and Space Shuttle
terminology, this is the third intact abort
mode available to the
Shuttle in the event of a failure during ascent
. It is the least likely intact abort mode due to the physics
of the Shuttle launch process. More on this in a moment.
As the name suggests, this mode has the Shuttle orbiter return to Earth after making a single orbit around the globe, or (in some cases) less than a full orbit. Unlike the Trans-Atlantic Abort Landing procedure, available earlier in the ascent, the Abort Once Around (AOA hereafter) requires the use of OMS engines at two points in the Shuttle's flight. Essentially, the AOA is a complete Shuttle flight cycle minus any on-orbit activity - the orbiter reaches MECO and performs an OMS burn (OMS-1 in Shuttlespeak) to roughly circularize the orbit. Later on in the same orbit cycle, it performs a de-orbit burn (OMS-2) just as it would when returning from a complete mission, and then follows a deorbit and re-entry procedure, aiming to end up at one of three landing sites in the CONUS:
This process is highly unlikely to ever be used. The reason for this lies, as mentioned earlier, in physics. It lies between the TAL option and the Abort to Orbit option. The prior would be more attractive if the Shuttle has suffered failures such that the functioning of the OMS system, or the functioning of the life support systems for a full orbit are in question. The latter would be more attractive in almost any other case - upon reaching orbit, the Shuttle crew and ground support would then have time to plan for the Shuttle's return (or even continuation of the mission, if possible). Given that the Shuttle would have to undertake a re-entry in the case of an AOA anyhow, it would be better (if possible) to do so in a more considered, careful fashion, after a more thorough evaluation of the orbiter; in that case, an ATO is more helpful. One possible failure that would strongly call for an AOA is a failure of the bay doors - if the doors are not opened immediately upon reaching orbit, the Shuttle cannot expose its radiators and dump heat from its systems. This is why the doors must remain open for the entire mission. If such a failure occurred, the orbiter would need to return to earth quickly (before more than a few orbits had been completed) and an AOA might be the preferred option.
The physics problem can be seen by looking at NASA's own flight support pubs. There is a flight decision card used during ascent, known as the 'AOA OMS 1/2 targeting card'. It shows a graph of Shuttle flight parameters, and contains guidance on which abort modes are available in each regime. Although impossible to reproduce here, interested parties can find this card on the Internet or through NASA's public affairs office. The relevant numbers on it for our purpose can be found at the bottom of its graph. There, several thick lines traverse upwards; each of these lines represents the boundary of an abort option regime. On this graph, the X-axis contains the shuttle's speed at MECO and orbital parameters, expressed in two fashions - feet per second (fps) and HA/HP (Orbital Apogee at MECO / Orbital Perigee at MECO). The y-axis indicates how many seconds, if any, the shuttle has undergone an OMS fuel dump (which it might be performing in the event of trouble). This is also expressed as the percentage of remaining OMS fuel on one side of the orbiter.
In any case, the point is that this region is extremely narrow! This reflects the fact that there just isn't that much difference between the minimum energy required to circle the Earth once and the minimum energy required to reach a mostly-stable orbit. In one specific example,
using the STS-90 targeting card, the region within which an AOA abort is indicated is bounded by a shuttle speed at MECO of 25,228 fps at the lower end, and 25,353 fps at the high end. This means that the shuttle, at MECO, must be traveling at a speed inside this narrow window (125 fps!) in order to be forced into an AOA. Below this speed at MECO, the TAL is indicated; above it, the shuttle has enough energy to press on to abort to orbit. As we have seen above, these are both more attractive than the AOA option.
In any orbiter which has undergone the flight controls upgrade overhaul, the cue card is a secondary backup to an improved flight computer and display mode, which is constantly recalculating the abort option regimes during ascent and can generate/display this graph upon command. In addition, the flight computers will now automatically select and fly the indicated abort mode if instructed to enter an abort state by the crew or by ground control.