from our point of view, the various
advanced technologies that have to come together to make VentureStar
work - the aerospike engines, the multilobe composite tanks, the
metallic thermal protection - all can be useful to other SSTO
configurations, if as we suspect VentureStar turns out (even at best)
less operationally flexible than optimum for a competitive general-
purpose commercial space cargo ship.
Enough cavilling. On to the design of this X-33 single-stage reusable
space rocket demonstration vehicle.
L-M's X-33 is a "lifting body", a blunt triangular wedge-shaped wingless
vehicle that, when it is in horizontal aerodynamic flight, gets its lift
largely from the airflow around the fuselage. X-33 is designed to
takeoff vertically and fly into space under rocket power, then re-enter
the atmosphere as a relatively high performance (high lift-to-drag ratio
and thus high maneuverability, high "crossrange") hypersonic glider.
Once slowed to subsonic speeds, it is designed to still have good enough
glide characteristics to make an unpowered runway landing with
reasonable reliability and safety.
This combination of good hypersonic re-entry and subsonic glide
performance is one of the keys to making this X-33 design work - L-M
claims to have a proprietary aerodynamic shape that will provide both.
This is one of the first places we come to where X-33 is running into
problems. L-M may well end up meeting their aerodynamic performance
claims - but it seems likely from the significant vehicle shape changes
we've seen that L-M didn't know as much as they claimed back when they
were bidding. The small tip fins of earlier iterations have grown to
small wings, and the overall vehicle shape has changed markedly. More
on this when we talk about the internal structures.
This X-33 is powered by a pair of Rocketdyne "linear aerospike" rocket
engines, burning liquid oxygen (LOX) and liquid hydrogen (LH2).
"Aerospike" is an unconventional type of rocket engine that gets thrust
by expanding gases against the surrounding air (if any) and the outside
of the engine, rather than against the inside as with conventional bell-
nozzle rockets. A "linear aerospike" is one where the combustion
chambers are arranged in two straight rows, one along each side of the
wide base of a truncated-wedge aft-facing expansion surface, rather than
in a circle around the base of a truncated-cone expansion surface (an
L-M chose linear aerospike engines primarily because they integrate well
into the lifting-body vehicle shape chosen - they blend into the tail
better and don't extend as far aft as bell-nozzle engines, reducing the
center-of-gravity problem this sort of vehicle has from engine weight in
the tail. The secondary reason was that aerospike engines provide good
performance from sea-level to vacuum without either going to very high
operating pressures (SSME's work at ~3000 psi, the X-33 aerospikes at
around a third of that - high-pressure pumps tend to be heavy, fragile,
or both) or mechanically changing the expansion nozzle geometry with
Aerospike engines have never flown, but they have been built and run on
ground test stands, the best-known occasion being in the early seventies
when Rocketdyne did considerable work on linear aerospike as a potential
Shuttle engine. After that fell through, the project was shelved until
L-M settled on the concept for their X-33 bid.
The X-33 engines are direct descendants of those 70's test-stand
engines, with new combustion chamber feeds but otherwise little changed.
(We hear Rocketdyne has had to track down and hire some of retirees from
that project for their lost expertise.) The propellant pumps are still
taken from the old Saturn 5 J-2 upper stage engine. Unlike the test
stand versions, the plan is that X-33's engines will each produce about
the same thrust as the J-2 bell-nozzle engine their pumps came from,
something over 200,000 lbs thrust per engine. As best we can tell, the
70's tests used only part of the J-2 pumps' capacity.
X-33's main method of steering in powered flight will be "thrust
vectoring" via differential throttling of the engines - no mechanical
gimballing. The main engine combustors are arranged in four rows or
banks - looked at from the rear of the vehicle as it sits horizontally
on a runway, the (horizontal) banks are top left, bottom left (left
engine), top right, bottom right (right engine.) X-33 won't really have
two completely separate engines; there will be side-to-side propellant
cross-connects, both for side-to-side steering and so one set of pumps
can feed both sets of combustors and keep the ship flying if the other
pumpset fails. X-33 would be able to handle an engine-out at up to 90%
propellant load, assuming the original planned vehicle weight and engine
thrust values and a 20% power reserve on the pumps.
X-33 thrust vectoring will be via diverter valves on each side between
top and bottom banks, plus diverter valves between the two sides. This
speeds response time and saves thrust-losses over throttling the pumps.
We have heard Rocketdyne is having a hard time getting sufficient
predicted thrust out of the X-33 engine design so far - we might hazard
a guess this is related to the propellant plumbing in this application
being far more complex than in the J-2 engine the pumps came from.
There was a news item recently that Rocketdyne wants to eliminate the
crossfeed ducting between the engines (and thus the engine-out
capability) but that NASA won't let them - this could be related.
