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Flying the Shuttle - Launch: Difference between revisions
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→Three, two one... ignition and... liftoff - a launch tutorial
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What's the state of the spacecraft now? The SRBs are very powerful devices, we've been rising with close to 3 g for two minutes. Now they're gone, and the remaining thrust is not even enough to hold the Shuttle with the near-full ET against gravity. But it doesn't need to, we're climbing with almost a kilometer per second vertically. Even if we had no thrust, we'd go above 100 km altitude now. | What's the state of the spacecraft now? The SRBs are very powerful devices, we've been rising with close to 3 g for two minutes. Now they're gone, and the remaining thrust is not even enough to hold the Shuttle with the near-full ET against gravity. But it doesn't need to, we're climbing with almost a kilometer per second vertically. Even if we had no thrust, we'd go above 100 km altitude now. | ||
We don't want to be much higher, but we need to be much faster - so the Shuttle needs to pitch down and start building horizontal velocity now, and it needs to do it such that when gravity has finally depleted the push from the SRBs, we fall right into the desired trajectory at 130 km altitude. For such a maneuver, you absolutely need the predictor dots. | |||
It's even a bit more complicated, because unlike the previous display, this shows inertial speed. The difference is the effect of the velocity of Earth's rotation, which depends on launch latitude and azimuth. So to capture the desired trajectory, we need to pitch down just the right amount accounting for the rotation effect as well as for any payload. Luckily guidance does this automatically. | |||
So you should watch the predictors indicate the drop into the trajectory while the Shuttle gets faster and the triangle eventually reach close to the horizontal part of the trajectory. | |||
Actually, the symbol will not always be spot on the trajectory, but it should remain reasonably close. If it ever diverges a lot, guidance has failed and you need to take over manually. | |||
* Okay, time to take a breath and scan the status of the APUs. There should be plenty of fuel left (we run them for a few minutes now, during entry they must run for close to an hour). Cooling water should still be full - they don't get hot until we are in orbit. And hydraulic pressure should be a good 3000 psi. | |||
[[File:Launch tutorial08.jpg|600px|Launch tutorial 1]] | [[File:Launch tutorial08.jpg|600px|Launch tutorial 1]] | ||
If an APU fails, it means we have an engine that can no longer be gimbaled and throttled ('hydraulic lockup'). That's not as bad as a failed engine, but it does mean we will have to switch it off manually some 30 second before the others. | |||
* The late part of the launch is a waiting game. As the tank depletes, acceleration increases back towards 3 g, and the DAP will gradually throttle the engines back when this happens. | |||
[[File:Launch tutorial09.jpg|600px|Launch tutorial 1]] | [[File:Launch tutorial09.jpg|600px|Launch tutorial 1]] | ||
But wait - what's that? Why do the predictors move upward? Remember that they tell what will happen if we do nothing? As the Shuttle accelerates, finally the centrifugal force which will keep us in orbit starts building to significant strength, and that means we'll start to rise unless we constantly gradually pitch down to keep altitude, So it's not worrisome at all because the DAP will do something like an altitude hold. | |||
* Very close to MECO, also the horizontal velocity scale will see movement. As inertial speed reaches above 25.000 ft/s, the indicator will move towards the CO point. Once the indicator is there and the Shuttle is at the end of the trajectory, all engines will be cut automatically and you are in orbit. Verify that you're going to a 320 km apoapsis. Then you can disconnect the ET and start preparing your ride for orbital operations. | |||
[[File:Launch tutorial10.jpg|600px|Launch tutorial 1]] | [[File:Launch tutorial10.jpg|600px|Launch tutorial 1]] | ||
Note that the Shuttle under automatic launch guidance does not rotate into upright attitude. That was indeed the case for the early missions, in the later Shuttle missions a rotation would be performed in the late stages to get an uplink to the TDRS com network. | |||
* Now that you've seen how it should be performed, you can start to practice doing it manually. | |||
If you need a real challenge, try switching the DAP via {{Key press|m}} to 'Thrust Vectoring (gimbal)' on the launch pad. This changes your stick input from a bias to the rate controllers flying the Shuttle to direct control of the engine gimbal angles. It is possible to fly the Shuttle into orbit this way, though really a tough job, which will make you appreciate the work done by the ascent DAP. | |||
== Further reading == | == Further reading == | ||