Why not land SpaceX's Starship like a plane?

Question

I get the reasoning behind designing Starship to land upright. It doesn't require landing gear and the turnaround is expected to be much faster. But I'm beginning to question that logic.

I think the cost/benefit of an upright landing is acceptable for unmanned vehicles like the booster. Most of the time it will land just fine, and if it craters once in a while, that's ok. But for a manned vehicle, it's far too risky. If those engines don't light in time, you and your 100+ fellow passengers are dead.

Is the turn-around time that much better? The main difference is that it's horizontal, right? Surely SpaceX can find a quick and easy way to get Starship vertical and in position for the next launch.

Is it really saving any weight? How much more or less does landing gear weigh compared to header tanks? Could you remove an engine or two if you don't need any to land with? How much could you save in fuel weight?

Answer 1

You're basically describing the Space Shuttle.

The Space Shuttle wasn't even a good solution when it was designed. It had precisely one goal - to look like a plane for the image of the Air Force. As far as engineering goes, the Big Dumb Booster was already well proven, and is what every other solution to space has used. But in order to get Air Force funding, it had to look like a plane. The justification at the time was reuseability - but even at the time, the level of reuse and cost of refurbishment made this questionable, and it was never a problem which actually could be solved because the design simply didn't make it possible.

Your first problem is reentry. Wings are really bad for that, because they're exactly the kind of shape you don't want, especially around the wing root. The leading edge of the wing is also a very vulnerable point. Wing-shaped heat shields basically aren't a good thing, and Columbia demonstrated what happens when you get unlucky. How many Columbias can your winged spaceship program tolerate?

Your second problem is landing it. The Space Shuttle was actually a glider - and as every glider pilot can tell you, landing is stressful. If the pilot of a powered aircraft gets approach wrong, they can push the engines up and go round again. If a glider pilot gets approach wrong, something is going to get broken, and that something is often the occupant(s). There are no second chances. So instead of a highly-automated system of engines and sophisticated control systems, you've replaced this with a pilot who can and will screw up, with no backups or any way to recover the situation. Sweet.

Ah, you'll say, but gliders don't crash that often. Firstly, you're probably not going to hear about it on the news unless someone dies. And secondly, gliders have a glide ratio of around 30:1, are ridiculously manouevrable, and land at about 40mph (fast but not too much) or slower if they can land into a headwind. The Shuttle had a glide ratio of 4.5:1, was notoriously hard to control (because a wing which forms even a partly-acceptable heatshield is not a good wing to fly), and landed at over 200mph. For reference, a glide ratio of 4.5:1 is substantially worse than any hang-glider - in fact it's about the same glide ratio as a skydiving parachute - and that landing speed is faster than an F-16 touches down. This is not a happy place to be, and it's a testament to the insane skills of the pilots (and a healthy dose of luck!) that none of them were lost on landing.

Thirdly, you need somewhere to actually land the damn thing. The big bonus of landing vertically is that you only need a flat patch of ground the size of your landing pads. The Shuttle needed a 3 mile runway to land on. That's 3 miles of perfectly flat ground, with the Shuttle initially rolling at 200mph when it touches down. Good luck with that on Mars.

And fourthly, you need an atmosphere. Earth has a thick enough atmosphere that wings work fairly well. Mars's atmosphere is a whole lot thinner, and designs for aircraft on Mars (there's a helicopter due to land next week!) need to pay serious attention to this. Glide ratios and speeds would be correspondingly worse - as if they weren't already bad enough for the Shuttle on Earth. And the Moon of course has no atmosphere at all.

If you really, really want a wing, then you can go back to Rogallo's work instead. Re-entry uses a normal heatshield, atmospheric braking with drogue chutes gets the speed down, and then a flexible wing is deployed. Since the invention of the ram-air parafoil, it's probably more practical to use that instead though. It may not look as cool, but you can still fly it as normal - it is perfectly practical, fairly robust, and easy to control. It even lands slowly. On the downside though, you have an extra thing to deploy, and any skydiver can tell you that chute deployments do fail, even before you add the extra ways that something can fail when you need to deploy it mechanically from a space craft. SpaceX did consider this, but their assessment was that firing the rockets (which after all are known to work, because they got you off the ground in the first place) is more robust than adding something else which can go wrong.

TLDR: It's not because of the weight of the landing gear at all!

Answer 2

There is not enough air on Mars. You would need absolutely humongous wings. There is no air at all on the Moon.

Surely SpaceX can find a quick and easy way to get Starship vertical and in position for the next launch.