L-M is also committed as part of their X-33 bid to have Rocketdyne
produce and run a test-stand demo version of the 400,000 lb thrust
super-lightweight linear aerospike engine for their proposed Venture
Star commercial SSTO cargo transport.
(Policy note: In large part, Lockheed-Martin won X-33 because their bid
included enough money to develop and demonstrate these new engines.
These engines are a major reason we still support X-33; they're
applicable to a range of other potential vehicle configurations. We'd
be very unhappy indeed to see either aerospike engine dropped from the
project after they were major factors in L-M's winning the bid.)
- Propellant Tanks
X-33's propellant tanks are another significant new technology required
to make the package work, and in this case we're pleased to report that
from what we know, the tanks are coming along well.
Some background... Generally, the largest single load rocket propellant
tanks have to deal with is internal pressure. Even pump-fed rocket
engines tend to need several tens of pounds of inlet pressure, and the
propellant tanks have to handle that pressure over huge surface areas.
You can keep a pressure tank extremely lightweight, as long as you have
thin high tensile strength tank wall materials, and as long as you then
don't fight a thin-walled tank's natural tendency to assume a round
shape under internal pressure. Build your tank square and you'll need
massive braces to keep it from inflating into a circle anyway when you
pressurize it... So most rocket propellant tanks are "figures of
rotation", shapes that are always circular in cross-section, with some
mix of straight, conic, and spherical sections viewed from the side.
The problem with this is that it limits what shape you can make your
rocket and still keep the tanks light. For a circular cross-section
rocket, no problem. For a squashed-wedge lifting body, well... The
solution is something called a "multi-lobed" tank.
A simple multi-lobed tank might be built up from two identical
cylindrical tanks. Slice one-third off each tank lengthwise, then
attach the pair of sliced tanks together side-to-side, butting together
the openings where you took off the slices. You'll have one tank with
two lobes, with a cross-section like a sideways "8".
Put pressure in this tank, and it'll try to expand into an "O" - you
have to add some sort of tension structure between the halves to hold
the sides of the "8" together. Now you have a stable two-lobe tank. A
lightweight stable two-lobe tank? Only if you can figure out how to
build it without a heavy flange where the two halves and the tension tie
join. These are non-trivial manufacturing problems; multi-lobed (two
lobes is just the simplest case) propellant tanks have stayed on the
wish-list till now. But they would be hugely useful in rocket lifting
bodies and other non-circular vehicles...
L-M has apparently solved the manufacturing problems. X-33 will have a
pair of 4-lobed graphite-epoxy liquid hydrogen tanks (the LOX tank will
be old-fashioned aluminum for now). The plan is to build the tanks in
four sections, "fiber-placed" by machine on forms, with a border of
"green" (unepoxied) fiber left on the mating edges. The edges of the
sections will be "woven-Y" joined along with a centerline tension-tie
truss, then epoxy-impregnated, then the entire tank will be place in a
large autoclave (at least 15'x25'x40', our estimate of the tank
dimensions) that Skunk Works just happens to have lying around, and the
entire tank will be cured into one piece with no heavy flanges.
As of last winter, the techniques had been tested on small sections and
there seemed to be no show-stoppers. Cryogenic insulation and
stiffeners (where required) will be on the outside of the tanks.
Much of the elegance of L-M's X-33 design lies in the fact that it has
very little structure per se. Of the four main structural elements,
three are propellant tanks. The liquid oxygen (LOX) tank forms the
ship's nose, the two liquid hydrogen (LH2) tanks are connected to the
LOX tank to form the ship's two sides (with the payload bay in the space
between them), and the aft ends of the LH2 tanks are connected to a
cross-truss that also serves to mount the engines and various
X-33 has no solid outer hull as such - just a relatively light
assemblage of latticework and standoffs that carry the metallic thermal
protection shingles that define the ship's aerodynamic shape.
X-33 has been having serious weight-growth problems, however, and some
of them relate to the structure. One problem is that thermal loads are
turning out to be higher than anticipated, and the TPS shingle attach
structures are getting heavier. Another seems to be that the
aerodynamic shape has changed since the tank shapes were fixed, so
considerably more standoff structure is required to make up the
difference. And another is that the center-of-gravity ended up too far
aft - engine weight growth? aerodynamic changes? weight of the larger
aerosurfaces? - and (in the current design iteration at least) has to be
compensated for by several thousand pounds of lead ballast in the nose.
- Thermal Protection
X-33 TPS is supposed to be advanced lightweight metallic "shingles" on a
lightweight composite standoff structure. The shingles are supposed to
be a thin metallic outer layer, over a honeycomb core for stiffness,
with a bottom layer of ceramic insulation to reduce heat transmission to
the interior of the ship.
