Reply to post: Re: 2 stroke, 4 star, Diesel?

Orion 'Mars' ship: Cosmic ray guard? Go. Parachutes? Go. Spacerock shield? Go!


Re: 2 stroke, 4 star, Diesel?

"I wonder what type of fuel they used to blast off from the Moon for Earth back in the Sixties?"

The Apollo Service Module and both ascent and descent stages of the Apollo Lunar Excursion module used Aerozine 50 (a 50/50 mix of hydrazine and UDMH) and nitrogen tetroxide.

"On earth they use Liquid Hydrogen, a volatile substance that has to be watched over by hundreds of technicians and ground staff and yet on the moon they need nobody"

The Saturn V used kerosene and liquid oxygen in its Saturn IC first stage, then hydrogen and oxygen in its second (Saturn II) and third (Saturn IVB) upper stages.

The Delta IV used by Orion is unusual in that it uses hydrogen/oxygen for all of its ground (booster, core) stages. You typically want lower impulse engines like solids or kerosene/oxygen on the first stage for a couple of reasons. One, you shed weight quickly and thus reduce gravity losses during ascent. Two, you get more thrust from dense fuel motors than low density motors for a given engine weight. (The best hydrogen/oxygen engines manage a ~75:1 thrust-to-weight ratio, while SpaceX is flying kerosene/oxygen engines with ~150:1 thrust-to-weight ratios, and even the overbuilt F-1 engine of Apollo managed ~100:1.) Delta IV's engine selection was a compromise based on costs, engine availability, and engine simplicity.

"....funny that."

Not really when you look at the different situations and engineering requirements.

Regarding Earth launch vs Moon launch engines, the situations are radically different on several counts. The overriding issue for Earth launches is the huge amount of delta-V (and thus fuel) you need to get into Earth orbit. That calls for high-efficiency fuels like hydrogen/oxygen, at least in weight-sensitive upper stages. The moon's delta-V launch requirements are radically lower, and the engineering impact of being so much further down the exponential Rocket Equation curve is fascinating: you don't need multiple stages to reach orbit; you don't need high thrust-to-weight ratios (so you can over-build and simplify your engines for reliability); and you don't need high-efficiency fuels.

On the other hand, NOT having hundreds of techs and engineers and a vast support infrastructure on hand for a launch from the lunar surface means you need to make compromises in the name of reliability. The Apollo LEM used toxic propellants that can be stored without heavy insulation and boil off that hydrogen and, to a lesser extent, liquid oxygen suffered. While the LEM could've used kerosene (or ethanol) and liquid oxygen (or hydrogen peroxide) with only modest storage problems, Aerozine 50 and nitrogen tetroxide had the advantage of easy ignition: they ignited when they touched each other. Weight limits meant the LEM could only have 1 engine, so it HAD to work without adding complicated ignition systems. Hence, toxic, hypergolic propellants were used. NASA has been trying to eliminate those propellants for decades, even suggesting overhauling the shuttles to use ethanol / oxygen. They're nasty enough that even Russia has taken steps to reduce the amount of hydrazine, UDMH, and nitrogen tetroxide it splashes across Siberia from spent stages.

In the past...heck, even now, in Russia...some rockets do use the Apollo LEM's propellants because of their simplicity and storability. They were the liquid propellants of choice for early ICBMs, but much-safer solid rockets replaced them. (They're storable without refrigeration, but a small corrosion leak between hypergolic, toxic propellants can turn a missile silo into a roman candle, or at least kill the technicians with their fumes. Hence: solids.) For civilian rockets that don't need to be stored in silos for years, you have additional options like kerosene/oxygen and hydrogen/oxygen, which give better performance without as much instant lung-burning death as the LEM's propellants.

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