Once something gets in orbit, it has a very finite amount of fuel to make very minor course or orientation adjustments.
This was interesting context for the aborted test the other week. The SLS is reusing shuttle booster tech, which on the face of it is extremely questionable as the shuttle only launched a tenth as often as it was originally designed for (iirc around five times a year instead of every week)
The perils of liquid hydrogen. After the SLS rocket's scrub on September 3, Ars wrote about the challenges of working with liquid hydrogen and the first stages of large rockets. Over its lifetime, due to this complexity, the similarly fueled space shuttle on average scrubbed nearly once every launch attempt. Some shuttle flights scrubbed as many as five times before finally lifting off. For launch controllers, it never really got a whole lot easier to manage the space shuttle's complex fueling process, and hydrogen was frequently a culprit.
Keeping the workforce intact ... So why did NASA continue using liquid hydrogen as a fuel for the SLS rocket if it is so difficult to work with and there are easier-to-handle alternatives such as methane or kerosene? One reason is that hydrogen is a very efficient fuel, meaning that it provides better "gas mileage" when used in rocket engines. However, the real answer is that Congress mandated that NASA continue to use space shuttle main engines as part of the SLS rocket program. This allowed traditional space contractors to continue to win lucrative awards from NASA for their work on shuttle-related hardware.
Edit: and a quick explanation of why hydrogen is hard:
Because it is so tiny, hydrogen can squeeze through the smallest of gaps. This is not so great a problem at ambient temperatures and pressures, but at super-chilled temperatures and high pressures, hydrogen easily oozes out of any available opening.
To keep a rocket's fuel tanks topped off, propellant lines leading from ground-based systems must remain attached to the booster until the very moment of launch. In the final second, the "quick-disconnects" at the ends of these lines break away from the rocket. The difficulty is that, in order to be fail-safes in disconnecting from the rocket, this equipment cannot be bolted together tightly enough to entirely preclude the passage of hydrogen atoms—it is extremely difficult to seal these connections under high pressure and low temperatures.
Politics and pork barelling. It makes everything better.
Last edited by jgl; 09-09-2022 at 08:09 PM.