Space Shuttle delta-V: How much is available at the ISS altitude?

[dotancohen]

I recently read that the STS orbiters have up to 300 m/s of delta-V available considering the weight of the orbiter and the quantity of propellant in the OMS. I also recall that leaving orbit requires some 100-150 m/s of delta-V depending on altitude.

If deorbiting takes 100-150 m/s, then is it safe to assume that entering orbit takes a similar amount? Do the SSMEs provide a way to circularise the orbit? I'd think not as the OT is ejected while the SSMEs are pointed towards the back.

I ask because if entering orbit takes 150 m/s of delta-V and deorbiting takes another 150, and the orbiter has only 300 m/s available, then how much is available for the mission? For docking with the ISS, or with Hubble in a much higher orbit? How much excess delta-V is available at the end of a mission, in the event of an emergency for instance?

Thanks.

 

[eachus]

A lot of questions from a co-incidence of numbers. First, technically all of the SSME burn, plus the solid rocket boosters is required to enter orbit. Once the SSMEs are off they cannot be restarted in orbit. If you look at the base of the shuttle during take off, you will see "sparks" (actually burning particles) from the starters firing up into each SSME. These starters stay on the pad.

Once the shuttle is in orbit, an OMS burn is used to circularize the shuttle's orbit. The point at which the main engines shut off (MECO) is below the intended orbit. As I recall on some of the early shuttle launches the OMS burn was required to stay in orbit, but that was a precaution. The OMS pods are one of the most reliable parts of the shuttles. (And there has always been talk about leaving the tanks in orbit, for example for use as components of a space station.)

Anyway, the 150 m/s or so required to get out of orbit is required to change the orbit into one that intersects the earth, not just goes deeper into the atmosphere.

Finally, the easy answer. On every Hubble repair/component replacement mission, the last thing done is as much of an OMS burn as is prudent is used to put the Hubble in the highest orbit possible. So on those missions, there is no extra delta-v left in the bucket. Also on some missions to the space station, the remaining extra OMS fuel has been used to boost the station. Again this has to be done periodically to compensate for drag. But the (highest) space station orbit is determined by the Van Allen belts, not the desire to re-boost as infrequently as possible.

 

[dotancohen]

A lot of questions from a co-incidence of numbers. First, technically all of the SSME burn, plus the solid rocket boosters is required to enter orbit. Once the SSMEs are off they cannot be restarted in orbit. If you look at the base of the shuttle during take off, you will see "sparks" (actually burning particles) from the starters firing up into each SSME. These starters stay on the pad.

Yes, I knew that MECO occurs before orbit, this is why I figured that the OMS is responsible putting the shuttle in orbit, that is, matching the velocity vector with the orbital velocity vector for the desired altitude. I meant to ask if decelerating the shuttle from orbital velocity to an Earth-crossing velocity requires 100 m/s of delta-V, then is it reasonable to assume that on launch the shuttle's Earth-crossing velocity must be corrected with a similar amount of delta-V.

Once the shuttle is in orbit, an OMS burn is used to circularize the shuttle's orbit. The point at which the main engines shut off (MECO) is below the intended orbit. As I recall on some of the early shuttle launches the OMS burn was required to stay in orbit, but that was a precaution. The OMS pods are one of the most reliable parts of the shuttles. (And there has always been talk about leaving the tanks in orbit, for example for use as components of a space station.)

I should hope the OMS pods are reliable! Actually, I've heard that the lines can be purged only about ten times, even though the pods can fire for hundreds of times. But isn't it actually one of the OMS tanks that are limiting further STS flights as the tanks are well beyond their designed fill/discharge cycle quota and the manufacturer no longer exists? I'm googling, but can't find the info right now.

Anyway, the 150 m/s or so required to get out of orbit is required to change the orbit into one that intersects the earth, not just goes deeper into the atmosphere.

I see. I sort of suspected that was the case, though I didn't give it much thought.

Finally, the easy answer. On every Hubble repair/component replacement mission, the last thing done is as much of an OMS burn as is prudent is used to put the Hubble in the highest orbit possible. So on those missions, there is no extra delta-v left in the bucket. Also on some missions to the space station, the remaining extra OMS fuel has been used to boost the station. Again this has to be done periodically to compensate for drag.

Ah, yes, I knew this! I should have though about that! Well, that answers the question for ISS missions! I wonder if they still do leave at least some propellant for emergencies, though. I wouldn't even know where to research that if not here.

But the (highest) space station orbit is determined by the Van Allen belts, not the desire to re-boost as infrequently as possible.

I did not realize that! Thanks, I need to go read about the Van Allen belts, then, I did not realize that they were so close to LEO that they would be a concern for the ISS.

Have a peaceful weekend.