A question on liquid propellant as it relates to stage size and fuel weight. . .

I'm not sure if this is the right group or not but I've been messing
around with some back of the envelope figures (actually a little more
exact but not much) and it seems that even though the H2/O2
combination has a higher ISP than LOX/Kerosene, in the former case the
stage ends up being so huge that you get lots of extra weight in the
structure. True since the H2 is relatively light the structure can be
lighter but this is beginning to look rediculous. I mean the second
stage is FAR larger than the first and the total ISP for the second
stage (H2/O2) is only about .85 that of the first. So I guess the
question I'm asking is is it better to go with the higher ISP and
accept the larger structure or go with a lower ISP but a much more
compact structure assuming you have the same total lb/seconds?

Remember that kerosene has limited use in reusable stages, because of the
hydrocarbon coking problem. Hydrogen is a much "cleaner" fuel

Matthew Ota

"Christopher M. Jones" wrote:

"Scott Ferrin" wrote:
I'm not sure if this is the right group or not but I've been messing
around with some back of the envelope figures (actually a little more
exact but not much) and it seems that even though the H2/O2
combination has a higher ISP than LOX/Kerosene, in the former case the
stage ends up being so huge that you get lots of extra weight in the
structure. True since the H2 is relatively light the structure can be
lighter but this is beginning to look rediculous. I mean the second
stage is FAR larger than the first and the total ISP for the second
stage (H2/O2) is only about .85 that of the first. So I guess the
question I'm asking is is it better to go with the higher ISP and
accept the larger structure or go with a lower ISP but a much more
compact structure assuming you have the same total lb/seconds?

This is a good question, and it's a very active debate at
the moment (so to speak). The consensus among those on
these newsgroups is, I think, that LOX/Kero is the best
way to go and that the higher Isp of LOX/LH2 is not worth
the trouble of decreased density of LH2 (to speak nothing
of the increased trouble of an ultra-cryogenic fuel).

The most telling point to me is that LOX/Kero technology
is already just about at the level of SSTO capability. The
dry mass fraction required by the fuel combination is very,
very close to if not past the dry mass fractions achievable
in LOX/Kero stage construction. Whereas there's still a
substantial gap with LOX/LH2 and LOX/LH2 stage construction.

In article ,
Scott Ferrin wrote:
...it seems that even though the H2/O2
combination has a higher ISP than LOX/Kerosene, in the former case the
stage ends up being so huge that you get lots of extra weight in the
structure...

Correct. Between the bulk of the stuff, and the need for insulated tanks,
and the much higher pump power required (pumps pretty much pump volume,
not mass) which drives down the engine thrust/weight ratio, LH2-based
stages are notorious for poor mass ratios.

Much of the religious fervor for LH2 dates from days when these issues
were not well understood, and everyone expected that with a bit of work,
LH2 would be just like kerosene only with a much higher Isp. The drastic
reduction in propellant density simply was not recognized as an important
issue. By the time the truth started to surface, LH2 religion was solidly
established.

...this is beginning to look rediculous. I mean the second
stage is FAR larger than the first and the total ISP for the second
stage (H2/O2) is only about .85 that of the first...

One consideration you may have missed, if you're really starting from the
basics, is that LOX/LH2 stages generally do not run their engines at an
"optimum" mixture ratio. The LOX/LH2 mixture ratio for maximum Isp is
about 4.0, but even early LOX/LH2 stages ran at about 5.0, and modern ones
generally run at 6.0. You get optimum *stage* performance by accepting
some loss of Isp to reduce tank mass.

...is it better to go with the higher ISP and
accept the larger structure or go with a lower ISP but a much more
compact structure assuming you have the same total lb/seconds?

The answer is, it depends. :-)

If you do not care about the total mass of the stage, typically you are
better off with LOX/kerosene. The mass ratio needs to be higher, but high
mass ratios are easier to achieve.

If you do care about the total mass, the first question to ask is why you
care. There is a long tradition in the field that gross liftoff mass is
the preferred figure of merit for a launcher. This is utter nonsense;
most of that mass is fuel, which is dirt cheap. There is a better case
for dry mass as the figure of merit, but even that is open to question,
because hardware cost is only vaguely related to hardware mass.

