9
$\begingroup$

I was contemplating on how a de-orbit may be abandoned post completion of the orbit burn, say, from LEO. A situation in which this might be necessary would be detecting a breach in the heat shield after the burn or any other emergency situation.

Are there standard procedures to abort de-orbit? Or is the fate of the crew sealed once the burn was complete?

Note:

  1. I know one could apply thrust at any point in the descent from LEO and shake the orbit up, but I also do realize that burns at non-nodal points is not very effective in raising/dropping the apses.

  2. While I am looking for standard procedures from the space shuttle program, it would be great if someone could detail the mechanics behind why it would work or not, as a bonus.

Improve this question
$\endgroup$
7
  • 2
    $\begingroup$ The soyuz reentry jettisons the engine shortly after the deorbit burn. I don't believe it has enough fuel to reestablish orbit once the deorbit burn is complete. $\endgroup$ – Innovine Apr 17 '20 at 9:56
  • 8
    $\begingroup$ I suspect that initiating a burn is considered a "no going back" step, and thus massive redundant checklists are performed prior to that point. $\endgroup$ – Carl Witthoft Apr 17 '20 at 11:03
  • 1
    $\begingroup$ Post completion of de-orbit burn is likely too late for an abort. $\endgroup$ – GdD Apr 17 '20 at 11:29
  • 8
    $\begingroup$ Note that heat shield damage may be survivable, vehicles have come back despite cracked shields, damaged/missing tiles, etc. Being stranded in orbit without sufficient remaining propellant to perform a controlled deorbit is not particularly survivable. $\endgroup$ – Christopher James Huff Apr 17 '20 at 13:12
  • $\begingroup$ @CarlWitthoft Yes, this crossed my mind while writing the question. $\endgroup$ – William R. Ebenezer Apr 17 '20 at 14:14
17
$\begingroup$

For the shuttle, technically the deorbit burn could be aborted, but the window to do so was extremely limited.

Once the deorbit burn was started, one of the key parameters monitored by the crew was the current height of perigee "HP" displayed on the maneuver display. This was a number in nautical miles and would have started out roughly equivalent to the height of apogee "HA".

(remember that

The deorbit burn was not intended to reduce the Orbiter's velocity to a small value, but rather to change its orbital parameters, so that its orbit intersected the sensible atmosphere. Specifically, it significantly lowered the orbital perigee.

)

enter image description here

(note this screenshot is from a orbital insertion burn so the TGT/CUR HP numbers are backwards from that expected for a deorbit burn)

As the burn progressed the HP got smaller and smaller. Before the burn, the ground would have read up to the crew a key number, "safe HP", which the crew would enter into the Deorbit Burn pads.

As long as the "current HP" was greater than "safe HP" the burn could be aborted. This can be seen in the Deorbit Burn Flight Rules card in the Entry Checklist. POST TIG means after ignition, the few problems serious enough to stop the burn are listed in the right hand column.

enter image description here

Below "safe HP" the vehicle is committed to entry. If the deorbit burn fails for some reason below safe HP, it must be continued by some of the many redundant means available: single Orbital Maneuvering System (OMS) engine completion, crossfeed, Reaction Control System (RCS) completion.

If while perigee is still above SAFE HP a failure occurs that severely impacts OMS capability, the crew will stop the burn. SAFE HP guarantees at least 24 hours of orbit time, which MCC can use to properly retarget the deorbit burn and assess the impacts of the failure.

On the other hand, if the failure occurs below SAFE HP, the crew must do what they can to complete the deorbit burn, which includes using the aft and forward RCS and possibly a recovery prebank.

Safe HP was generally around 80 nm if memory serves.

Sources

Generic Entry Checklist

Space Shuttle Flight Rules

https://space.stackexchange.com/a/12014/6944

Shuttle Crew Operations Manual

Improve this answer
$\endgroup$
4
  • $\begingroup$ Great answer. 80 nm is certainly a generous height though. If I understand correctly, say if the shuttle has a HP = 80 nm, the orbiter is calculated reach a minumum altitude of 80 nm above MSL(though it will be lower due to aerobraking). Is that right? $\endgroup$ – William R. Ebenezer Apr 17 '20 at 14:01
  • 1
    $\begingroup$ And I know this is out of scope of the question, but what is the HP (immediately after burn completion) at which the burn is sufficient to make a good reentry angle? I have tried deorbiting in Orbiter 2016, but most of the times end up bouncing back into space(followed by several bounces in subsequent orbits) or getting the hull temperature super hot and the crew deep-fried :( Any answer links or books that you could name would also be great, if you're busy at the moment. $\endgroup$ – William R. Ebenezer Apr 17 '20 at 14:05
  • 3
    $\begingroup$ @WilliamR.Ebenezer the linked stack exchange reference answers the second comment: HP after the burn was 28 nm. As far as the first comment, I think you are right, minimum altitude would be 80 nm but dropping, that orbit was only "guaranteed" to be OK for 24 hours. $\endgroup$ – Organic Marble Apr 17 '20 at 14:09
  • 1
    $\begingroup$ @WilliamR.Ebenezer to address "further reading" I'd suggest the Shuttle Crew Operations Manual linked in the answer. There is a LOT of information in there about the deorbit burn in there - just searching it for "deorbit burn" turns up hundreds of hits. $\endgroup$ – Organic Marble Apr 17 '20 at 14:19
2
$\begingroup$

It's less efficient to circularize when you're not at periapsis but it can be done and in the case of aborting a de-orbit the inefficiency isn't all that great anyway since you're near periapsis. The limiting factor is that fuel is a very expensive commodity, rockets don't carry much extra. Once your periapsis is in the atmosphere you won't have the fuel to raise it and then lower it again.

Improve this answer
$\endgroup$
3
  • 1
    $\begingroup$ On the other hand, if you have the posited cracked heat shield, you won't want to lower it again, ever. $\endgroup$ – Joshua Apr 18 '20 at 0:00
  • $\begingroup$ @Joshua With a cracked heat shield it's take your chances in the fire, or face certain death when your life support runs out. $\endgroup$ – Loren Pechtel Apr 18 '20 at 23:24
  • $\begingroup$ Or potentially get rescued. A new shuttle flight could have theoretically been prepared in 10 days. $\endgroup$ – Joshua Apr 19 '20 at 2:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.