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The SpaceX Grasshopper vehicle (formally a VTVL vehicle) tests have been increasing the height that their tests can climb to before doing the vertical landing. Throughout all these tests it keeps upright.

But in practice, operating as a rocket stage, this vehicle will fall from a substantial height. Before it gets close to the ground, would it keep vertical as it falls, or would it tumble, or fall oriented horizontally?

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  • $\begingroup$ Is this even answerable yet? I.e. SpaceX has not directly said. Elon has commented that they do not intend to do a swoop of death or K turn, but that is not very official. $\endgroup$ – geoffc Jul 23 '13 at 14:41
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As the concept video originally released shows, the Grasshopper System will be able to stabilize itself, and land vertically. No doubt there will be some loss of complete control in places, but in the end, the spacecraft must be controlled, and will land vertically. See the YouTube video below:

This makes sense, for a lot of reasons. The primary reason is that the rocket is more stable in a vertical configuration than a horizontal, allowing for more accurate control. No doubt some re-orientation will occur, but as a whole, the system will work better if operating in a vertical configuration.

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    $\begingroup$ I look forward to seeing that work as well as it works in that video. Will be the dawn of a new era. Also, I now feel compelled to simulate it in KSP... $\endgroup$ – Erik Jul 23 '13 at 16:58
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    $\begingroup$ @Erik: KSP's aerodynamics modeling isn't all that great yet. Someday it will be, but it might take a while... $\endgroup$ – PearsonArtPhoto Jul 23 '13 at 17:05
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    $\begingroup$ Enter Ferram Aerospace. $\endgroup$ – Vedant Chandra Mar 1 '14 at 13:19
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This is not meant to be cheeky, but if I understand your question correctly, then the answer seems to be contained in the name of the vehicle.

VTVL: Vertical Takeoff Vertical Landing.

I think that even if the Grasshopper had no active flight controls, a large gyroscope would suffice to keep it vertical throughout flight, but the article makes it clear that it does have active flight controls. The Wikipedia article mentioned that:

  • "...a third flight in December 2012 of 29 seconds duration, with extended hover under rocket engine power, in which it ascended to an altitude of 40 metres (130 ft) before descending under rocket power to come to a successful vertical landing."
  • CEO Musk said in November 2012: "Over the next few months, we’ll gradually increase the altitude and speed. ... I do think there probably will be some craters along the way; we’ll be very lucky if there are no craters. Vertical landing is an extremely important breakthrough..."
  • During a Phase 1 test, the Grasshopper RLV would be launched and ascend to 240 feet AGL and then throttle down in order to descend, landing back on the pad approximately 45 seconds after liftoff.

So in short, I think the answer to your question is: "vertical."

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    $\begingroup$ I do not have a source, but Musk was asked on Twitter if they would rotate (I.e. deorbit as nose down for heat shield, then rotate to engines down for propulsive landing) and he commented that it would remain the same orientation all the way down. Which is quite interesting as well. Wonder if they will run low thrust through an engine to act as a buffer for heat control? $\endgroup$ – geoffc Jul 23 '13 at 15:41
  • $\begingroup$ You misunderstand the question--of course it lands vertically, but what orientation does it use to get to the landing spot? It could be that it travels horizontally, then uses its propulsion systems to reorient itself for its vertical landing. $\endgroup$ – Gwen Jul 23 '13 at 16:19
  • $\begingroup$ What did you mean by "large gyroscope"? A stabilizing gyroscope would be an incredibly inefficient way of maintaining the attitude of a launch vehicle, which is massive and has a massive moment of inertia, and which is subject to significant aerodynamic forces. If you mean a gyro for the purposes of attitude knowledge then it would have to be massive, plus all rockets have attitude determination systems anyway for powered flight, so landing would likely impact this little, if at all. $\endgroup$ – Adam Wuerl Jul 28 '13 at 15:49
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The full flight profile of a VTVL Grasshopper-like vehicle hasn't been shared by SpaceX, but it certainly won't "stay vertical" for the entire flight — assuming that vertical is defined as 90° flight path angle.

Ground launch rockets typically start by flying more or less straight up. Because they are at low angle of attack this keeps lateral aerodynamic forces manageable and it gets the rocket out of the thick lower atmosphere as quickly as possible. But as soon as possible (exactly where depends on all sorts of parameters but above 100,000 ft would be an order of magnitude altitude) they begin what's called a gravity turn. The trajectory begins to turn over and the flight path angle decreases.

So it is likely that the vehicle will have started to turn over by staging and thus will no longer be vertical.

Assuming the vehicle falls ballistically before restarting the main engines for landing, there are really only two high-level options:

  1. Actively control attitude: use an attitude control system to control how the vehicle is pointing
  2. Let the vehicle attitude float (i.e. not controlled).

Which is best to choose would be a trade buried in the details of the rocket and trajectory design, and which we'll only know the answer to when SpaceX decides to share it.

Either way, the vehicle will have to be properly re-oriented for engine ignition before it got too deep into the atmosphere. Depending on the aerodynamic stability of the vehicle this could be done in part passively or with (or augment with) active control. Because it's only a first stage it will only be going several Mach, and thus probably won't need a thermal protection system as heavy, complex, or expensive as what was used for Shuttle or even the Dragon or Apollo re-entry vehicles.

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The booster is probably stable without controlled thrust once the landing legs are deployed, the spaceX description says the engine(s) will be fired 3 times after separation.

The booster will only turn around backwards to direction of flight during the decel and landing phases. It will use aerodynamic drag to reduce the majority of its speed and not use extra fuel.

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    $\begingroup$ Could you please edit to include some links to the information you're alluding to in your answer? I'd edit this in myself, but am not sure which SpaceX description you meant. Cheers! $\endgroup$ – TildalWave Jan 6 '15 at 23:03

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