Flying the Shuttle - Entry: Difference between revisions

== Going home - a tutorial ==

<i>In this tutorial, you'll learn to use the Shuttle's avionics to fly the hypersonic trajectory from entry interface to TAEM interface. The tutorial assumes that you have (by doing the on-orbit tutorials) gained basic familiarity with the avionics and the systems such that you can configure the MFDs to your needs and enter commands.</i>

You'll find yourself right in the upper atmosphere on a sinking trajectory. Let's bring up the trajectory map from the menu to see where we are going:

As you can see, the trajectory ends close to Vandenberg, which is where we'll land. The next thing is to tell that to the avionics (in real life, we would have done that before leaving orbit, but we just started FG...) - so let's bring up SpaceShuttle -> Entry guidance and confirm Vandenberg as the landing site by pressing 'Activate'. Don't bother with the other options like the runway, TAEM approach selection or entry point at this point, they're relevant for TAEM guidance only.

Now, let's take a minute to consider our situation. The red trajectory line is a ballistic trajectory, and it's endpoint is where the Shuttle would hit the surface <i>if there wasn't an atmosphere.</i> Since there is an atmosphere however, we may end up somewhere else - which is the whole point of entry trajectory control.  

* We have a few minutes time before we really reach the dense atmosphere, so we can configure the cockpit to our needs. Bring up a DPS screen in GNC mode (it should show the ENTRY TRAJ 1) before you, next to it the PFD might be handy, and the APU/HYD and the SPI displays to your right. You may find the information from the FG-native HUD useful, though if you want to be realistic, switch the layer off (before you do so, check that the DAP selection in the upper left reads 'Aerojet' - if not, you'll be in big trouble...)

The whole configuration might look like this:

To the upper left of the leftmost panel, at the edge of the console, there's four pushbuttons. The upper two change select CSS or AUTO for the pitch channel - change the selection from CSS to AUTO and verify that the button lights up. The Shuttle will respond by automatically pitching up to about 40 degrees.

The purpose of this is as follows: During entry, thermal protection requires that we turn the blunt end of the Shuttle into the plasma stream. This translates into a rather strict Mach-dependent AoA requirement - for instance at Mach 23 we <i>must</i> fly 40±3 deg AoA or we'll burn and break up. Setting the pitch channel to AUTO makes the flight computer control AoA very precisely - which gives us the room to worry about trajectory control.

Of course, that means that all we can control is the roll channel, so we need to use that.

This is a display of remaining range (x-axis) versus velocity (y-axis) - be warned, there's no altitude shown on this display! Moving left means you're closing with the site, moving down means you're slowing down.

Thus, if we get below the nominal trajectory, we want to fly small bank angle, if we get above we want to fly large bank angle. At the same time, we want to bank into the direction to keep ΔAz small, thus if we're right of the target, we bank left till we reach a -10 deg ΔAz, then we reverse attitude (that's called a 'roll reversal') and bank right to let ΔAz grow to +10 degrees, then we do another roll reversal.

If you're used to how this feels in an airplane, it'll feel exceedingly weird because the guidance holds 40 deg AoA all the time - in essence you'll do this:

Observe how with small changes in bank angle you can make vertical speed increase and decrease - there's really control here. Utilize it to maneuver the Shuttle symbol back to the trajectory (you'll inevitably go a bit below initially for the chosen trajectory).

That gives us time to check on a few other things - APU status and SPI (surface position indicator). You can take in APU status at a glance - if all is green you're fine, if anything shows red, you might have to shut down the APU. Airfoils will still work with only one APU running (just in case) but the response of the Shuttle will be less crisp and immediate.

SPI will show the need for trim. Usually the elevon indicators should be close to zero - if they're not, you're gradually losing roll control. Below Mach 10, you can use the body flap and the speedbrake to try to ease the trim load of the elevons. For the loadout of this entry scenario, you can trim body flap up if you like.

Once the craft slows down further, the computer will gradually reduce AoA (no need to be alarmed) and the Shuttle will become more responsive - which unfortunately means you'll hit the ΔAz limits rather sooner than later, so roll reversals become more frequent.

* Close to TAEM interface, you can start to see the color of the sky change to a deep blue. If you've stayed with the nominal trajectory and kept the azimuth under control, you should be some 85.000 to 100.000 ft altitude with about Mach 3 with some 60 miles to site on the indicator.

* If you get much below the nominal trajectory, you need to aim at the target at the first reasonable opportunity (perhaps around Mach 14 when you have plenty of speed left and the Shuttle actually can already change direction) and otherwise fly zero bank angle for optimum glidepath. If you aim too early, it costs a lot of lift without changing direction, if you aim too late the additional distance you have to fly due to the azimuth offset is too large.