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From: "Jeff Greason" <>
Subject: Re: X-33 Tank Failure Report
Date: Fri, 1 Sep 2000 19:53:41 -0700

Robert Lynn <> wrote in message
> Nasa decides that it has a long history of using LH2/LOX so any SSTO
> vehicle must use LH2/LOX.  They are pretty much the only people in the
> RLV game who believe this, most analyses of RLV's indicate that LOX/LH2
> is a bad fuel combination for SSTO when compared to the developmentally
> and operationally cheaper and easier to use hydrocarbon/LOX.  LH2 really
> only makes sense in upper stages of multistage designs.  Using LH2
> (apart from the difficulties of a multilobed tank design) was the chief
> reason for the tank failure and a consequently massively overweight and
> underperforming X33.

Certainly, I personally believe that LOX/hydrocarbon is a superior
choice to LOX/LH2 for most applications, and I've frequently explained
my reasoning here in the past.  However, I believe this statement is a bit
over the top.

If you try to settle the argument by "counting noses", you would find
that most studies favor LOX/LH2.  Of course "most studies" are NASA
studies (publish or perish!), and the look at LOX/hydrocarbon, if
done at all, is often done with *very* questionable assumptions (I've
seen several, for example, which assume that tank mass is independent
of density and that engine T/W is constant for any propellant, which
are pretty fundamental and flawed assumptions).

When you filter out the noise here, you're left with a much smaller set
of studies.  Because of the prevailing hydrogen mythos, very few
studies taken to the level of preliminary design (actually looking at the
structure in some detail) have been done with hydrocarbon fuels for
SSTO.  We really *don't know* the answer.

If you do a little sensitivity analysis on the parameters, you find that
the benefits of hydrogen are very sensitive to your assumptions about
tank mass, drag, and TPS acreage mass.  The benefits of
hydrocarbons are very sensitive to your assumptions about "parasitic"
masses which don't scale with engine and tanks -- things like landing
gear, aerodynamic controls, crew cabin & life support, etc.

IMNSHO, this explains a lot of the enthusiasm for LH2 in government
organizations and in the two remaining U.S. "design bureaus" -- it's
a lot easier to pitch for lots of $$ to study big problems like lighter
tanks, super-lightweight-unobtainium TPS, and your fancy new
aerodynamic shape.  It's a lot harder (in spacecraft or aircraft) to get
a big budget for shaving a few more ounces off of the APU's, the
control actuators, life support, etc.

On the composite tanks question, a lot of nonsense is frequently
spoken.  Having actually tried to go out and buy these recently, my
current assessment of the state of the art:

Hydrocarbon tanks: metal and composite tanks are available from
multiple vendors in bodies of revolution (spheres, cones, cylinders,
and so on).  Multilobed tanks of funny shape can be had in metal
or composite, cheaply if the pressure is low, more expensively if
the pressure is high.

LOX tanks: metal tanks are available from multiple vendors in bodies
of revolution.  Multilobed metal tanks can be had, but you're
restricted to "normal" materials such as weldable aluminum alloys
or appropriate stainless steels unless you want to get into R&D.
True all-composite tanks (no liner), are right on the cutting edge
for commercial suppliers -- I know of 2-4 companies (depends what
you call "credible") who can do, but I wouldn't characterize any of
them as a slam dunk -- you'll need to work the program carefully.
However, composite axisymmetric LOX tanks are right now climbing
out of the research phase for commercial suppliers.  Microcosm
flew one recently. A multilobed composite LOX tank
would be a research project, not a product order.

LH2 tanks: there are a *few* vendors who can do lightweight metal
LH2 tanks with confidence as long as they're bodies of revolution.
Most lightweight LH2 tanks out there are "legacy" designs and
a lot of the expertise in doing this has retired or died off, so it's
not clear to me that you could do (for example) an S-IVB tank
today without having to pay to re-invent some of the technology.
Composite LH2 tanks which are axisymmetric are on the bleeding
edge even from aerospace suppliers -- bring a *big* checkbook.
Multilobed LH2 tanks?  Do you *like* pain?  Multilobed
composite LH2 tanks?  Ah -- you *do* like pain...

"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <>                <>

From: "Jeff Greason" <>
Subject: Re: X-33 Tank Failure Report
Date: Tue, 5 Sep 2000 10:31:51 -0700

Phil Fraering <> wrote in message
> This brings up a question I've been meaning to ask.
> Would it be possible to build something vaguely like an X-33 efficiently
> without using multilobed tanks?

If "vaguely like an X-33" you mean a lifting body shape with LOX/LH2
fuel, the answer is no.  In a LOX/LH2 design, most of the volume is in
the LH2 tank, the vehicle density (wet) is so low that any reasonably weight
efficient solution requires the tanks to double as the aerostructure.
Therefore, if the aerostructure is not round, the tanks are not round.

If you mean "something VTHL with similar or greater delta-V using
LOX/LH2 fuel", then the answer is yes -- you do a wing-body,
with cylindrical body.  The cylindrical body is the LH2 tank, and then
(depending on more detailed trades) you either stuff the small LOX tank
 in the body (or probably tanks, for c/g management), or you do a
 "wet wing" wet with LOX -- this hasn't been flown, but was studied
  extensively for one of those late 80's near SSTO craft, RASV if
  memory serves correctly).

Of course, that pretty much was the Rockwell X-33 proposal.

"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <>                <>

From: "Jeff Greason" <>
Subject: Re: X-33 Tank Failure Report
Date: Wed, 6 Sep 2000 17:28:48 -0700

Henry Spencer <> wrote in message
> In article <>,
> Robert Lynn  <> wrote:
> >...I would be damned sure that a composite hydrocarbon tank
> >would not have failed in the same way - pretty hard to get cryopumping
> >without the cryo...
> Small caution here:  there are cryo hydrocarbons!  Some people think
> liquid methane or subcooled propane is a better choice than kerosene.
> (It's defensible; there are arguments either way.)

Of course, not all cryogens are created equal.  There's one set of
problems when you drop below 273K and have to keep water
vapor out of your lines. There's a big step up in difficulty below 90K
(colder than LOX) and a bigger step below 77K (colder than liquid
nitrogen).  Having the atmosphere condense out on your equipment
causes a host of problems.  This is what the cryopumping problem
in the X-33 tank appears to have been.

While there are "cryogenic" hydrocarbons, I can't think of any
which one would reasonably want to use with boiling points below

"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <>                <>

From: Doug Jones <>
Subject: Re: X-33 Tank Failure Report
Date: Wed, 06 Sep 2000 23:52:30 -0700 wrote:
>   Is methane cold enough to liquify air (which i thought was the cause
> of cryopumping...)?

Melting & boiling points of light alkanes at one bar, in kelvins:

Substance   MP     BP
Methane     91     112
Ethane      90     185
Propane     86     231
Butane     138     272

None of these can be stored below 90 K without active cooling with LN2,
and in practice would be used either at near boiling point or with an
uninsulated common LOX bulkhead so that the boiling LOX cools the fuel.
I dimly recall that there is an ethane-propane eutectic mixture with a
melting point around 80 K, but it would take great effort (and rather
pointless effort at that) to cause air to liquefy on a cryo hydrocarbon

(Propane has an amazingly wide range from melting to critical
temperatures- 86 K to 370 K, almost as broad as water's 273 K to 647 K.)

Doug Jones
Rocket Plumber, XCOR Aerospace

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