Flying the Shuttle - Abort Procedures Overview: Difference between revisions

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Flying the Shuttle - Abort Procedures Overview (view source)

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[[File:RTLS.gif|600px|Return to Launch Site]]
[[File:RTLS.gif|600px|Return to Launch Site]]


The abort is declared after SRB separation outside the atmosphere. From there, there are three distinct phases - powered flight, coast and ET separation and final glide.
Full explanation of this Abort here : [[http://wiki.flightgear.org/Flying_the_Shuttle_-_Return_To_Launch_Site_RTLS]]
 
The powered flight phase starts extremely low on thrust. At that stage, the main aim is to deplete propellant. Pitched highly upwards (up to 60 deg) to vector thrust downward and minimize loss of altitude, the Shuttle continues to accelerate downrange. During this time, the OMS propellant already has to be dumped (by burning it) — {{Key press|Ctrl|o}} activates the OMS fuel dump. If this is not done, the Shuttle's CoG will be off by the time it needs to glide, with usually catastrophic consequences.
 
At a turning point, the Shuttle then pitches around rapidly (about 10 deg/s) and starts to decelerate, still vectoring thrust downward. All the time the trajectory drops, and pitch should be adjusted such that by the time of MECO, it has come down to about 220.000 ft but not significantly lower
 
[[File:Shuttle RTLS abort01.jpg|600px|The Space Shuttle during an RTLS abort — after pitch around]]
 
After pitch around, the Shuttle eventually reaches a turning point and starts to accelerate back to the range. As propellant depletes and ET weight is reduced, acceleration reaches up to 3 g and the Shuttle becomes very maneuverable again, allowing it to reach up to Mach 7 pointed back towards the landing site.
 
Timing for the pitch-around is critical — wait too long and the Shuttle will not make it back to the site, turn too early and the Shuttle will overshoot. The proper indicator for the pitch-around is the amount of propellant remaining, dependent on the time when the engine failed. Refer to the following cue card:
 
{| class="wikitable"
|+ RTLS cue card
! failure time !! pitch around
|-
| 0:00 || 44%
|-
| 0:30 || 47%
|-
| 1:00 || 47%
|-
| 1:30 || 48%
|-
| 2:00 || 51%
|-
| 2:30 || 51%
|-
| 3:00 || 52%
|-
| 3:30 || 53%
|}
 
The next step is to fly the Shuttle at MECO to a safe attitude to separate the external tank. Since the tank is a huge device, it feels quite some atmospheric drag and should ideally be separated as high up in the atmosphere as possible. However, since the Shuttle is very far from orbital velocity, it should ideally make the transition from powered to gliding flight as low in the atmosphere as possible to avoid dropping very rapidly.
 
The result is a compromise — the idea is to separate the ET between a qbar of 1-2 psf, at an alpha of about -2 deg (for this the Shuttle needs to be pitch down in the last moments of the flight) with no more than 2% propellant remaining to avoid sloshing and in addition all rotation rates nulled (use the RCS attitude hold mode quickly).
 
[[File:Shuttle RTLS abort03.jpg|600px|The Space Shuttle during an RTLS abort — after ET separation]]
 
Since the Shuttle is just feeling the fringes of the atmosphere, it will start to drop rapidly (i.e. almost accelerating 9.8 m/s downward every second) and plunge into the dense atmosphere. As soon as the atmosphere catches, forces will ramp up quickly. The aim of the next few seconds is to survive this. Short of a contingency abort, the plunge into the atmosphere from RTLS MECO is probably the most dramatic maneuver you will ever experience in the Shuttle.
 
Pitch needs to be brought up rapidly to 50 deg and the Aerojet DAP needs to be on. The Shuttle may reach vertical speeds in excess of 500 m/s before the atmosphere shows significant effect. Then watch acceleration ramp up - as soon as the acceleration reaches 2.2 g,  pitch has to be reduced to lessen the forces. Pitch needs to be managed such as to keep the acceleration on the vehicle close to that number till the acceleration lessens and the vertical speed reduces to less than 100 m/s.
 
If the Shuttle survived the contact with the atmosphere, the remaining task is to manage azimuth and range to the site. For this, one returns to the normal techniques of flying Mach-dependent AoA and using bank angle to manage sink and drag or S-turns for ranging, all intended to bring the Shuttle to a regular TAEM interface.
 
To initiate an RTLS abort in FG, the entry guidance computer needs to be set to the launch site and as abort mode 'RTLS abort' needs to be selected. In response to this, the HUD symbology changes to show key information needed to pilot the abort, such as the distance and relative velocity to site and the percentage of propellant remaining.
 
[[File:Shuttle RTLS abort02.jpg|600px|HUD symbology of the Space Shuttle during RTLS abort]]
 
The groundtrack of an RTLS abort is an elongated loop, heading out into the launch direction before returning almost the same path:
 
[[File:Shuttle RTLS abort05.jpg|600px|Groundtrack of a Space Shuttle RTLS abort]]


== Transatlantic Abort Landing ==
== Transatlantic Abort Landing ==
Gingin
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