Question STS-51F Space shuttles Abort to orbit question(s)

[thumper235]

Picture it 7-29-1985, STS-51F has a successful launch. Roughly 3 and a half minutes into flight experiences an SSME shut down and was given an ATO command.

I was looking this up on wiki and it kinda seemed like a trainwreck of information. Could someone possibly elaborate on this one just a bit? My questions mainly are around when the ATO was declared and when they landed. Did they orbit a few times before landing? Im just confused. Thanks!

 

[Urwumpe]

Picture it 7-29-1985, STS-51F has a successful launch. Roughly 3 and a half minutes into flight experiences an SSME shut down and was given an ATO command.

I was looking this up on wiki and it kinda seemed like a trainwreck of information. Could someone possibly elaborate on this one just a bit? My questions mainly are around when the ATO was declared and when they landed. Did they orbit a few times before landing? Im just confused. Thanks!

ATO is no abort of the mission as such. ATO means that you aim for a different orbit if normal orbit insertion is not possible. When one engine fails for example late in the ascent, you can still reach orbit on the remaining engines, but you need a slightly longer burn time because of only two engines providing thrust. This difference between planned MECO and what you can achieve means usually a small difference in the orbit parameters at MECO, usually you also get a lower orbit because of the lower engine performance. But you are still in orbit and can continue your mission.


It is much better explained than Wikipedia in the "Intact Ascent Aborts Workbook 21002".

You also can't just continue with your ascent without telling the flight software, that you want to perform an ATO in the situation. The guidance needs to adapt new targets maybe, aiming for an "ATO MECO target" instead of the nominal target if the velocity is less than a value defined before launch and written on the software tapes with the ascent software. Also, ATO means dumping OMS fuel depending on how early the abort is initiated. From "PRESS TO MECO" on, no OMS fuel will be dumped, from "PRESS TO ATO" to "PRESS TO MECO", the OMS dump is reduced with increasing velocity from full dump to zero.

 

[DaveS]

The ATO was at around 6 minutes into the mission with center engine failure occurring at 5 minutes, 47 seconds.

 

[Urwumpe]

The ATO was at around 6 minutes into the mission with center engine failure occurring at 5 minutes, 47 seconds.

More important would be the speed at which this happened, than the pure time. The typical VI for the reduction of the ATO dump is 11500 fps, the cutoff of the OMS dump happens typically at 18000 fps.

A full ATO dump for example could be 56% propellant with interconnect, taking 139 seconds with 2 OMS engines and 24 RCS thrusters for the dump. Or 377 seconds with only the OMS engines (and no interconnect).

 

[gwiz]

My questions mainly are around when the ATO was declared and when they landed. Did they orbit a few times before landing? Im just confused. Thanks!

The mission was actually extended by a day over the pre-flight plan. The main effect of the ATO was that the orbit was lower than planned and this meant that there was residual atmosphere overhead that interfered with the astronomical observations.