# Space shuttle reentry: what altitude/speed for aerodynamic control?

At some point during the space shuttle’s reentry process, attitude control transitioned from using its reaction control system to using its aerodynamic flight control surfaces. In NASA’s own words:

OK, so they’ve specified two important control transitions (ailerons/elevators) in terms of dynamic pressure. Makes sense, but dynamic pressure is a function of both altitude and speed. anyone know roughly what altitudes/speeds these transitions occurred at?

I’m not an expert, but IIRC when the Air Force was testing the 100 series fighters to their limits, the surface controls became ineffective around 100,000 feet. Famously, Chuck Yeager ran into this climbing in an F104, could no longer control the plane’s orientation, and ultimately lost the plane when it went into an uncontrollable spin. His plan didn’t have reaction controls. I can’t say how speed plays into it, except that once the plane is in near-vacuum, I don’t know that it matters at all, because there is no longer what might be thought of as “stall speed”.

His plane, the third NF-104A, did have reaction controls. He did two runs over 100k feet. The first one went okay. The second one didn’t quite go so high. That’s the one that went bad. The first one went high enough that the reaction controls were effective in the very thin air and he was able to set the proper angle for descent. On the second run, he didn’t go as high and the reaction controls weren’t strong enough to set up the angle. (The regular controls were useless at that altitude.)

I seem to remember reading that on the flight that didn’t work out, they hadn’t taken into account the air at altitude being warmer on a flight later the same day, therefore a bit thicker.

Sample re-entry profile can be found here: http://www.orbiterwiki.org/wiki/Intuitive_Atmospheric_Entry#Entry_Profile
And working from that:
[ul]
[li]20 psf is at 24,000 ft/s and 245,000 feet[/li][li]10 psf is approximately at 24,200 ft/s and 260,000 feet[/li][/ul]

Dynamic pressure (q) is a function of static pressure and Mach number (the ratio of flow velocity to the speed of sound in the medium); specifically, q = (k * p[SUB]s[/SUB])/2 * M[SUP]2[/SUP]. The altitude at which q = 10 psf depends on how fast the Orbiter Vehicle (OV) is going at a given altitude and what the local density of the atmosphere is, which because this is in the thermosphere or upper mesosphere, can vary considerably with solar heating. Given a reentry speed of around 7.5 km/s (orbital speed minus the ~300 m/s delta-V to descend from orbit) and assuming a range of densities around the mesopause gives q=10 psd at somewhere between 79 and 85 km AGL.

However, there is no discrete transition between propulsive flight and lift/drag flight; as the dynamic pressure gets higher the ailerons and elevators gain more authority, and then the rudder, but the RCS yaw thrusters are still in use by the control system until the transonic regime because the OV doesn’t have sufficient control authority in supersonic flight to bank using aerodynamic forces without breaking or burning up. From Denis Jenkins excellent Space Shuttle: The History of the National Space Transportation System:The RCS turns the Orbiter’s nose forward for reentry which occurs at 400,000 feet, slightly over 5,000 miles from the landing site. The velocity at reentry is approximately 17,000 mph with a 40-degree angle-of-attack. The FRCS thrusters are initiated by the GPCs immediately prior to reentry, and the aft RCS thrusters maneuver the vehicle until a dynamic pressure of 10 pounds per square foot, which is when the Orbiter’s ailerons become effective. The aft RCS roll thrusters are then deactivated. At a dynamic pressure of 2 pounds per square foot the Orbiter’s elevators become effective and the aft RCS pitch thrusters are deactivated. The Orbiter’s speedbrake is used below Mach 10 to induce more positive downward elevator trim. At approximately Mach 5 the rudder is activated, and the aft RCS thrusters are deactivated at Mach 1.

Stranger