Starship is not structurally capable of being in a horizontal position. It will simply crumple and/or break in half.

Could you remove an engine or two if you don't need any to land with?

Considering you need only one engine to land but 6 to launch, obviously not.

Answer 3

In addition to the other very valid answers, at the moment we see Starship flying alone and can easily visualize adding wings, but in orbital mode it will be stacked atop the superheavy booster. Adding large wings to the top of a rocket makes it very unstable, akin to making a dart fly backwards source, and would need to be stacked at an odd angle to zeroise the wing lift during ascent.

Space shuttle and Buran solved these problems by stacking the winged element on the side of the rocket to keep center of lift and drag close to center of mass. The X-37B launches inside a fairing. Both of these solutions would have substantial side effects on the intended Starship design if incorporated.

It would theoretically be possible to use a very fast acting control system to manage the dynamic instability, but this starts to look very similar to the problems being solved to do a powered landing in any case.

Answer 4

Wings are heavy. Surprisingly so.

As Jorg pointed out there is no air on the Moon, and Mars's atmosphere is pretty thin.

In fact, Starships payload to the Moon is surprisingly low, because it has to carry all the fuel to land entirely propulsively.

Cheating by using air resistance is important.

Answer 5

On Earth, you need only build a suitable runway. Have a look at the Fly Back Booster concept from the 1990s. Gliding can lower rate of descent even better than parachuting, and at high landing speeds not a lot of wing is needed.

One can only imagine a time traveler going back to the 1970s and meeting von Braun.

We might have some idea what the NASA SLS would look like today.

But one may also expect SpaceX to forge ahead with their groundbreaking work, and, in an age of advanced computers reacting in milliseconds, the "HoverSlam" may yet reach an acceptable safety level for passenger transport.

Answer 6

A better question to ask might be: why do planes not land vertically, and instead require a long runway? The answer is that the engine (or more precisely the propeller or fan stage it drives) lacks sufficient thrust to overcome gravity, and therefore the plane relies on air passing at speed under its (large) wings to maintain lift.

Rockets have enormous thrust, so lack of engine thrust absolutely isn't a problem. The nine engines of Falcon 9 have sufficient thrust to lift the rocket with a full load of propellant off the launch pad and accelerate it up to thousands of km per second. When it lands, it uses only a single engine, and even when this is throttled right down, it produces so much thrust that the booster is unable to hover. Instead the engine must be controlled carefully to ensure it decelerates to 0 m/s just as it reaches the ground - and then the engine is shut down at just the right moment. I understand the engines on Starship will be able to throttle down low enough to be able to hover (indeed this was demonstrated already with the initial Starhopper prototype.)

Using the rocket equation we can work out the weight penalty for a powered landing. Rearranging the formula as shown below we get the following

$dv$ = required change in velocity (assume "wing" flaps slow it to 50m/s fall speed, 180km/h, 113mph)

$v_e$ = exhaust velocity (3200m/s according to Raptor article on Wikipedia)

$\frac{\text{m}_o}{\text{m}_f}$ = mass ratio before and after burn

$\frac{\text{m}_o}{\text{m}_f} = \text{e}^{dv/v_e} = \text{e}^{50/3200} = 1.016$

That means that if the flaps slow Starship to a fall rate of 50 m/s, the weight penalty for landing propellant to reduce velocity from 50 m/s to 0 m/s is 1.6% of the total dry craft weight.

Making the flaps big enough to act as proper wings to enable a safe landing would be a massively higher weight penalty.

It should be remembered that the header tanks are larger than required just for landing, as they will also contain propellant for re-entry burns, which require higher $dv$ (change in velocity.)

Answer 7

This is not a happy place to be, and it's a testament to the insane skills of the pilots (and a healthy dose of luck!) that none of them were lost on landing.

The fact there were no incidents on landing the Shuttle is a tribute to the genius of the design engineers and the skill and training of the pilots.

Luck had nothing to do with it.

Answer 8

But for a manned vehicle, it's far too risky. If those engines don't light in time, you and your 100+ fellow passengers are dead.

There are plenty of ways to kill 100+ passengers dead in a horizontal, airplane-style landing as well. The right answer is to make the time and effort to make the system safe and reliable, which will happen after they dial in the final design. The development flights happening in Boca Chica right now are just figuring out how to make the system work - once they've figured that much out, they can spend time refining and bulletproofing.

We've done the Frankenrocket with the Shuttle, and we've learned some painful lessons from it. We don't need to learn them again.