SpaceX lands their booster rocket vertically, this seems complicated. The space shuttle landed with wings which seems more simple. Why is it preferable for the rocket to land vertically as opposed to with wings?
Why is it preferable for SpaceX to land their booster vertically rather than fly it down with wings?
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3 Answers
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Wings are heavy. Control surfaces are hard to move at reentry speeds, so need powerful hydraulics. These must be triply redundant. More weight to carry.
Wings are not useful in orbit and are dead mass carried up, and then back down.
The Shuttle could not go around for a second pass, so any goof up during landing and the crew was all dead. Thankfully that never happened, but I seem to remember one Shuttle did land short and was close to a problem. A story of short landing shuttles is interesting.
Whereas vertical landing seems complex, the reality is, the engines and fuel are already there. They are needed on the way up. If you wish to recover them, you need to bring them back, so might as well use them on the way down. The fuel is already in the booster, just use some for return instead of all to orbit. (Reduces payload, yes, but may be worth it, time will tell if it pays off).
SpaceX is nicely lowering risk with incremental testing via Grasshopper, F9R-Dev1, and then real tests on otherwise paid for commercial flights.
Thus the point is, yes vertical landing requires extra fuel, and has payload costs. But so does a set of wings. Wings look simple, but as you get deeper into it, you add more and more subsystems, until it eats up much more payload than you would have expected.
Revisiting this answer almost three years later, they have successfully landed 8 or more stages either on the ASDS barge fleet or on land. Thus it is clear, the proof is in the pudding. (And the pudding is delicious and fun to watch). Mission after mission lands. Reuse has happened. Whether reuse is quick and affordable remains to be seen at this time. I guess I shall have to revisit this in another two years, based on what comes next.
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1$\begingroup$ avweb.com/news/safety/183035-1.html?redirected=1 $\endgroup$– PearsonArtPhoto ♦Aug 3, 2014 at 1:36
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3$\begingroup$ @AnthonyX Almost all the fuel is gone by the time it is time to return. Thus the stage weighs a tiny fraction of launch mass. Then aerodynamic breaking can slow the stage down enough that only reasonable amounts of rocket braking is required. It costs payload, but it may be worth it. Time will tell. $\endgroup$– geoffcAug 3, 2014 at 2:36
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5$\begingroup$ One other thing is that it would introduce significant lateral pressure on the stage body so it would have to be reinforced to withstand that, adding to its weight. At which point, you're really designing a SSTO and not a reusable first stage. And we all know which ones currently fly and which ones don't. $\endgroup$ Aug 3, 2014 at 9:06
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5$\begingroup$ Considering SpaceX's martian ambitions, an additional consideration might be that they can use the same rockets to land (a Dragon) anywhere. Aerodynamic landing through the thin atmosphere and lower gravity of Mars would need a different shape (and be much more challenging) than landing on Earth. Further, they don't need to build huge landing strips, especially on Mars. And vertical precision landing anywhere helps safety during an emergency also on Earth. Same system is used as a launch escape system, which the shuttle lacked. Mars lander, escape system and airstrips are all replaced. $\endgroup$ Aug 3, 2014 at 9:45
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2$\begingroup$ @ChrisR Lots of elements of the vertical landing are novel. Throttleable engines (Which the SSME's, RD-180 to some degree, RS-68A, etc are as well) are not something new. Of course, this would never work with solids, which are mostly fire once, and then burn to completion. Restartable is probably a better point than throttleable. $\endgroup$– geoffcAug 3, 2014 at 15:30
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A vertical landing system, versus a winged one, does several things:
- increases available landing sites (see note 1)
- encourages more agile boosters (which enables better course corrections)
- requires very little additional hardware
- preserves a larger fraction of disposable launch payload (see note 2)
- doesn't preclude a disposable use if payload maximization is essential (See note 3)
Note 1:
The vertical landing system needs about a 100m radius circle to land in. (Due to engine blast, mostly.) It needs that to be a relatively firm surface. Any flat spot with a concrete pad
The winged landing requires at least 20m wide, by at least a kilometer long. It needs to be nearly flat, fairly smooth, and clear of approach obstacles. Further, there is the consideration of approach directions - NASA had a network of some 30 approved runways for different landing approaches.
Note 2:
THe only additional equipment needed for the vertical launch are the landing gear and the additional gimbals for the thrust. All the additional equipment can be radially symmetrical for both mass and atmospheric drag purposes. The primary mass penalty is the combination of the fuel mass for landing fuel and the loss of that same fuel for payload launch.
For winged landing, the additional equipment needed includes the wings, the landing gear, and the control surfaces. The wings inherently are not balanced - the produce a sideways force during a vertical takeoff, and are not readily made multi-axis symmetrical. The fuel penalty is primarily due to the additional mass of the wings - but it's a significant penalty because the the mass of the wings is so profound - counting control surfaces, the wings are often most of the dry mass.
Note 3:
The mass penalty for disposable use in a vertical landing craft is only the landing gear and gimbals.
The mass penalty for disposable use in a winged design still includes the massive wings, and the extra fuel needed to lift them; disposable use gains no benefit.
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1$\begingroup$ Wings also add some drag. I'm not sure how much (compared to the drag of the rocket/payload/fairing that you have in wingless launches anyhow), but it's non-zero. Similarly, I'm not sure how much the drag affects payload capacity vs. the wing's mass' effect on capacity (if you took a winged vehicle and could make the wings either drag-less or mass-less on ascent, which would be more beneficial?), although I suspect the drag is less of a penalty than the mass (partially because it only applies in atmosphere). Still, pushing more air out of the way does cost something. $\endgroup$ Apr 13, 2017 at 6:21
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Rockets are cylindrical similar to airliners, etc. but the structural design is geared to vertical flight (and as you know; now vertical landing). A contemporary answer on Aviation.SE shows the structural design of a horizontal cylinder, in this case a 777 airliner.
Unless orbital vehicles develop from a horizontal takeoff-to-orbit design philosophy, a given vehicle would need to be designed to stand up to both vertical take off and horizontal landing. This adds weight and complexity, breaking some pretty fundamental economic rules of space flight.
