Tuesday, November 20, 2018

Horizontal landing for the BFR on Earth. UPDATED, 12/15/2018

Copyright 2018 Robert Clark

 In a recent discussion of the BFR, Elon Musk has discussed giving the BFS upper/landing stage extended-size fins, to the extent they resembled wings, to help with the landing on Mars:

Elon Musk reveals updated design for future SpaceX Mars rocket.
By Loren Grush@lorengrush  Sep 17, 2018, 9:38pm EDT

 In fact in this video Musk alternately called them wings and fins:

 Primarily these would be for control during reentry on Mars. The final landing would be through a propulsive, vertical landing.

 The idea has been that full wings would be too heavy, as for the Space Shuttle for example. However the X-37 provides an example that shows a reentry stage can use short, stubby, and therefore lightweight wings, to do a full horizontal landing on Earth:

 Boeing has not given the breakdown of the masses of the structural components of the X-37. But the Skylon gives another example of an orbital stage doing a fully, horizontal landing with short stubby wings:

 The designers of the Skylon have given the weight of the carbon fiber wings as only 2% of the gross takeoff weight. Because the Skylon does a horizontal takeoff, the wings have to support the full gross takeoff weight. But for the BFS assuming it flies a non-lifting trajectory to orbit, the wings would only have to support the dry(empty) weight for when it returns from orbit. This would mean a much smaller weight for the wings.

 For example, if we applied this to the Falcon 9 upper stage, the gross takeoff mass is about 100 metric tons, but the dry weight only about 4 metric tons. The 2% of the dry mass would only be about 80 kg for the wings.

 It may even be possible to get even smaller weight for the wings. A remarkable new development is lightweight but strong materials is the isotruss:

 From the graph it is nearly twice as good as carbon fiber in bending strength on a per weight basis. So a Skylon-type wing might only have to be 1% of the landed weight or only 40 kg for the Falcon 9 upper stage.

Such small-size wings still would not provide a fully horizontal landing in the thin atmosphere of Mars, but they would cut down the amount of propellant needed for the propulsive landing.

  Bob Clark

UPDATE, 12/15/2018:

 The latest update on the SpaceX BFR is they intend to use metals for the tanks and other structures:

 Then SpaceX succeeding in this could give impetus to resurrecting the X-33/Venturestar with high strength metal structures rather than carbon composite.

 Additionally, I suggested above using the isotruss for its bending strength for the structures such as wings. It is notable that the isotruss obtains its strength from its unique geometry since while it is composed of carbon composite it is twice as strong as a standard carbon composite tube.

Isotruss Tower
280' tower installed in Spanish Fork, Utah

 This suggests that a metal isotruss also could be twice as strong as a standard metal tube, and should therefore be even stronger when made of the specialty high strength metals SpaceX is considering.

 Another intriguing possibility is suggested by this. A recent development is scifer steel wire:


 Like carbon fiber this is only available in wire form, not sheets. However, it has from two to three times greater strength than the high strength steels available in sheet form. So the scifer wire could still be used to form a steel isotruss, possibly doubling its bending strength.


  1. The X37, and other similar vehicles get a lot of lift from the body. Consider the weight of the X23/X24/Prime vehicles. No wings at all and horizontal landing.

    And don't forget, 250 knots over the numbers is not a soft field landing ! Don't get me started on density altitude at the equivalent Earth altitude of 100,000 feet.

    1. It would be great to have a lifting body as a reusable orbital vehicle since you would not need extra weight for wings. However, a key problem is non-cylindrical shapes really suffer in weight efficiency for the propellant tanks. Recall this is what doomed the X-33/VentureStar SSTO.

      I did speculate on some possible ways to solve this for the X-33 though:

      DARPA's Spaceplane: an X-33 version, Page 3.

      Agreed the wings would not be enough for a fully horizontal landing on Mars. But a very key point is eliminating the propellant that must be kept on reserve and the resulting lost payload for a reusable first and second stage on Earth with the vertical landing approach. Likely, even on Mars the required propellant that needed to be kept on reserve would be reduced, if not eliminated.

      I don't see where you're getting the 250 knots number. Were the X-37 or the supposed to land at this speed?

      Bob Clark

    2. I fully agree. I had read your comments on the tankage previously (done the propane tanks myself with hoseclamps) and have considered the M2 shaped lifting body (half cone, Six Million Dollar Man crash footage) as a possible starting point. The bi-conic shapes offer some cylindrical options, but not good enough for landing without deployable structures.
      I suppose we shouldn't really fear the deployables, Retractable gear are deployables after all.

      The 250 knots is just a thought. 250 kias is the max speed in US airspace below 10,000 ft. 200 kias actually for landing. The X15, with brush main gear, landed at about 240kias with 174 the targeted full stall flair final touchdown speed.

      Mars Needs an Airstrip ?


  2. As near as I can tell from stuff Musk has said and Spacex has posted, the two near-belly fins plus canards provide pitch control at high AOA for reentry, and rotate to a triangular 3-leg landing configuration during the retropropulsive touchdown. The entry sequence for Mars is unlike the entry sequence for Earth, because Mars's atmosphere looks like Earth's atmosphere above 105 kft. Withstanding broadside air loads during Earth reentry will be their biggest design headache. -- GW