Oribits episode 2: Shuttle ascents and aborts
I started writing this post a week or so back and an unfortunate combination of circumstances caused it to go missing... the well-educated amongst you will possibly feel that you know all this stuff already, so apologies in advance. You can always go and do a complicated poll for evilmattikinz instead... Oh, and do ask questions if you want!
I promised in the last episode to offer some numbers, and this entry will be full of shuttle-related stuff so you can follow the launch (expected this week)- so to substantiate my statements about Shuttle trajectories, here is a picture (from the Ascent/Aborts Flight Procedures Handbook[1], courtesy nasaspaceflight.com):
I'll explain the blobs on the line first (a brief explanation of shuttle propulsion systems follows for those unfamiliar...). SRB staging is the point (around 2 minutes into flight) where the solid rockets start to burn out and are jettisoned, leaving the shuttle flying on main engines only - this continues until main engine cut-off (MECO on the graph). The point at which the engines are shut down at MECO is determined by the shuttle's velocity rather than time (this fact will be relevant later). In the case of missions to the space station, MECO is 25819fps (feet per second - the shuttle isn't metric).
Just to clarify the propulsion set-up - the shuttle has 3 main engines (SSME - space shuttle main engines), 2 orbital maneouvering system (OMS) engines, many small thruster jets (RCS - reaction control system) and the two solid rocket boosters (SRB). They are notable because once lit, they can't be turned off. This means that the launch sequence is careful to fire up the SSMEs first and check they are at close to full power before igniting the SRBs[2].
Returning to the graph - after MECO we have the external tank seperation[3] followed by OMS-1, the first burn of the orbital maneuvering system. In fact, this is rarely used now - most missions are flown to orbit on the main engines (sometimes using the OMS engines to assist them while they are running) and a second burn of the OMS (not shown) is used to attain the final orbit.
The interesting thing to note on the diagram is the other labels - these are an outline of the possible "abort" modes - the intention of the shuttle design is to incorporate a high degree of redundancy, and that means planning for various kinds of failures. The most common ones would be "systems" failures (cabin leeks, loss of cooling capacity for example) or engine failures (1, 2 or 3 main engines failing). The most common case is a single-engine failure, and the marks on the graph indicate (in general terms) the abort mode to be used for a single engine failure at different stages. During an actual flight the flight controllers will call out the boundaries to the crew ("Negative Return", "Single Engine Press 104" etc.) as the ground have much better tracking information and insight into performance. The crew have backup data to use if communications with the ground are lost.
During the first stage an abort can be declared, but nothing will happen until the SRB seperation. Then (and for some time after), the first abort option is RTLS (Return to Launch Site). This is comparativeky exotic. The intention is to land the shuttle back at the Kennedy Space Center - because it is be some distance away, they gradually pitch the shuttle around to fly back (as a way to get rid of some of the velocity they have been picking up. Once it has too much energy, that stops being an option.
TAL (Transatlantic Abort Landing) is the next option - they land in Spain (Zaragosa and Moron) or France (Istres) - this is a little like a normal re-entry. The lower line on the graph shows the profile up to MECO for that mode. The next option, AOA (Abort Once Around) involves landing in the USA after doing almost a whole orbit. Finally, ATO (Abort to Orbit) is the state where they can reach an orbit, even if it isn't the orbit planned. It is possible to make the nominal orbit (and thus have a successful mission) with two engine failures if they happen late enough - this is the significance of "Press to MECO" and "Single Engine Press to MECO" - because the three SSMEs share the same hydrogen and oxygen supply, a longer burn on the remaining engine(s) can be sufficient to achieve the desired orbit. The 104 is just the standard throttle setting (in percent of the nominal rating of the engine - they are normally run at 104% now) - it is possible to use 109% under some circumstances.
All these options are considered "intact aborts" - the intention is that the shuttle will land normally at a prepared landing site; there are also a large number of "contingency abort" situations - 3 engines failing in first stage would probably result in the crew bailing out over the ocean - but there are procedures to cover these cases. The "busiest" case is (I gather) an AOA for systems problems such as lack of cooling - there is a lot of equipment to turn off (to avoid generating heat and using power) and a lot of reconfiguration in a short space of time. One of the training staff described them as "sporty"!
I'll try ot get another one up here before the shuttle launch... though you may be fed up of lengthy essays by then :)
[1] The book used by the ground staff to explain the systems and the rationale behind the procedures. The crew have a much shorter checklist to work from (the whole 8 minute nominal ascent is a single page - they don't have a lot to do).
[2] The SSMEs use liquid hydrogen and liquid oxygen, stored in the large brown external tank.
