Moonshot, CEV Modifications

DarkNemesis618 writes "In the latest round of budget cuts, NASA introduced plans to modify the CEV for the planned Moon landing in 2018. The original plan called for an engine used on the space shuttle to be modified for the CEV. The new plan is to use an updated J-2 engine. The J-2 engine was first used on the Saturn V rocket which took the Apollo astronauts to the moon in the late 60's early 70's. It is not expected to save any money in the near-term, but in the far term, it should be a cost saver since the technology already exists and is proven. In the 10 Apollo launches aboard the Saturn V rocket, there were no problems with the launch vehicle."

Not really a surprise

[AKAImBatman]

Note that this isn't really a surprise to those who have been following the CEV development. The original plan called for a modification to the SSMEs for multiple restarts as the J-2 (the upper stage engine for the Saturn V) is no longer in production. However, there was a lot of discussion inside NASA that restarting production on the less powerful J-2 would be cheaper, faster, and easier than trying to modify the more powerful (but far more complex) SSME to do the job.

To give quick rundown on which engines are which:

SSME (Space Shuttle Main Engines) - LHOx Fuel - 1.8 MN
SRB (Solid Rocket Booster) - Solid Fuel - 14.7 MN
J-2 (2nd and 3rd stage Saturn V) - LHOx - 890 kN
F-1 (1st stage Saturn V) - Kerosine - 6.7 MN

The SSME and J-2 are directly comparable, and the SRB and F-1 are directly comparable.

Re:2018?

[AKAImBatman]

I'm a bit confused as to how it takes us longer to get to the moon now than it did in the 60s.

1. Money. If we spent as much today as we did on the Apollo program, we'd be able to get a craft ready in a very short period of time. (Note that while NASA receives more than enough money, most of it goes toward the Space Shuttle's maintenece and other projects.)

2. Technology. The industry that produced the Saturn V doesn't exist anymore, so it is not really possible to produce it again. We can produce a new rocket like the Saturn V (or buy off the Energia, take your pick), but that would just give us another moonshot rocket. What we want to build this time is an infrastructure that will keep us on the moon instead of merely sending up a few tons there and back.

If there was an emergency, I imagine we could get to the moon inside two years. Most of the lander equipment can be remanufactured and lifted by the Space Shuttle, and strap-on boosters could be lifted to propell the module. But that's not the point. That's why we're doing this the right way this time. Or to put it in perspective, the Apollo missions started out with 2,900 tons of hardware. They came back with about 6 tons. That means that they expended 2,300 tons of hardware to get 3 people to the moon and back. That's a hell of a lot of waste! :-)

Re:Let's see...

[Burdell]

The SSME is higher efficiency in terms of thrust vs. mass. However, that isn't the only measure to be considered. Each SSME costs a lot more to build, because they were designed to be reused (IIRC the current plan is to not reuse the new vehicle's engines). The SSME is throttleable, but if that is not needed, it is an added complexity and expense. The J-2 was designed to be throw-away.

Also, there are some questions about the SSME for the new vehicle. The SSME would be used in upper stages that are lit in-flight. The SSME has only ever been lit sitting still, on the ground, at sea-level atmospheric pressure and temperature. The J-2 was used on upper stages of the Saturn V, so it is proven in that capacity.

Re:Let's see...

[Anonymous Coward]

Yes, but the shuttle's engines only worked 19 times before one of them failed in flight. And only a fast thinking controller prevented a second engine from also shutting down, which was a very good since the Shuttle would of probably ended up in the middle of the Atlantic if a second SSME had gone down.

Re:Not really a surprise

[AKAImBatman]

Did you see those differences in performance on those engines? The SSME is about twice as powerful as the J-2, but it also weighs twice as much. Thus it makes sense to use two J-2s. The SRBs OTOH, get over twice the power of the F-1 for far less weight. Since they're almost entirely fuel (not much engine, just light 'em up), they have an incredible thrust to weight ratio.

