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Thursday, February 16, 2023

Clamshell wings for hypersonic reentry of rocket stages.

 Copyright 2023 Robert Clark

(Patent pending)

 It is known that large wings can reduce the speed and therefore aerodynamic heating a stage can experience during reentry. But such wings would induce high drag on ascent in addition to their high weight.

 A proposal to solve both of these issues: wings that open up from the stage sides or from the fairing for the upper stage, clamshell wings.


An overview of research on waverider design methodology

  • August 2017
  • Acta Astronautica 140

    •    The curved shape of the wings around the cylindrical rocket when opened up would provide both high lift and drags, important for the hypersonic reentry.

     For a reusable lower stage, though it would be difficult to maintain the structural integrity of the tanks for reuse when they open up to be wings, especially for maintaining a leakproof seal for the next flight. If this is to be used for a lower stage, the clamshell wings would have to be added around the stage. This would reduce the weight efficiency of the stage. However, quite likely they would still weigh quite a bit less than the propellant that has to be kept on reserve, unused, during ascent, for use for return to launch site.

     For the SuperHeavy it’s to be 7% of the propellant mass or 250 tons being kept on reserve for return to launch site. This high amount of propellant kept on reserve is a large part of the reason why the reusable versions of the Starship/SuperHeavy, just as with the Falcon 9 lose so much on reusability, 30% for partial reusability, and 50% for full reusability. 

    The clamshell wings around the tanks likely can be designed to be well less than the mass of the tanks, which for the SuperHeavy is in the range of 80 tons. Wings in general commonly weigh in the range of 5% to 10% of the aircraft weight to be carried. This would be quite heavy if they had to support the fully fueled weight of the vehicle, as is normally the case with aircraft. Note though the wings would be closed around the tanks on ascent and would only open up to support the dry mass on return. Elon has estimated the dry mass of the SuperHeavy as less than 200 tons, so only at most 10 to 20 tons would be added to the stage weight, far less than the 250 tons of propellant needing to be carried now as “deadweight” during ascent. 

     For this to work you would want the clamshell wings to have high lift and drag at hypersonic speeds. The Space Shuttle for example has been described as a flying brick at hypersonic speeds having a hypersonic L/D at about 1, though its subsonic L/D was better at about 4.5. The clamshell wings will quite likely have high hypersonic L/D because of the prior research done on caret-wing hypersonic waveriders:

       The clamshell wings would be analogous in shape to the caret-shaped waveriders able to achieve high hypersonic L/D. During the return flight, we can also imagine achieving high levels of control by varying the angle on each side of the wing.
      If you can make this extra surface be lightweight then you would get low wing loading. The importance of low wing loading for reentry for spaceplanes is discussed here:

      Wings in space.
      by James C. McLane III
      Monday, July 11, 2011
      http://www.thespacereview.com/article/1880/1

      At the end of the article there is this passage:

      Wing loading (the vehicle’s weight divided by its wing surface area) is a prime parameter affecting flight. The antique aluminum Douglas DC-3 airliner had a big wing with a low loading of about 25 psf (pounds per square foot of wing surface). At the other end of the spectrum, the Space Shuttle orbiter has a high wing loading of about 120 psf. This loading, combined with an inefficient delta-shaped wing, makes the orbiter glide like a brick. A little Cessna 152 private plane features a wing loading of about 11 psf and modern gliders operate down around 7 psf. A space plane with huge lifting surfaces and a very low wing loading might not require any external thermal insulation at all. Building a space plane with a wing loading of, say, 10 psf should not be an impossible proposition. Perhaps some day it will be done.

      {emphasis added}

      Moreover, because of their curved shape they should be even more effective at slowing down the descent during reentry at high angles of attack, like a parachute.

      I estimated the wing loading using this clamshell wing idea for the new Falcon 9 FT first stage, assuming they added a proportionally small amount to the weight. I used the specifications here:

      Falcon 9 FT (Falcon 9 v1.2).
      http://spaceflight101.com/spacerockets/falcon-9-ft

      The dimensions given there are listed as 42.6 meters long and 3.66 meters in diameter, at a dry mass of 22,200 kg.

        Regarding the stage horizontally, you would have to put the swing points along the sides, rather than at the top, so that the clamshell wing on each side could open without blocking the opening of the clamshell wing on the other side. This means the wing area would be half that of the full surface area. So the surface area is (1/2)*Pi*3.66*42.6 = 244.9 m^2, 244.9*3.28^2 = 2634.86 ft^2.

      The dry mass is 22,200 kg, 22,200*2.2 = 48,840 lbs. So the wing loading is 48,840/ 2634.86 = 18.5 pounds per square foot(psf). This is not 10 psf, but it is significantly better than the shuttle, and with the reduction in descent due to the curved surface this might still be enough to require minimal thermal shielding.

      Also, we might be able to get additional wing area by putting clamshell wings on the upper surface, though not the same size as the lower ones so that all can open fully. This would essentially be a hypersonic biplane. It is known biplanes increase left at subsonic speeds. Recent research shows this also happens at hypersonic speeds

      The hypersonic I Plane has a unique biplane configuration to increase its payload and reduce drag. China Science Press


       For attitude control we allow the swing points to be moved up or down.

      For making upper stages reusable.

       SpaceX has wanted to make the Falcon 9 upper stage reusable but has been unable to do so. This method can allow the upper stage to be reusable as well as providing a simpler approach to recovering the fairing.

       The upper stage dry mass of the Falcon 9 is about 4 tons. Then a 5% wing mass using the clamshell approach would only be an additional 200 kg, a small reduction in the payload mass. Note also the high wing area afforded by a clamshell wing approach would reduce the reentry heating as well.

       For the fairing we could allow the fairing itself to open up to form the clamshell wings. Unlike the case for the propellant tanks, you don’t have the need for the high degree precision and accuracy for closing up the wings for reuse for just the fairing.

       Another possibility might not detach the fairing at all. The fairing would be carried to orbit along with the upper stage. It would open up like a clamshell to release the payload. Then it would remain in the open position for fly back to the launch site, serving as the wings for the upper stage as well. This has the advantage of not having to recover and reintegrate the fairing and upper stage separately, but more importantly it's a simpler task than attaching variable clamshell wings to propellant tanks without damaging the structural integrity of the tanks. At a mass of the fairing at about 2 tons for the Falcon 9, this would subtract about 300 kg from the payload instead of just 200 kg for the case where the fairing was detached and clamshell wings applied to the upper stage. Actually, it would be a little better than this since the fairing being jettisoned so high in the flight, it subtracts nearly it’s weight from the payload anyway.

       Fully aerodynamic landing at Mars, aerobraking.

       Aerobraking is the proposal to slow down at Mars aerodynamically only, not using thrusters which would require carrying extra propellant at arrival. For several years hypersonic waveriders have been proposed to accomplish it:

      Hypersonic waveriders for planetary atmospheres
      •  This would be especially important if we want to reduce the travel time to Mars to limit the health effects due to high energy cosmic rays and long exposure to zero gravity, rather than the commonly proposed 6 to 8 months.
          Interestingly the SpaceX Starship upper stage if fully refueled in orbit could achieve a 12 km/s delta-v. This would be sufficient to get a small habitat for a small exploration team to Mars in 35 days.
           Such high departure speeds would result in high arrival speeds at Mars as well, in the range of 20 km/s. I’m proposing clamshell wings emulating caret-shaped waveriders perhaps in biplane format, by approaching at low altitude, “skimming the tree-tops” so to speak, can accomplish aerobraking at Mars to land without propellant burn.
           Further modeling needs to be done to confirm this.
            Robert Clark
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