[3] The flight path for the shuttle has to be designed to avoid dropping the external tank over populated areas or shipping lanes, even though it will disintegrate as it re-enters the atmosphere.
I promised in the last episode to offer some numbers, and this entry will be full of shuttle-related stuff so you can follow the launch (expected this week)- so to substantiate my statements about Shuttle trajectories, here is a picture (from the Ascent/Aborts Flight Procedures Handbook[1], courtesy nasaspaceflight.com):
I'll explain the blobs on the line first (a brief explanation of shuttle propulsion systems follows for those unfamiliar...). SRB staging is the point (around 2 minutes into flight) where the solid rockets start to burn out and are jettisoned, leaving the shuttle flying on main engines only - this continues until main engine cut-off (MECO on the graph). The point at which the engines are shut down at MECO is determined by the shuttle's velocity rather than time (this fact will be relevant later). In the case of missions to the space station, MECO is 25819fps (feet per second - the shuttle isn't metric).
Just to clarify the propulsion set-up - the shuttle has 3 main engines (SSME - space shuttle main engines), 2 orbital maneouvering system (OMS) engines, many small thruster jets (RCS - reaction control system) and the two solid rocket boosters (SRB). They are notable because once lit, they can't be turned off. This means that the launch sequence is careful to fire up the SSMEs first and check they are at close to full power before igniting the SRBs[2].
Returning to the graph - after MECO we have the external tank seperation[3] followed by OMS-1, the first burn of the orbital maneuvering system. In fact, this is rarely used now - most missions are flown to orbit on the main engines (sometimes using the OMS engines to assist them while they are running) and a second burn of the OMS (not shown) is used to attain the final orbit.
The interesting thing to note on the diagram is the other labels - these are an outline of the possible "abort" modes - the intention of the shuttle design is to incorporate a high degree of redundancy, and that means planning for various kinds of failures. The most common ones would be "systems" failures (cabin leeks, loss of cooling capacity for example) or engine failures (1, 2 or 3 main engines failing). The most common case is a single-engine failure, and the marks on the graph indicate (in general terms) the abort mode to be used for a single engine failure at different stages. During an actual flight the flight controllers will call out the boundaries to the crew ("Negative Return", "Single Engine Press 104" etc.) as the ground have much better tracking information and insight into performance. The crew have backup data to use if communications with the ground are lost.
During the first stage an abort can be declared, but nothing will happen until the SRB seperation. Then (and for some time after), the first abort option is RTLS (Return to Launch Site). This is comparativeky exotic. The intention is to land the shuttle back at the Kennedy Space Center - because it is be some distance away, they gradually pitch the shuttle around to fly back (as a way to get rid of some of the velocity they have been picking up. Once it has too much energy, that stops being an option.
TAL (Transatlantic Abort Landing) is the next option - they land in Spain (Zaragosa and Moron) or France (Istres) - this is a little like a normal re-entry. The lower line on the graph shows the profile up to MECO for that mode. The next option, AOA (Abort Once Around) involves landing in the USA after doing almost a whole orbit. Finally, ATO (Abort to Orbit) is the state where they can reach an orbit, even if it isn't the orbit planned. It is possible to make the nominal orbit (and thus have a successful mission) with two engine failures if they happen late enough - this is the significance of "Press to MECO" and "Single Engine Press to MECO" - because the three SSMEs share the same hydrogen and oxygen supply, a longer burn on the remaining engine(s) can be sufficient to achieve the desired orbit. The 104 is just the standard throttle setting (in percent of the nominal rating of the engine - they are normally run at 104% now) - it is possible to use 109% under some circumstances.
All these options are considered "intact aborts" - the intention is that the shuttle will land normally at a prepared landing site; there are also a large number of "contingency abort" situations - 3 engines failing in first stage would probably result in the crew bailing out over the ocean - but there are procedures to cover these cases. The "busiest" case is (I gather) an AOA for systems problems such as lack of cooling - there is a lot of equipment to turn off (to avoid generating heat and using power) and a lot of reconfiguration in a short space of time. One of the training staff described them as "sporty"!
I'll try ot get another one up here before the shuttle launch... though you may be fed up of lengthy essays by then :)
[1] The book used by the ground staff to explain the systems and the rationale behind the procedures. The crew have a much shorter checklist to work from (the whole 8 minute nominal ascent is a single page - they don't have a lot to do).
[2] The SSMEs use liquid hydrogen and liquid oxygen, stored in the large brown external tank.
[3] The flight path for the shuttle has to be designed to avoid dropping the external tank over populated areas or shipping lanes, even though it will disintegrate as it re-enters the atmosphere.