Landing a spaceship

Question

I was trying to think of a way to land a spaceship. The best I could think of was a ridiculously long airstrip for landing, and lots of catapults for takeoff, but then there was the problem of having land good enough to support the weight of a spaceship (i.e. it'd be better if you didn't have to rebuild your airstrip every time something lands), plus the landing gear would have to hold the shock.

Then I had a bit of literal shower brilliance, and it makes sense because it involves water. What if there where no landing gear? What if there weren't even land? What if instead of landing, it was a splashdown?

Assume a fairly standard sci-fi ship about the size and weight of a modern destroyer. We're also assuming it can sustain atmosphere reentry (otherwise, what's the point of landing, am I right?). As far as I'm concerned, for atmospheric flight, all it needs to do is be able to fly about straight-ish until it reaches either orbit or the ocean. The kind of planets we want to land on are Earth-type, all of them, no exception.

Here are the questions:

• Is this really a more sensible way to land a spaceship than on land?
• What systems/design features would it need to be able to land, float on seawater, and takeoff?

Answer 1

First of all: Landing in an ocean has its benefits. The biggest one is clearly, that water is 'soft' in contrast to solid land (by being less dense and quite fluid in contrast to rigid crystals of rock), giving way quite easily on a hard impact and thus allowing a higher landing speeds without ripping the ship apart. However, decellerating to a 'reasonable' speed of maybe 30 to 40 feet per second like the Apollo is still very much advised. The latest land-landing Soyuz TMA only makes a touchdown at less than 5 feet per second. Being allowed a splashdown velocity of factor 6 to 8 larger than for a ground impact will seriously help in emergency landings (and is already doing so for airplanes).

Also, sealing the ship airtight to sustain space means, you made it watertight the same moment. Just keeping the overall density below 1 ton/m³ (density of water) by having enough hollow space in walkways and labs and the whole thing will float.

As a bonus, the ocean could cool the heated hull of the ship after reentry, especially if the atmosphere is very dense or the reentry very fast and thus aerobreaking heats up the ship quite much in the lower atmosphere.

However, landing is easy, as a ship just has to make sure of 3 things: not burn up in the atmosphere, not get crushed on impact and don't break your cargo on landing. Placing the landing in the ocean serves 2 of those targets, as shown above.

So, we landed. Landing obviously comes at a cost: you go deep into the gravity well of the planet, so you have to overcome it again to get away again. To do so, you need to go to escape velocity, which is:

$v_e=\sqrt{\frac{2GM}{r}}$

In this $G = 6.67×10^{−11} \frac{m^2}{kg \times s^2}$, M the planetary mass, r the position the ship rests at, so normal nill, which is usually the water level. Now, accelerating from rest to that speed is usually fatal (in case of earth: 11.2 km/s!), but one can use a trick: accalerate over time and go up on the way, and as you go up, reduce also the needed escape velocity. Just make sure to accelerate enough over time. This is what rockets do. Now, we parked our ship in the ocean — where do we get fuel from to reaccelerate up and away?!

Luckily, the basic answer is pathetically easy: from the ocean itself! The most simple rocket fuel is $2H_2+O_2=2H_2O$, which is a highly exothermic reaction. To get to the needed Oxygen and Hydrogen, one can simply crack the water, for example with a battery or by applying the current from solar panels or the ship's reactor. With a bit work on the hydrogen, it can be refined to even better storeable fuels, such as Hydrazine ($N_2H_4$).

However, we still should get to at least a shallow spot to launch our spaceship: our engines may not be submerged to burn our fuel and the acceleration of the initial blastoff is much more effective if the exhaust gases get propelled downwards and it is pretty hard to keep the exhausts facing downwards and out of the water while floating.

To launch from shallow water, having launch-legs would be a good feature, errecting the ship to launch position and retracting in flight. To launch floating, retractable legs with pontons/floaters at the end that do the same and get the engines over the sea surface would be needed.

Answer 2

I say landing and taking of from planets will cost a spaceship a massive amount of energy. Gravity is strong and the ship is likely not designed aerodynamically.

Even if you were to fly around in an aerodynamic spaceship you have different atmospheric pressure, composition and plain old gravity on every planet you visit.

Sure it's sci-fi, even something the size of a Carrier can land and take off if you want it to in your world. But does it have to?

