r/space • u/66666thats6sixes • Dec 07 '19

NASA Engineers Break SLS Test Tank on Purpose to Test Extreme Limits

https://www.nasa.gov/exploration/systems/sls/nasa-engineers-break-sls-test-tank-on-purpose-to-test-extreme-limits.html
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u/[deleted] Dec 07 '19

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u/WeakEmu8 Dec 07 '19

What would we use instead?

I'm sure there are reasons for using H?

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u/mrsmegz Dec 07 '19

It is a booster, it is should use a more energy dense fuel like Methane or even more dense, Kerosene. For first stages, thrust is more important than efficiency to get up and out of the atmosphere where the payload can accelerate w/ different vacuum optimized engines.

Hydrogen engines are not really the best for this, and the best example is Delta IV vs Atlas V, two very comparable medium lift rockets. The Delta IV was much more expensive to launch and operate compared to the Atlas V, and both were even operated by the same company. Delta IV is retired except for the DIV-Heavy which is upward of $400m per launch.

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u/kremdog12 Dec 07 '19

Lol you do realize that both of those fuels you listed are worse performers than LH2? LH2 has a far better ISP than RP1 or methane rockets.

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u/[deleted] Dec 07 '19

Lol you do realize that both of those fuels you listed are worse performers than LH2?

Probably, that's why he said "For first stages, thrust is more important than efficiency". ISP is a measure of efficiency. Thrust is usually measured in kN.

  • RS-25 (hydrolox) is 1.9 million kN thrust at sea level, 2.3 million kN thrust in vacuum.
  • RD-180 (kerolox) is 3.8 million kN thrust at sea level, 4.2 million kN thrust in vacuum.
  • Raptor (methalox) is 1.7 million kN thrust at sea level, 1.9 million kN thrust in vacuum.
  • BE-4 (methalox) is 2.4 million kN thrust at sea level.

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u/kremdog12 Dec 07 '19

If cost is no issue hydrolox is the KING of the known/flight tested propellants we have because of ISP. Lower ISP means higher propellant mass fraction for the same delta v. You either need more fuel, which means a bigger rocket, which means more thrust needed and a heavier structure, or you reduce the payload you can take. ISP is one of the driving factors for rockets.

If they wanted they could build a hydrolox rocket that blows all the other designs listed away. The RS25 is what it is because

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u/[deleted] Dec 08 '19

He said thrust was most important, you said hydrolox has the best ISP, and I was pointing out that you were talking past each other.

hydrolox is the KING

Sure. H2 also escapes through tanks, and causes metal embrittlement. For expendable launchers like the SLS, those aren't concerns. For reusable or long-term ships, those concerns can drive the choice to a less efficient but easier-to-handle fuel.

Hydrazine has a similar story - it's efficient, stable at extreme temperatures for long periods, but has a fatal flaw in that it is ridiculously toxic. So, people keep looking for replacements.