The one case where total mass pretty clearly does matter is for an upper
stage being put on an existing launcher. There, the lower total mass of a
LOX/LH2 stage can be a win, because it permits a rather bigger upper stage
with rather higher payload.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Matthew B. Ota wrote:
Remember that kerosene has limited use in reusable stages, because of the
hydrocarbon coking problem.

This would surprise the Russians, who built Buran with a LOX/kerosene RCS
and intended-to-be-reusable LOX/kerosene boosters. Coking is a design
constraint, not an inescapable curse; reusable LOX/kerosene stages are
perfectly feasible.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

...this is beginning to look rediculous. I mean the second
stage is FAR larger than the first and the total ISP for the second
stage (H2/O2) is only about .85 that of the first...

One consideration you may have missed, if you're really starting from the
basics, is that LOX/LH2 stages generally do not run their engines at an
"optimum" mixture ratio. The LOX/LH2 mixture ratio for maximum Isp is
about 4.0, but even early LOX/LH2 stages ran at about 5.0, and modern ones
generally run at 6.0. You get optimum *stage* performance by accepting
some loss of Isp to reduce tank mass.



I was going off of SSME mix ratios.





...is it better to go with the higher ISP and
accept the larger structure or go with a lower ISP but a much more
compact structure assuming you have the same total lb/seconds?

The answer is, it depends. :-)


Dang. :-) I figured as much.



If you do not care about the total mass of the stage, typically you are
better off with LOX/kerosene. The mass ratio needs to be higher, but high
mass ratios are easier to achieve.

If you do care about the total mass, the first question to ask is why you
care. There is a long tradition in the field that gross liftoff mass is
the preferred figure of merit for a launcher. This is utter nonsense;
most of that mass is fuel, which is dirt cheap. There is a better case
for dry mass as the figure of merit, but even that is open to question,
because hardware cost is only vaguely related to hardware mass.


I don't really care about the mass that much.





The one case where total mass pretty clearly does matter is for an upper
stage being put on an existing launcher. There, the lower total mass of a
LOX/LH2 stage can be a win, because it permits a rather bigger upper stage
with rather higher payload.


Basically what I'm looking at is a two stage to orbit with ALL of the
second stage minus the propulsion module going into orbit and using
the things to make a REAL space station out of. It's just an exercise
for kicks. I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example. (After you cooled it down a bit
LOL) Didn't want extra kerosene sitting around doing nothing and the
coking issue was a consideration also although the 1st stage would be
reusable and uses LOX/kerosene.

In article ,
Scott Ferrin wrote:

I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example. (After you cooled it down a bit
LOL)

Fuel cells, anyone . . . ;-)

--
Herb Schaltegger, Esq.
Chief Counsel, Human O-Ring Society
"I was promised flying cars! Where are the flying cars?!"
~ Avery Brooks

On Fri, 25 Jul 2003 12:19:13 -0500, Herb Schaltegger
wrote:

In article ,
Scott Ferrin wrote:

I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example. (After you cooled it down a bit
LOL)

Fuel cells, anyone . . . ;-)


DOH!! Well I think I'll figure it both ways (H2 and kerosene) and
see what I come up with. That LH2 2nd stage is looking mighty big
though.

On Fri, 25 Jul 2003 18:25:48 GMT, (Henry Spencer)
wrote:

In article ,
Scott Ferrin wrote:
Basically what I'm looking at is a two stage to orbit with ALL of the
second stage minus the propulsion module going into orbit and using
the things to make a REAL space station out of.

As I noted, upper stages are one place where LH2 may be defensible.
But the case is much weakened if you're designing the lower stage to
fit, rather than coping with an existing lower stage.

It's a judgement call, but I suspect you are better off with kerosene
throughout.


Yeah I'm starting to lean that way.




...I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example.

Trouble is, any mass you budget for that is probably better spent just
launching water. The tanks are going to be essentially empty when you
reach orbit anyhow, so there won't be very much left.


Yeah I was thinking that too. Water is much easier to deal with than
keeping them seperate.

