As I know, Space Shuttle used the same amount of fuel for every mission. And used OMS for finishing orbit. Then how it is possible that, for example STS-1 which hadn't payload and was in roughly 266km*271km orbit, used same amount of fuel as STS-31 who carried Hubble Telescope (approximately 11t) into orbit of 585km*615km?
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$\begingroup$ You assume they run the OMS pods to depletion. Without actually knowing the answer I would assume they have fuel left over, and either vent it before reentry as needed. $\endgroup$– geoffcCommented Dec 21, 2016 at 22:57
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2$\begingroup$ The question needs to be definitized as to what type of fuel is being asked about. MPS fuel (LOX/cryogenic H2), SRB fuel (solid rocket fuel), OMS or RCS fuel (hydrazine with an oxidizer)? I do know that OMS fuel loading, for one, was dependent on the mission objectives. $\endgroup$– DiggerCommented Dec 21, 2016 at 23:56
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5$\begingroup$ Citation needed that those two vehicles used the same amount of fuel. From what I can tell, the STS-31 Orbiter carried 2.6 times the amount of fuel compared to STS-1. $\endgroup$– David HammenCommented Dec 22, 2016 at 2:09
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1$\begingroup$ @geoffc Venting OMS fuel before reentry? You'd better hope that whoever did the math for that got it right, lest you may be in a tricky spot (incorrect orbit and not a lot of means to correct it). $\endgroup$– userCommented Dec 22, 2016 at 14:53
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1$\begingroup$ The forward RCS was dumped at EI-18; that was the only OMS/RCS dump routinely performed in flight. $\endgroup$– Organic MarbleCommented Dec 22, 2016 at 15:06
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2 Answers
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If the question refers to the Orbital Maneuvering System (OMS) propellants, as Digger commented, it is based on a false assumption that the same amount of propellant was used for every mission.
Many old Space Shuttle Mission reports are available on the NTRS server. Here are two examples showing the different usages.
answered Dec 22, 2016 at 15:28
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Three things I can think of:
- Fuel loaded does not equal fuel used.
The shuttle was loaded with nearly the same amount of fuel for every launch (that being "full tanks"), but that doesn't mean the same amount was consumed. For a normal launch, main engine cut-off would be determined by the orbiter reaching a target velocity. Any fuel remaining in the tank at that point is discarded.
For some missions, there would have been a wider margin between necessary and loaded fuel. For other missions, the margin was very tight.
- Different orbital inclination
Besides altitude, different inclinations require different amounts of fuel to reach. From Florida, any inclination other than 28.5 degrees requires more fuel. For Hubble, STS-31 launched into this most efficient orbit. I don't know what the mission reason was, but the STS-1 orbit was 40 degrees.
- Flight profile
Early shuttle flights flew a "standard insertion" profile, while later flights (including STS-31) flew direct insertion, which was more efficient.
Because the flight profiles were different, comparing OMS burns is difficult, but STS-1 used 161s of OMS and STS-31 used 305s of OMS to reach orbit. So at least some of the difference is there.
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1$\begingroup$ Things to add: STS-1 flew on Columbia, the heaviest of the orbiters, while STS-31 used Discovery, which weighed approximately 7,000 lb less. Due to redesigns, the external tank on STS-31 weighed approximately 12,000 lb less than the external tank on STS-1 -- nearly all of that can be translated into available payload mass. $\endgroup$– TristanCommented Dec 22, 2016 at 15:41
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$\begingroup$ While true, I didn't mention it because cargo and other ate up all the difference and more. STS-1 launch weight was 219,000 lbs, STS-31 launch weight was 249,000 lbs. They didn't get higher than STS-1 by being lighter. $\endgroup$ Commented Dec 22, 2016 at 16:42
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$\begingroup$ This is true, but they were able to get that much payload mass up there in part due to the inert mass reductions. $\endgroup$– TristanCommented Dec 22, 2016 at 17:13