Return To The SSME

Alfred Montestruc wrote:
On Jan 5, 11:30 pm, kT wrote:
AlfredMontestrucwrote:
On Jan 3, 8:15 pm, kT wrote:
AlfredMontestrucwrote:
On Dec 28 2008, 8:41 am, kT wrote:
Brian Gaff wrote:
Just observing all this makes me wonder if anyone actually looked back at
things that occurred before. Seems that the obvious problem of the plume
environment was never considered until far too late, yet it presumably has
been considered in the past as the smes design was made to cope with close
proximity of engines.
It's a first order 'obvious' engineering consideration, an engineering
'**** test' would have stopped this in committee on the very first day.
1) RS-68 is a massive and inefficient engine compared to the SSME.
2) RS-68 is a fluid hog at startup, and has a well known hydrogen flare.
3) If you want to fly RS-68s, you can fly the Delta IV Medium and Heavy.
If one can simplify the design, why reinvent the wheel, or use a different
whell less well suited to the road, so to speak.
Because Michael Griffin is a ****ing idiot engineer. He is incompetent.
Clearly this idiotic idea was originally rammed through the committee.
Who would be in any position to do such a thing? Michael Griffin!
One thing does worry me though. Short termism. OK so the engines might work
out to be cheaper if the Shuttle carries on a bit longer and the political
decisions of keeping workers is in there, but does this mean that the long
term cost of a program which is supposed to go on far a long time is sound?
Sounds like someone out in the future could say this is far too costly.
Someone in the past already commented on the engineering failure of it.
The only reason I was enthusiastic about it was the fact that they were
going to use hydrogen, they were going to build a ten meter tank, and
that tank could be reengined with SSMEs, which with the addition of the
SRBs, would easily be capable of delivering that core stage to orbit.
I commented on this way back in early 2006 when it was first proposed.
The problem still remains :
1) the foam insulation.
2) the expendability of it.
Expendability is easily solve. Deliver the core stage to orbit where the
engines can be recovered and gigantic spaceships can be constructed.
The foam problems still remain as the fundamental engineering science of
rocket science, something America has abandoned with Michael Griffin.
Ya know, if the SSME is such a hot rocket engine design, and not
counting upgrades it is over 20 years old, why not outsource
manufacture of it to the Russians?
They have their own version, the RD-120, sitting in mothballs.
It is not like they have a
shortage of rocket engines that they could lob ICBMs at us with, so I
do not see a real national security issue.
I'd rather take the RD-120s out of mothballs.
I bet they could make it a bunch cheaper than we can.
They do - RD-120s, they even have channel wall nozzles.
They aren't quite as bleeding edge on performance though, and I've heard
they're a real bitch to start, as are most things of this nature.
Ok so we buy the RD-120s and if we need more we buy more.
Oops, that's a typo, they're actually called RD-0120s.

As long as the ISP, thrust/weight and reliability are all up to snuff
(even if not quite as good as SSME).
Unless you think we are going to war with Russia in the next couple of
generations that is the smart move.
If I had the cash I would privately secure the SSME assets as well.

Also the foam insulation is a non-issue for a top mounted spacecraft
as others have pointed out.
Read the rest of the thread, I've already pointed out why the foam is
always an issue if you wish to make the most out of the rocket equation.
Humm, I am not going to read the whole 90 odd posts on this thread,
sorry.
Can't help you there, I'm not about to explain it all over again.

Physically the structural skin is designed to work at the temperature
of the cryogenic liquid, and the insulation goes on top of that. You
have insulation mainly to prevent excessive pressure buildup and ice
forming on the tank while you are parked on the ground during your
countdown, once the engines are lit you have a big modo pressure
relief in that the cryogenic propellant is now being consumed at a
high rate. Probably it is a good idea to still have some insulation
on the tank in that phase, but it is much less critical than while
sitting on the pad.
Actually, no, as I explained, there are hypersonic heating effects.


Yeah, but by the time you are hypersonic you *should* be really high
up in the atmosphere, and the ambient temperature will start out much
less (till you get really high then the density is really, really
small), and the density is tiny.

Also that is going to be more pronounced near things that cause
shocks,and those are predicable and so you can beef up the insulation
there.



The shaking that causes insulation to come off only starts at or just
before lift-off so it is not an issue till then. That leaves as
possible reasons to worry about insulation falling off,
I already explained it, the problem is UV catalyzed hydrocarbon polymer
chain degradation and microshedding on orbit.

Ok, if the big tank gets that high.




1 - possible increased aerodynamic drag
2 - localized weight changes that mess with calculations of both total
burn time (not a big player as you will net gain delta-vee ) and
guidance (also not a big player unless you do not have much margin for
steering)
3- loss of a lot of insulation might still cause excessive pressure
build up, which can be dealt with by a pressure relief valve, also if
it becomes significant the effects of 1 & 2 will become more serious.
I honestly do not see this as a big deal unless you start losing large
% of the total insulation early in the burn.
Whatever. The goal is to balance tank pressure with gravity forces of
the extreme terminal acceleration encountered with high performance
cryogenic hydrogen engines in extremely aggressive SSTO and stage and a
half flight profiles, where the core stage is delivered to orbit as
payload. I'm not about to give up even the 15-30% payload hit of an
upper stage, to say nothing of the 60-75% payload hit of a core stage.

The foam must go.

And be replaced by????

Metal. Aluminum - lithium alloy.

Insulating the tanks on the pad is trivial. Probably the oxygen could be
skipped altogether. As pointed out numerous times by the idiot crowd,
the ice falling off the vehicle makes no difference in a straight stack.

Clearly fast hydrogen fills can speed things up. The hydrogen tank can
then be insulated by retractable panels while sitting on the launch pad.

The fun begins at launch. As the vehicle begins to accelerate the
cryogens will begin to boil. As the drag increases the metal tank will
begin to heat. That heating will become more apparent as the cryogens
begin to deplete. The increased pressure in the tank from the boiling
gases will have to be accommodated, preferably by adding thrust in a
direction opposite of motion. Boiloff and heating rates will need to be
precisely quantified, particularly in the hypersonic regime where the
structural integrity of the tankage will most likely be compromised.

Are you following me here? As the vehicle transitions to horizontal some
of the hypersonic heating will be on the top of the tank, since the
vehicle is still rising. As terminal acceleration increases, pressure of
the tanks will have to be adjusted to compensate for structural issues.

As I have clearly pointed out, this is a highly nontrivial problem which
has not been adequately addressed since the beginning of the space age,
and is the primary barrier to order of magnitude improvements in launch
costs and operations, including the more obvious - space colonization.

I would think that some treatment or coating of the tank would suffice
for the hypersonic heating aspects of the problem, since it is brief.

After engine shutdown, sustainer engines (for OMS and roll control) can
settle the residual fuel, and it will have to be scavenged into smaller
tanks for immediate on orbit use. Then both the tanks will have to be
repressurized for orbital stability, and eventual retrofit operations.

We used to sit around, for decades actually, and think that everyone
else must be thinking about these things, but now we sit around with the
stark and abject realization that nobody is thinking about these kinds
of things, but us. That's how dumbed down the world has become nowadays.

With SSMEs around, you've got a chance. Without SSMEs, you got nothing.