jack1983 Smack-Fu Master, in training
12y
93

The Lurker Beneath said:
Do it on a windy day?
There are risks with too high wind speeds as well. High airflow causes static buildup and turbulence. Hydrogen requires very little ignition energy (well below 1mJ at stoichiometric conditions). So even a gust of wind can set off a combustible mixture.

Cryogenic hydrogen is an absolute nightmare to work with.

paulfdietz Ars Scholae Palatinae
7y
1,168
Using hydrogen as the fuel in the first stage was never a good idea. The density is just terrible, making the first stage much larger. Because the first stage is disposed of so quickly, Isp is less important; what's more important is "density impulse" (density x Isp), and LoX/LH2 is inferior to LOX/hydrocarbon by that metric.

Low density also makes the engines more expensive, as more pumping power is needed for a given thrust. //

paulfdietz Ars Scholae Palatinae
7y
1,168

pokrface said:
Right, but the "first stage" of SLS (and STS) is properly the SRBs.
It's more like the first "half stage". What matters for the argument is the velocity at which a stage is done. Even with the SRBs there, that's not that high (although higher than say the Falcon 9, which ends the stage 1 burn at an unusually low velocity so it can be recovered.)

The SRBs do allow the thrust of those LH2 engines to be somewhat lower, since they don't have to lift the fully fueled stack off the pad themselves. By the time the SRBs drop some propellant is gone. This ameliorates to some extent the relatively poor thrust/weight ratio of the engines (73.1 vs 184 for the Merlin 1D.)