Scott Ferrin writes:

Basically what I'm looking at is a two stage to orbit with ALL of the
second stage minus the propulsion module going into orbit and using
the things to make a REAL space station out of. It's just an exercise
for kicks. I was thinking of using the LOX/LH2 upper stage because it
seems like it would be useful for other things too. You could burn it
to get drinking water for example. (After you cooled it down a bit
LOL)

You could run it through a fuel cell and get electricity.

Didn't want extra kerosene sitting around doing nothing and the
coking issue was a consideration also although the 1st stage would be
reusable and uses LOX/kerosene.

If I were thinking along these lines, I'd look at the TSTO version of
the Saturn V (e.g., the one that launched Skylab). You'd replace
the smaller diameter Skylab with a second stage diameter payload
section, but this is essentially a Skylab "wet workshop", only scaled
up. The killer is they don't build Saturn V's anymore.

Something else to look at would be a modified shuttle ET (engines on
the bottom, payload on the top). This is pretty big too, but still
able to fit in the VAB. You'd have to modify a launch platform to get
the engine exhaust to go through it, not on it.

After that, I'd look for an existing launch vehicle with a suitable
second stage. Atlas V? Looks like it has LOX/kerosene first stage
with a LOX/LH2 upper stage (one or two RL-10A-4-2 engines on the
second stage). Second stage diameter of about 3 m.

Jeff
--
Remove "no" and "spam" from email address to reply.
If it says "This is not spam!", it's surely a lie.

"Henry Spencer" wrote:
As I noted, upper stages are one place where LH2 may be defensible.
But the case is much weakened if you're designing the lower stage to
fit, rather than coping with an existing lower stage.

It's a judgement call, but I suspect you are better off with kerosene
throughout.

That would be my guess as well. One of the huge problems of
the past way of doing things in rocketry has been that
tendency to patch and patch and patch and very, very rarely
build something new from scratch. I think a lot of that had
to do with the rush to win the "space race" and the ICBM race
early on, especially mixed with the historically high rate of
failure of new rocket stages, even now. So there's been a
tendancy, I think, to be scared of change to a very high
degree and to cling desperately to the old "if it ain't broke,
don't fix it" mantra. How many new first rocket stages have
we built in the last few decades? Very few. And, as we've
seen, rocket builders are more likely to patch on fixes to
rockets that aren't really up to snuff anymore, such as with
solid boosters and enhanced upper stages, etc.

My opinion, and I'm no expert but I doubt I'm alone in
holding it, even among those experts, is that rocketry is not
inherently difficult, and if you're willing to do it right
you can develop brand new systems fairly easily. For
example, one of the big no-nos that I've seen has been the
tendency to skip intermediate test steps. And that's just
plain idiotic. Even more so when they turn the first full up
test into a discounted launch. They really ought to test
individually and incrementally. And they really ought to
design for mass production rather than for finicky one-off
batches of quasi-unique configurations. The first stage is,
obviously, an important piece, so test it thoroughly on its
own. Hell, run flights with it and with dummy upper stages
which are just dead weight for proper balance and the tiny
bit of sophistication necessary to let the first stage do
its thing. If it blows up or veers off course, no big deal,
it was just one stage, and it's tremendously foolish to
expect, much less rely on!, early tests to go exactly as you
planned. Planning a robust system to deal with failure, to
*expect* failure and to plan for it shows maturity and
experience. Then you can proceed from there all the way up
to full-scale, integrated tests blending into shakeout and
validation.

Another huge problem is that current designs and designers
use different systems far too often when they could just as
easily use common systems. More unique components means
more opportunities for failure, and often there's no reason
for uniqueness. For example, most multi-stage rockets use
different engines on *every* stage. There's very little
reason in my opinion to do that, especially when the
requirements of several of the stages are pretty similar
(such as the first and second stages, often). As Henry has
pointed out, there has been a huge tendency in rocketry to
transform the quirks of rocket designs of the past into
dogma. That's why you don't see many partial launch tests,
that's why LH2 is a favored fuel, that's why GLOW is
considered important, that's why they have "inaugural
launches", that's why new first stages are rare, that's why
solid boosters have become a staple of modern rocket design.
In short, that's why rocketry is so screwed up today. All
the mistakes of the past have been added to each other and
codified into law, it's a wonder we have spaceflight at all
with that going on.