5
$\begingroup$

SpaceX's Starship uses a unique flight control scheme during descent that I have not seen anywhere except with skydivers: it falls straight down belly-first using four aerodynamic control surfaces at the nose and the bottom to induce drag and thus provide pitch, roll, and yaw control, somewhat like a skydiver uses their four limbs.

Starship SN9 Flap Test Time Lapse [Time lapse by @asy on Twitter, original source footage by LabPadre.]

This is a pre-flight actuation test of the aerodynamic control surfaces. The Starship is in its launch attitude, i.e. tail-down / engines down. On descent, it would be falling belly-down, where the belly is the side facing away from the camera, i.e. if the vehicle were falling right now, we would be looking at it from the top.

These aerodynamic control surfaces have been called lots of names by the spaceflight community: fins, flaps, wings, flings, wing-dings, flippety-flappety-bits (yes, really). Tim Dodd, the Everyday Astronaut, sometimes jokingly calls them Elonerons. SpaceX themselves only name them once on the Starship website, where they use the term flaps. Elon Musk uses both flaps and body flaps in tweets.

They are, however, different from many of the concepts mentioned above, in that they act perpendicular to the airflow. The closest analogy I can think of, other than a skydiver's limbs, is how the Northrop Grumman B-2 Spirit uses differential speed brakes for yaw control instead of a rudder.

Given that we have so many names for so many different types of control surfaces (e.g. flap, slat, spoiler, aileron, elevator, rudder, speed brake, horizontal and vertical stabilizer) as well as combinations of them (spoileron, flaperon, stabilator (all-moving tail), "ruddervator" (V-tail)), etc., there must be some term we can apply to these?

What I am looking for is a term that concisely conveys what these control surfaces do, in much the same way as everybody knows what an "aileron" or an "elevator" does. Or, alternatively, an answer could be that such a term does not exist.

Improve this question
$\endgroup$
1
  • 1
    $\begingroup$ I like "attitude control variable drag brakes". $\endgroup$
    – John K
    Dec 23, 2020 at 20:21

2 Answers 2

Reset to default
3
$\begingroup$

As far as I know, the only other example of this flight profile in aviation is post-stall maneuvering, especially for flat spin recovery. For planes, control in these flight regimes is mainly accomplished by thrust vectoring and canard/stabilator actuation. The Starship flaps don’t match these.

Another close sibling of this flight profile is the Shuttle reentry. The Shuttle used a body flap as well as conventional Delta-wing control surfaces in combination with RCS to maintain a specific, high AoA during hypersonic reentry, then transitioned to gliding for approach and landing. The Shuttle body flap (under the engines) has the closest resemblance to the Starship flaps out of any space vehicle I know. It has a much smaller actuation range, though.

Of course, the control surface naming depends on how you define the vehicle frame; with thrust direction-forward, the flaps are variable-dihedral. With velocity direction-forward, the flaps are variable-AoA with a wide actuation range.

The noun I would go with would be variable-drag flaps. Spoilers aren’t correct; there isn’t another airfoil to spoil. Airbrakes usually don’t contribute to attitude control.

Improve this answer
$\endgroup$
1
  • 2
    $\begingroup$ The Space Shuttle body flap also came to my mind. The hypersonic entry profile is indeed very similar between Space Shuttle and Starship, using the "broad" side of the vehicle to maximize drag, but of course what happens after most of the velocity is bled off, is the major difference. (Although I believe SpaceX did test cross range capability using the body and flaps for lift during the SN8 test flight. They are "gliding" the F9 after all.) The Shuttle would keep a horizontal velocity component and glide, whereas the Starship cancels the horizontal component completely and falls straight down. $\endgroup$
    – Jörg W Mittag
    Dec 23, 2020 at 9:48
2
$\begingroup$

Elonerons is perfect, although birds beat him to it years ago.

Birds "tuck" a wing to slip/fall to one side while the opposite wing is extended. They can also vary the geometry of their tails (opening and closing the fan) for pitch control.

Elon's 4 finned craft simply does the tuck for pitch and roll, or combinations of both.

The Starship in "belly flop" bore a striking resemblance to an incoming shuttle, and it is not hard to imagine later iterations with a central wing. This would not be unlike the "jib, main, and jigger" rigs of sailboats.

But, for now, it "flops" its way home, using drag. Hopefully, next landing approach may be stabilized enough for a softer touchdown. Go SN9!

Improve this answer
$\endgroup$
6
  • 2
    $\begingroup$ Stability wasn't the problem -- Elon has tweeted that it was lack of pressure in the header tanks, causing loss of thrust at literally the last second. $\endgroup$
    – Zeiss Ikon
    Dec 22, 2020 at 18:38
  • $\begingroup$ @Zeiss Ikon The SN8 prototype will be basal to many possible iterations. An earthly point to point design may have wings, and indeed a recoverable 1st (rocket) stage. This design would closely resemble the Nike Hercules. I have my doubts the "flop/flip" will be comfortable for passengers, but it is a start. $\endgroup$
    – Robert DiGiovanni
    Dec 23, 2020 at 2:30
  • $\begingroup$ The belly flop is actually likely to be more comfortable than other vertical landing options -- G forces are either through the floor, through the seat backs (assuming nose-facing seats as in an airliner), or somewhere in between. No hanging from the straps. And the high drag configuration allows gradual deceleration, high up, so no high G slow-down in the lower atmosphere. $\endgroup$
    – Zeiss Ikon
    Dec 23, 2020 at 12:06
  • $\begingroup$ "Gradual deceleration from higher up": Earth = wings, Mars = large parachute, Moon = bigger header tanks $\endgroup$
    – Robert DiGiovanni
    Dec 23, 2020 at 12:17
  • 1
    $\begingroup$ Doesn't have to be wings. Any large, low density object will decelerate from air drag, and the more cross section area for its mass, the higher (=thinner air) this can effectively be managed. Wings are a mass penalty, carrying heavy structure that's only used for a tiny fraction of the flight. If you can land without them, you can save that mass -- and every ounce of dry mass you can save is an ounce more payload you can carry with the same engines and fuel. $\endgroup$
    – Zeiss Ikon
    Dec 23, 2020 at 12:26

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .