Few flying machines manage to capture the imagination quite as much as the Space Shuttle - but rightfully so; few craft also experience quite such an enormous range of aerodynamical conditions. With the ability to launch like a rocket, operate in orbit as a spacecraft, enter the atmosphere on a near ballistic trajectory and gradually transit via hypersonic trajectory management to subsonic airplane-like flight, the NASA Space Shuttle is a truly unique flying experience.

The FlightGear team is proud to present a highly detailed Open Source simulation of this fascinating craft for the 2016 'San Francisco' stable release and later versions of FlightGear. The simulation covers all mission phases from launch to touchdown and has a strong focus on realistic orbital operations, including payload handling, proximity ops and docking. For the most part, it can literally be flown by the original NASA crew manual.

       

While the development focus is on simulating the experience of how the Shuttle is piloted (which in reality is a fallback mode), there's also substantial support for (semi-) automated flight. In addition, there are also various 'educational' modes which are not part of the real Shuttle, but illustrate certain aspects of its operation. For instance, it is possible to not use the digital autopilot support and directly control airfoils during entry like for an airplane which allows to directly appreciate how difficult aerodynamical control during the early entry phase is.

Flight Dynamics

Spanning the full range of a rarified thermosphere rushing by at Mach 27 to the dense lower atmosphere through which the Shuttle glides at subsonic speeds on final approach, the orbiter probes a huge variety of conditions. Through this range, its aerodynamical characteristics change drastically. The entry into the atmosphere is flown at a high AoA of 40 degrees, creating a detached shockwave of hot plasma which limits the direct heat transfer to the fuselage, while at lower Mach numbers the AoA is gradually reduced, leading to a more aircraft-like behavior.

While the craft is yaw-stable at subsonic speeds, at supersonic and hypersonic speeds the high AoA mandated by thermal considerations places the vertical stabilizer essentially outside the airstream, making the craft dynamically unstable. Yaw stability in this regime is then purely managed by the flight control systems. For a similar reason, at high Mach numbers the response to the craft to elevon movement is strongly asymmetric: Upward deflection causes much less pitching moment than downward deflection by the same angle. Only at lower airspeed and smaller AoA, the situation becomes symmetric, which is more usual for normal aircraft.

The simulation utilizes a huge amount of aerodynamical data from NASA technical documents in the public domain. For the nominal envelope of operations, most of this data has been measured during actual Shuttle missions. To describe aerodynamics outside this envelope, wind tunnel data has been used, both for the orbiter alone and the full launch vehicle including solid rocket boosters and external tank. For the major aerodynamical coefficients (such as pitching, yawing and rolling moments), the full non-linear dependence on Mach number, AoA and degree of airfoil deflection is included in the simulation. This allows effects such as the suction felt by the aft fuselage caused by elevon movements at high Mach numbers or the Mach dependence of the cross couplings between the various control channels.

       

From a piloting perspective, the entry phase with its change from near-ballistic trajectory management to truly aerodynamical flight is perhaps the most interesting. As in reality, the simulation provides automatic helpers. In particular, an AoA auto-hold can be activated to help keep the Shuttle in a thermally safe regime, and the rate-controlled DAP makes the Shuttle respond crisply and controlled to each input, regardless of whether in near-vacuum, in hypersonic descent at Mach 16 or in the lower atmosphere at Mach 2. In fact, while the Shuttle is yaw unstable and needs to be constantly nudged back to zero sideslip, you'd be hard-pressed to observe any trace of this.