In addition, NASA has no infrastructure for Kerosine fuels, making the switch from the SRBs to the F-1 more difficult. They *do* have an infrastructure for LHOx fuels, so the change from the SSME to J-2 is a fairly easy one.

Re:Not really a surprise

[Profane MuthaFucka]

You can't throttle the F1. It's either all-on or all-off. The Saturn V dealt with Max-Q issues by completely shutting down the center engine.

Re:Not really a surprise

[LWATCDR]

"In addition, NASA has no infrastructure for Kerosene fuels,"
Atlas V?
Also what is the specific impulse for an F1 first stage VS an SRB?
Yes the SRB has more static thrust but I think the F1 is equal to it in specific impulse. Plus the F1 allows for an on pad shut down and is probably easier the vector than an SRB.
BTW the Specific impulse for the SRB is 268.8 For the F1 it is 304.8.

Using modern AlLi alloys for the tanks an F1 powered first stage might still be a good option. The real reason is cost. The SRBs are cheaper short term.

some numbers

[Quadraginta]

Apollo cost about $135 billion [wikipedia.org] in 2005 dollars, and the CEV is expected to cost $15 billion [wikipedia.org].

Re:Let's see...

[AKAImBatman]

The Saturn 1B flew an addition 10 flights on the J-2s, which was based on the Saturn 1 that flew 10 flights on the RL-10s, which was based on the Jupiter IRBM that flew about 17 flights. Each Saturn 1B had 1 J-2 while each Saturn V had 6 J-2 engines.

Or in other words, the J-2 engine has a long history and has proven itself highly reliable. Its reliability isn't really in question.

Re:Not really a surprise

[ericwhy]

Actually, you can, and NASA does, throttle the SRB's. Actually, throttling isn't really the correct word, but by changing the shape of the solid fuel, SRB thrust is reduced by about 33% 50 seconds into the burn. After MAX Q, it is returned to maximum thrust.

See the section on SRB's here http://www.daviddarling.info/encyclopedia/S/Space_ Shuttle.html [daviddarling.info]

Re:No problems?

[rcw-work]

I seem to rember in Apollo 13 the center 2nd stage engine, a J-2, went out early.

Dangerously strong pogo oscillations [yarchive.net], which could have ripped the engine off the rocket, happened to trip a pressure sensor which caused the computer to shut down the engine.

Pogo was reduced to tolerable levels by the end of the Apollo series, and later engines such as the SSME were designed to eliminate it entirely.

Re:Not really a surprise

[LWATCDR]

"There have been a handful of Atlas V launches. Nothing near the scale of what the Shuttle flys today, and what the CEV *will* fly."
What are you talking about? The shuttle flew 12 times a year at it's peak? The CEV will fly maybe that many? The Atlas V is going to be used for commercial and military launches for how many years?
I also do not believe that the F1+Fuel is much heavier then an SRB. The difference in the specific impulse means close to 10% less fuel mass for the F1 than the SRB.
Without a complete study with more data then you or I have the choice of the SRB over an F1 is at best a guess.
I will say that Boeing did design a replacement for the SRB that used... The F1 for a fly back booster.

Re:Not really a surprise

[AJWM]

The startup sequence for the F1 is hairy as all get out (taking about 7 seconds from "ignition sequence start" to full power). Furthermore, while it may be apocryphal, Harry Stine once told me that the ignition sequence was controlled by a patch-panel programmed computer and that the documentation for the patches was long since lost and those people that knew how to do it have mostly died off.

The F1 ignition sequence includes steps like pre-filling the inlet tubes with a hypergolic mix to actually light the thing, diverting some of the fuel (kerosene) to the hydraulic system for the gimbal actuators, a controlled chill of the lox plumbing without getting the kerosene plumbing too cold (don't want any frozen lumps in there), starting the gas generators to power the turbopumps, etc. -- not necessarily in that order. The SRB ignition sequence is basically just detonating a small bomb at the top of the hollow solid fuel core.

Personally I like the idea of resurrecting the F1, but the difference in experience and reliability levels between F1 and SRB vs J2 and SSME are considerable -- and in the latter case the J2 start is simpler than the SSME start.

