Do/why don't spaceplanes suffer from flutter?

Question

Spaceplanes tend to have beefier, stiffer parts than normal aircraft, at least partly due to drag-reduction being less of a concern. But they also experience far higher airspeeds during reentry.

Yet I've never heard much, if anything, about flutter, or any other form of aeroelasticity being a problem on any of them. Maybe that's because I don't know much about existing or past spaceplane programs? But still, I'd be interested to know.

I can imagine some candidates for the reason why flutter might be less severe:

• the frequencies are so high and modes so many and so uncoordinated that too much energy is dissipated as heat and sound, leading to simply vibration with no damage.
• the sheer stiffness of the vehicle moves flutter frequency out of reach. (Which sounds rather far-fetched for me)

Hoping to find an answer, I Googled about aeroelasticity of the Shuttle, and got this document. It's rather long, so ATM I haven't drawn any conclusions from it. It would be nice if someone has any background info on what the designers of the Shuttle (which I would guess is the best known spaceplane) faced during development in relation to flutter.

Answer 1

Apologies for trying to offer an answer to a nearly three year old question, but I'm unable to resist this one.

The truth is that all spacecraft that operate (however briefly) in our atmosphere do need to be concerned with aeroelasticity. It is only through careful design, analysis, and testing that a vehicle will be free from catastrophic aeroelastic effects -- it certainly doesn't happen by accident.

As you noted in your link (NASA TM X-2570), there was an entire session devoted to aeroelastic effects on the space shuttle at a single joint NASA/AIAA conference in 1972. Slide 28 of the PDF is particularly interesting because it clearly shows that the biggest challenges for shuttle flutter analyses were in the transonic regime, not necessarily the hypersonic regime. This is largely true today and the explanation is probably telling. In exterior hypersonic flow, the forces are generally fairly steady and (on surfaces facing the freestream) typically very readily obtained (e.g., using Newtonian aerodynamics -- see NASA TM X-53391). Transonic aerodynamics, however, are highly nonlinear and required modest computing power to evaluate that didn't really become available until the late 1980s (e.g., Silva and Bennett, "Using Transonic Small Disturbance Theory for Predicting the Aeroelastic Stability of a Flexible Wind Tunnel Model", 1990).

If the question is really "why don't we see aeroelastic failures of space planes?", the answer comes back to all that careful analysis, design, and test. Space programs in particular are extremely thorough about working through every possible issue before flight. I've heard this derisively called "analysis-paralysis" and some of the criticism leveled at the time and budget required for space vehicles may indeed be fair. However, unlike their air-vehicle counterparts, which have the ability to slowly expand a flight envelope guided by analysis during flight testing, space vehicles are usually an all-or-nothing. Thus, there is a ton of time, man-hours, and money that go into ensuring that these vehicles are flutter-free.

Having said all that, it's not that space vehicles are actually so well understood that they never suffer from unexpected aeroelastic problems. Indeed, it's not uncommon that they do encounter these types of issues as they transition from static, to low-speed, compressible, transonic, supersonic, and finally hypersonic flight regimes (or vice-versa on entry). Thankfully though, the issues encountered have historically not been catastrophic (did I mention the enormous amount of effort that went into design, analysis, and test?). However, when these issues do crop up, they are assiduously examined post-flight and the vehicle typically will not fly again until the issue is expected to be resolved.

Answer 2

Not Shuttle specific, but there are generally two ways to prevent flutter of a control surface; for a manually operated surface, you balance it to put the surface's CG at or slightly forward of its hinge line, so that motions of the parent surface have a neutral (if CG on the hinge line) or counteracting (if CG is forward of the hinge line) influence on exciting sympathetic motion in the surface (that is, if the wing flexes up, with the CG aft the surface it "trails" the motion and displaces down relative to the wing attachment, putting it on the hinge line neutralizes this effect, and putting it forward of the hinge line has the opposite effect, creating an inertial moment in the same direction as the wing motion).

If the surface is hydraulically powered using irreversible hydraulic actuators, balancing of the surface generally isn't required and flutter protection is from the rigidity of the hydraulic connection (both in holding the surface from moving, and also acting as a passive damper when unpressurized). You can usually tell an airplane with irreversible hydraulics because there are no tabs or balance horns, or offset hinge lines, and the center of gravity of the surface will be some distance aft of the hinge axis.

So jets with irreversible hydraulic controls typically have unbalanced control surfaces that depend on the hydraulic actuators themselves for flutter prevention, and may also require additional passive damper units if the number of actuators used doesn't meet the minimum damping requirements following loss of an actuator. Service backlash limits for surface hinge attachments and actuator connections (resulting in surface free play) are largely a function of maintaining the rigidity of the system for flutter resistance.

The Shuttle having hydraulically driven control surfaces, you can be sure that flutter resistance is ultimately coming from the hydraulic actuators and the stiffness of the connections and structure (I don't know if the shuttle uses dedicated flutter dampers to back up the actuators).