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.

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    $\begingroup$ Hopefully someone who understands aerodynamics better will answer, but the premise seems wrong to me. The shuttle only experienced about 1.5g on reentry, which any airframe could handle. With stiff stubby delta wings, it seems like flutter would be unlikely to develop. I could be wrong though. $\endgroup$ – Bret Copeland Sep 9 '20 at 16:59
  • $\begingroup$ @BretCopeland 1.5G? I thought it was 3 $\endgroup$ – Abdullah Sep 9 '20 at 17:18
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    $\begingroup$ 3g during launch. There's certainly a lot of vibration during launch. All launch vehicles and payloads go through vibration analysis. I'm sure if there were any issues, they simply would have strengthened areas as necessary. My impression is that the opposite occurred - the Columbia/Challenger airframes were stronger than they needed to be and Discovery, Atlantis, and Endeavor benefited from weight reductions. $\endgroup$ – Bret Copeland Sep 9 '20 at 18:36
  • $\begingroup$ Do/why don't spaceplanes suffer from flutter? I think it should just be "Why don't spaceplanes suffer from flutter?". "Do don't spaceplanes suffer from flutter" doesn't sound quite right. $\endgroup$ – Super Sep 9 '20 at 21:18
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    $\begingroup$ The old Orbiter Aerodynamic Design Data Book (archive.org/details/nasa_techdoc_19810067693) has lots of cool graphs showing aeroelastic effects on stability (Page A.1-7 fwd) but interpreting them is not something I can do. The whole of Appendix A is about aeroelasticity of the Orbiter. I think it's fair to conclude that aeroelasticity was considered in the design. $\endgroup$ – Organic Marble Sep 9 '20 at 22:12

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).

  • $\begingroup$ Is this about the difference between free-floating vs non-free-floating surfaces? $\endgroup$ – Abdullah Sep 10 '20 at 8:49
  • $\begingroup$ I'm describing how you prevent flutter in a non-hydraulic surface, one operated by cables from a control column, or operated by a servo tab on the trailing edge of the surface and can be moved by air loads, vs one with hydraulically powered surfaces (irreversible hydraulics refers to the fact that airloads can't back drive the surface) where balancing is not required, and inferring that a craft like the Shuttle with hydraulically driven surfaces, will get its flutter resistance from the "rigid" hydraulic connection and not by mass balancing. That, and basic structural stiffness, is about it. $\endgroup$ – John K Sep 10 '20 at 14:51

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