Flying the Shuttle - Launch: Difference between revisions

You'll find yourself on a pad in launch complex 39 at the cape, on a hot Florida morning. You have in fact just a few minutes to liftoff. (There's no launch gantry, flame trench, access road,... because no 3d modeler has put it there yet).

<i>If you toy with the idea that specifying a heading is silly, be warned - it's crucial for the launch guidance to find a viable solution that the tail fin of the Shuttle points into the Eastern hemisphere - keep this in mind if you want to launch from somewhere else in the world.</i>

Next, tell the Shuttle that you want the flight on autopilot by selecting the PITCH AUTO and ROLL/YAW AUTO pushbuttons on the forward panel (circled in red, underneath the transparent checklist).  

Flying the Shuttle manually into orbit isn't actually difficult at all - if you know what you have to do, because you need to anticipate a few things and that's what we're out to learn here. But if you want to take over from the automatic guidance at any point, push the CSS pushbutton for the channel (you can just take control of pitch and let the DAP handle yaw/roll for instance). <b> After activating CSS in-flight, do not attempt to hand control back to the AP - this will not work!</b>.

* While you wait, take a look at the FG-native HUD layer:

There's plenty of useful information displayed here, all items worth monitoring during ascent marked with red dots. While the Shuttle displays velocities in ft/sec and orbital altitudes in miles, you may find the option of getting some parameters displayed in metric units helpful (if you have a science background for instance).

You can actually watch the show from outside by using tower view. If you've done everything according to plan, the SRBs ignite three seconds after the main engine, and at that point Atlantis lifts off.

* If you stay in-cockpit view, what happens next feels and looks decidedly weird - as soon as the (currently not modeled, you remember?) launch gantry is cleared, the orbiter rotates and yaws to get onto the launch course, and then pitches down into inverted flight. That's right, you ride into orbit head-down.

One of the main reasons is the wing bending moment. While in terms of thermal stresses, entry is far more dangerous than launch, in terms of forces acting on the frame, launch is a good factor two more serious. If we would launch in upright position, the wings would be ripped off cleanly. The lowest stress on the wings occurs for negative AoA, so we pass the region of max. qbar (i.e. highest aerodynamical stresses) at the optimum alpha for the wings.

Look at the engine status.

The lower right shows chamber pressure of all three main engines. During max. qbar they get throttled back, otherwise they should show (at least) 100% of rated power. In the case of an engine loss, it's possible to throttle the remaining ones up to 109% of rated power via the override display, but perhaps let's just not try this right now. Above are helium regulator and storage tank pressures. What's that for?

* Let's take a look at the ascent trajectory:

This is a plot of ground-relative velocity vs. altitude. The Shuttle is the yellow triangle climbing along the line, and there's a predictor which tells us where the Shuttle is expected to be 20 seconds from now - if there's no change in attitude or thrust.

* After SRB separation, the ascent trajectory display changes to this:

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.

* 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.

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.

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.

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.