Re:Not really a surprise

[AJWM]

Saturn 1C Empty: 135,218 kg
SRBx2 Empty: 174,000 kg
Saturn 1C Fueled: 2,286,217 kg
SRBx2 Fueled: 1,180,000 kg

That's not quite an apples-to-apples comparison. Initial thrust of the 2 SRBs is about 5 million pounds, of the S1C, about 7.5 million pounds. The Shuttle launch is also augmented by the thrust of the 3 SSMEs, and the whole thing puts about 65,000 pounds in orbit. The Saturn lower stages (S1C followed by SII) could put about 200,000 pounds in orbit.

The 2 SRBs don't have quite the same thrust as the 5 F-1s of the S1C, and don't have nearly the total impulse. The S1C stage engines burn for almost a minute longer than the SRBs.

Misleading.

[DerekLyons]

In the 10 Apollo launches aboard the Saturn V rocket, there were no problems with the launch vehicle.

This is a bit misleading, the summary starts out talking about the engines, the swaps to the launch vehicle. In fact, the J-2 engines had considerable problems on the flight of Apollo 6 [wikipedia.org]. The pogo problem was not cured until Apollo 14. (In fact, though it was overshadowed by later events, it came quite close to causing an abort on Apollo 13.)

In fact, when the Apollo series is looked at critically - one becomes astonished by the number of near misses and diving catches. NASA was lucky, very lucky.

Re:Let's see...

[Burdell]

Nope, you are wrong. SSMEs are lit on the pad six seconds before liftoff. They burn for about 9 minutes and then shut down for the rest of the flight. The SSMEs are fueled by liquid hydrogen and liquid oxygen stored in the big orange external tank, which is discarded right after SSME shutdown (so the rest of the flight there isn't any fuel for the SSMEs). The next time an SSME would be lit is after removal from the orbiter, refit, and either at a test facility or at another launch.

Orbital maneuvering is done using the (wait for it) Orbital Maneuvering System, or OMS, engines. These are the two smaller "pods" on either side of the tail above the 3 SSMEs. The OMS (as well as the smaller RCS used for attitude control) engines use hypergolic fuels, nitrogen tetroxide and monomethylhydrazine.

Re:Not really a surprise

[AJWM]

Where do you get 5 million pounds? I have 3.3 million pounds of force per SRB, giving a combined total of 6.6 million pounds of force, or 29.4 kN. That's pretty darn close to the 33.4 MN of the S-IC.

I was thinking 2.3 million pounds per SRB and doubling it. I stand corrected. However, the SRBs propellant is shaped to gradually reduce thrust over time (to compensate for reducing weight of the stack and limit overall acceleration). The F-1s gain efficiency with altitude and at just before center-engine cutoff, the 5 are putting out about 9.3 million pounds thrust, dropping to about 7.4 million with 4 (1.85 million pounds thrust each).

the whole thing puts about 65,000 pounds in orbit.

The STS has a theoretical maximum of 137.8 metric tonnes to orbit. Of that, 109 metric tonnes is the orbiter itself.

You're right. My bad. That 65K number is the original design payload of the Shuttle (actual closer to 50K pounds). The Orbiter itself is a couple hundred thousand pounds which also makes orbit.

I'm not sure what you mean by "nearly the total impulse". The SRBs fired in conjunction with the SSMEs gives the shuttle a far greater efficiency than the Saturn V.

Total impulse -- thrust times time. Newton-seconds, if you like metric. "Efficiency" don't enter into it, without defining all your terms. If the Shuttle really had "a far greater efficiency than the Saturn V" (defining "efficiency" as "lift capacity"), it'd be able to put an Apollo CSM/LM combo (or equivalent mass) into trans-Lunar orbit, or a Skylab-equivalent into LEO. It can't do either, although arguably the Orbiter itself, with a Spacelab or Spacehab in the cargo bay, is nearly Skylab-equivalent. (Less roomy and shorter duration, though, although Skylab wasn't designed for reentry.)