I say unless you really want to have big ships on the ground for some reason just have them orbit the planet. The ship can dock with space stations and use shuttles/elevators/whatever to ferry goods and crew.

Answer 3

All answers missing very crucial point: there is a fundamental difference in approach to building exoatmospheric vessels compared to endoatmospheric.

Spaceships need to have reinforced hulls to prevent EX-plosion, while air- and sea- borne vessels are reinforced to prevent IM-plosion. One atmosphere of atmospheric pressure seems like not much, but in space it's actually a tremendous force just on it's own. Those two goals can be jointly pursued in construction, but that means excess mass. Space shuttle is not big because why?

There is also a world of difference in power requirements to move spaceship in space and to move it up (and down) gravity well. I know mass is mass, but for the purposes of the question it's suffice to say there is a difference. So that means high power requirements, which also means size of the PGU (Power Generating Unit), which cuts into capacity of a spaceship's hull, as is extra reinforcements and quite possibly some aerodynamic requirements.

Space combat vessel needs to be armored in some way. If you elect atmospheric braking it means that outer layer - i.e. armor - is exposed to extreme conditions not unlike combat (depending on weapons used in warfare, of course), thus weakening it enormously. Armor in spaceships has dual-purpose: prevent/absorb combat damage and shield from all sorts of radiation encountered in space. Current space tech relies on E-M shield of Earth to work in space, but spacegoing vessels have to have this sort of protection. Weakening that every time you enter atmosphere sort of defeats the purpose.

Thus we get to a bottom line: I recommend dropping the idea unless you give your civ enormous power source(s) (or other techs like force fields reinforcing hull or reactionless drive of some sort) to accommodate for all the requirements of the concept.

Answer 4

Why do you want to land?

First question you should ask is: "Do I really want to land this big thing?!". Why do the characters of your story want to do this? What is the benefit they are seeking with it? If there is no tangible benefit of landing, then do not land!

And even if there is, this benefit had better be pretty darned good because getting up into orbit again is a costly and difficult affair, especially with a monstrosity like that. What is it that landing this ship accomplishes that cannot be fixed with landers / shuttles?

And if you are determined to land this ship, well then landing is not a difficult thing, because the power you have getting up into orbit is much more than is needed to make a soft landing in water. Aerobraking will deal with most of the energy bleed for you. After that it is just a matter of setting down gently at sea.

However, as a courtesy do try to stay away from populated areas when aerobraking because large objects moving hypersonicly through the atmosphere tend to leave a lasting impression...

Answer 5

If your spaceships have the thrust and fuel capacity to reach orbit (with or without some help of a catapult), they could refuel in space and then use their thrust to slow down to a soft, powered landing. Theoretically, you could land the sci-fi way on a fixed landing pad, but water would be a lot more forgiving if you don't get your attitude or final velocity just right.

If your ships are launched from a catapult without adding significant thrust of their own, they might as well land them on the big equatorial planet-encircling rail track you built to launch destroyer-sized ships into orbit. Just aim the ship there, have a big maglev train get into position underneath it and catch the ship in the same magnetic fields you use to launch it.

Answer 6

Any spacecraft that is the size and mass of a modern destroyer and expects to be landing on Earth-type planets on a regular basis, must have access to massive amounts of power. Getting to Earth-type planets on this scale would require a highly developed form of long-range interstellar travel.

Spaceships with more than sufficient power could simply descend and land vertically to touchdown on a planet's surface. Of course, placing a large mass on unconsolidated ground has its problems. It's like a building without foundations and is sure to fall over.

However, spaceships with more than sufficient power might descend vertically and instead of actually touchdown it might simply hover just close enough to the ground for easy access to the surface. This could be done by ladders or extensible elevators or short-range landing craft, say, helicopters or even jet-packs.

The sensible and logical way to land on an Earth-type planet is to actually not touchdown. Park your spaceship in orbit around the planet and use spaceplanes to travel back and forth to the planet's surface. This also has the advantage of having your spaceship pass over every part of the planet and survey it from orbit. It's safer too (well, from direct attack by planetary surface based hostile forces, but not from disgruntled indigents armed with missiles).

The sight of a huge spaceship the size of a destroyer hovering above the surface of an alien but Earth-type planet with landing craft and explorers skimming through the air with their jet-packs would be a magnificent. Why bother touching down anywhere else when the sheer theatrically beats marine or land surface or giant airstrip touchdowns.