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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."
Here's the thing though: It worked 10 times. So what ? The space shuttle worked safely 24 times before it had a problem.
BTW there are 2 or so Saturn V still lying around to use- see here
The SSME is tricky beast. Converting a slightly more modern (30 year-old) but very large and complex lower stage engine vs. reviving an older upper stage design. They will re-design this engine somewhat, but they know the basic design works in the intended role.
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I'm a bit confused as to how it takes us longer to get to the moon now than it did in the 60s.
Project Apollo was announced July 28th, 1960. July 20th, 1969, we set foot on the moon. Just under 9 years. (My dates may be a bit off.)
Even if you say the new project starts now, that's still 12 years. How frustrating.
The shuttle engine is not currently capable of in-air ignition, while the J-2 engine did exactly that for Apollo. The shuttle engine would thus have to be modified, while the J-2 engine would not. So it's more like 10 times vs 0.
P.S. This is pretty clearly written in the article.
Modify a less-efficient, 40 year old design that hasn't been produced in several decades, or modify a more-efficient currently-used design. Choose the former because it "already exists?"
It's not quite a simple as that.
True, the J-2 is older and less-efficient, but it's a much simpler, more reliable design than the SSME.
The SSME is much more intricate, tempermental, expensive and operates at much, much higher pressures than the J-2. The reliability of the SSME in the Shuttle is more a tribute to the army of inspectors employed by NASA than to its inherent design.
Personally, if I were trusting my life to a new rocket , I'd prefer to sacrifice a little ultimate efficiency for an engine that has reliabilty designed in, not inspected in.
"In addition, NASA has no infrastructure for Kerosene fuels,"
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.
Yes the SRB has more static thrust but I think the F1 is equal to it in specific impulse.
Static thrust is what you want. The point of the F-1s and SRBs was to get the rocket off the pad, up to Max Q, and out of the thickest part of the atmosphere. From there the more efficient LHOx engines provide more than enough thrust to carry the weight into orbit. The plain and simple fact is that the F-1 is MUCH heavier, reducing the overall efficiency of the entire rocket. Thus the SSME/SRB combination will continue to be used for the liftoff phase. The J-2 will be used in orbit where its reliability and restart advantages make it a better choice than the SSME, and the F-1/SRB argument doesn't even enter.
Using modern AlLi alloys for the tanks an F1 powered first stage might still be a good option.
Then the program would be held up for years why they certify a new engine. That defeats the point of the entire CEV exercise. (i.e. Build an infrastructure quickly.)
The real reason is cost. The SRBs are cheaper short term.
The SRBs are always cheaper. Not to mention reused.
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The expendable portion of a lunar mission isn't necessarily waste. You took a bunch of survey equipment (including a golf cart) to the moon. Why would you expend structure and fuel to bring it back? Similarly, you need structure and tanks to contain the fuel you're using to get to the moon. When the tanks are empty, why would you haul that empty (and now useless) mass around? Nope, the most efficient method requires you to be an interstellar litterbug. As soon as a resource is depleted, you jettison as much as you can.
In space, mass is the one variable you can control. Escape velocity, orbital velocity, and a host of other parameters are dictated by the math - your orbital altitude is a function of your velocity, not your mass. So to obtain a particular orbit, you need to achieve velocity V. The energy required to do so is dictated by E = 1/2mv^2. There's a direct relationship between the mass of the vehicle and the energy required to achieve a particular delta-V.
If you look at the Rocket Equation, you'll see that the overall relationship between wet mass (fuel) and dry mass (structure + payload) is exponential in nature. At the bottom of the Wiki page (link above) there's an example comparing single- and two-stage-to-orbit vehicles. All things being equal, the two-stage vehicle can put more mass into LEO by shedding the first stage.
Besides, bringing back used equipment is usually pointless. All of the Apollo-era vehicles used ablative shielding techniques, and couldn't be re-used (by design.) The Shuttle is technically re-usable, but it's largely rebuilt in-between flights. It pays a huge penalty in payload mass in exchange for bringing the brick-lined wings on a round-trip journey.
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