We understand there are problems with the TPS so far - details are
sketchy. We mentioned heat loads to the standoff structure being higher
than anticipated previously. We've also been told that inconel is being
substituted for titanium aluminide for the shingle outer skins for cost
reasons - this would account for some weight gain, as inconel is not
light. We would guess that inconel foil outer skins would also have
some durability problems, denting easily under raindrop impacts and
such. We assume that inconel will be just a placeholder for X-33 and
that any commercial followon would require the lighter stiffer TiAl -
which we expect this X-33 program will still develop and test.
- Flight Control Software
X-33 flight control software has some difficult challenges to meet. In
particular, on the ship's first flight the software will have to deal
with keeping the ship stably on course through flight regimes where the
engine efficiency and thrust-vectoring responsiveness won't be known
precisely in advance. Part of the answer to this will be to increase
use of the ship's aerosurfaces for steering while under power in the
atmosphere. Part of the answer, we suspect, will be a lot of muttered
prayers and crossed fingers for the first few minutes of flight #1...
We are told that X-33 flight control software algorithms are being
designed at NASA Marshall, that Allied Signal Corp is then coding them
in C++ offsite, and that the code will then be tested at an Integrated
Test Facility (an "iron bird" ground test rig) at Edwards AFB. This
strikes us as a likely formula for Software Project Manager ulcers, late
code, Ariane 5 style fly-sideways code, or all of the above. Software
may yet end up as the long pole in the X-33 tent. We'll see.
- Flight Test Ops
The X-33 Cooperative Agreement calls for 15 flights starting in early
1999, culminating in several flights that will reach Mach 15 (about 60%
of orbital speed) by late 1999. The agreement also calls for two two-
day turnaround demos plus several more seven-day turnarounds.
X-33 flights will launch from a site west of the Edwards dry lake bed,
near the USAF Phillips Labs rocket test area. The X-33 will be returned
after flights on the back of a NASA 747 "Shuttle Carrier Aircraft".
The first two flights will cover ~100 miles, to Silurian Dry Lake Bed,
reaching max speeds near Mach 4 and max altitudes near 116,000 feet.
X-33's VTHL (vertical takeoff, horizontal landing) configuration makes
any less drastic first flight very difficult - the vehicle needs
considerable altitude and airspeed to safely make the transition to
horizontal flight so it can land. Less risky incremental "bunny-hop"
hover tests are right out.
The next series of flights, ten max, will be to Michael Army Airfield at
Dugway Proving Grounds in Utah, and will range from Mach 9-12 at up to
164,000 feet. The final series of up to three flights will be to
Malmstrom AFB in Montana, covering ~1000 miles, reaching ~250,000 feet,
at speeds of up to Mach 15 - if they manage to trim enough weight from
the vehicle to make that performance. The current design iteration is
projected to max out at Mach 13 or so.
- Summing Up, part 1
The preceding isn't an attack on the competence of the working engineers
actually trying to build and fly X-33. More on the engineer-management
divide in part 2... As best we can tell, these problems are a mix of
perfectly normal solvable teething troubles, plus the contractor top
management's skewing the bid toward new technology for its own sake to
win the bid, in turn a result of both the new-technology requirements
the White House imposed before giving the go-ahead, and of NASA's new-
technology uber-alles reflexes. There's also the contractor top
management's possible lack of commitment to ensuring X-33 succeeds now
that they've won the contract - more on that in part 2 also.
Meanwhile, we find it more than a little ironic that, while X-33 still
has a good chance of being a useful X-vehicle technology pathfinder, it
is turning out to be a very poor "Y-vehicle" prototype for Lockheed-
Martin's proposed Venturestar Shuttle replacement. X-33's problems
point out graphically how much trouble Lockheed would have been in if
they'd gotten what they were pushing for two years ago, government
market guarantees for their going straight to developing Venturestar
with no intermediate step. And yes, we told you so at the time, guys.
** continued in Space Access Update #71, part 2 **
-----------------------(SAS Policy Boilerplate)------------------------
Space Access Update is Space Access Society's when-there's-news
publication. Space Access Society's goal is to promote affordable access
to space for all, period. We believe in concentrating our resources at
whatever point looks like yielding maximum progress toward this goal.
Right now, we think this means working our tails off trying to get the
government to build and fly multiple quick-and-dirty high-speed reusable
"X-rocket" demonstrators in the next three years, in order to quickly
build up both experience with and confidence in reusable Single-Stage To
Orbit (SSTO) technology. The idea is to reduce SSTO technical
uncertainty (and thus development risk and cost) while at the same time
increasing investor confidence, to the point where SSTO will make sense
as a private commercial investment. We're not far from that point.
With luck and hard work, we should see fully-reusable rocket testbeds
flying into space before the end of this decade, with practical
radically cheaper orbital transports following soon after.
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Join us, and help us make it happen.
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