# Would making a last ditch effort to skip save the spaceship?

Imagine a spaceship entering Earth's atmosphere, it has lost control of its engines and is relying on its reverse thrusters to decelerate before crashing into the Pacific ocean.

So in an attempt to save everyone on board from an impending doom, the pilot turned off all the reverse thrusters located on the top of the ship while diverting all powers to the thrusters located at the bottom.

I was wondering would making the spaceship skips on the ocean surface be much safer than dropping straight into the water as gently as possible?

In many sci-fi blockbuster movies spaceship were seen skipping on solid ground and the pilots emerged out with a smile and often suffered minor superficial cuts.

If skipping increased the survivability then all future spaceship pilots are required to perform this manuver before they are licensed to fly.

• When you say 'spaceship' you mean a "solar system dingy" or "interstellar motor speedboat"? Because the different requirements the two need to answer will have an impact on their capabilities to respond to an emergency landing (perchance, do you consider FTL spaceships? Asking for a friend, Solo's the name; Han Solo if you heard of him. Sortava reckless guy, but still managed to stay in one piece; him and his Millennium Falcon contraption too. But you never know when crash-landing may be needed) – Adrian Colomitchi Mar 11 '20 at 8:12
• @AdrianColomitchi: I watched The Last Jedi, no FTL but that scene is awesome! ;D – user6760 Mar 11 '20 at 8:33
• I watched The Last Jedi I didn't. Something happened to my friend? ;p – Adrian Colomitchi Mar 11 '20 at 8:39
• Seriously tho' about the space capabilities of that spaceship, any chance to some clarifications? – Adrian Colomitchi Mar 11 '20 at 8:40
• I'd note that for high speed reentry often the big issue is not smashing into the ground, but not disintegrating in the atmosphere. If your speed is so low that you reach the ground intact, then you have already somehow shed the vast, vast majority of your energy, and any functional "space-strength" engines could be sufficient to land. – Peteris Mar 11 '20 at 20:22

It doesn't really make sense to talk of "forward engines" and "reverse engines" for a spacecraft. Use maneuvering thrusters to orient the ship, main engines to change orbit. As long as it is not in the atmosphere, the craft can reorient to fly backwards, sideways, or upside down. So the question would be more like this:

Can the RCS engines be used to control reentry?

The Reaction Control System in real-world spacecraft is sometimes considered a backup to leave orbit. This would take a longer burn since they are individually and collectively less powerful, and there could be problems if the RCS uses different fuel with a lower Isp.

The heroic piloting would not come at the last second before splashdown, it would come in orbit when the pilot calculates the best deorbit burn using limited engines.

• Lowering perigee is most effective when burning at apogee, so splitting long burns can help fuel efficiency, but there is limited amount o time in radiation belts one can spend safely - that's what needs to be calculated. Otherwise just burn retrograde and pray it'll suffice would be the way to go. – Mołot Mar 11 '20 at 12:15
• @Mołot there's a slight risk with that approach, that you'll dive into thick atmosphere while moving really fast, and heat up too much. – John Dvorak Mar 11 '20 at 14:22
• @JohnDvorak We are talking about "get to the atmo or die anyway", I guess. But yea, that's one more thing to calculate. – Mołot Mar 11 '20 at 14:31

Horizontal velocity only helps if your craft is aerodynamic. Consider skipping stones across a lake. The best skipping stones are flat and wide, like a wing. They're not round like a ball. Flat stones sink a little into the water, maybe a few millimeters, and because of their shape and velocity, the water applies lift to the stone. It picks up vertical speed and flies back into the air.

The same is gonna be true of your spaceship. If it is relatively flat and wide, it might skip like a stone. And that would help a little. But the problem with this, is that if you don't land perfectly level, your tail will hit first. This will torque your craft's nose down, hard. The faster you're going horizontally, the harder your nose will torque down. It could easily be hard enough to destroy the ship. If one side hits first, you tumble, and again you could easily destroy the ship. If the nose hits first, the aerodynamics actually hurt you. The aerodynamic forces will pull the ship down further rather than bouncing you back up.

In summary, if you can land very nearly perfectly level, it will help a little. If you can't land totally level, you die. Seems to me that keeping a high horizontal speed is more trouble than it's worth.

• And that's why we have flying saucers! – Guran Mar 11 '20 at 7:18
• But the problem with this, is that if you don't land perfectly level, your tail will hit first. Oh, come OOONNN! Are you are telling me that a spaceship won't have vectored thrust and the people of the future forgot how to design a PID flight attitude control (when amateurs can do it today at least for wheeled or propelled or even gas thrust vehicles)? – Adrian Colomitchi Mar 11 '20 at 7:45
• "22 October 1962: a Northwest Airlines DC-7C" : "The accident report called the ditching "an outstanding feat," citing several key factors in this water landing's success: pilots' skill, ideal conditions (calm seas, favorable weather, daylight), time to prepare for the ditching and the military passengers' ease with following orders." - Every time you hit the water again, you could easily destroy the ship; why risk it more than once. – Mazura Mar 11 '20 at 23:56
• "The runaway bounced landing (with at least 2 bounces after initial touchdown) is usually not recoverable." – Mazura Mar 11 '20 at 23:56
• @AdrianColomitchi the OP did say " it has lost control of its engines" in the introduction. – Mr Lister Mar 12 '20 at 8:56

Train your space pilots on gliders.

First, spacecraft must slow from orbital velocity. This is done either using the atmosphere, or if the atmosphere is too thin such as Mars or the Moon, they must use engines.

Once that's done, most real spacecraft entrusted with human lives land using parachutes. This is simple and safe.

There are a handful of space planes, the space shuttle and X-37. They have wings and landing gear. They land as unpowered gliders on a long runway.

Parachutes. Landing gear. Wings. Runways. All of these are missing or from your typical sci-fi spacecraft. Sci-fi spacecraft are often bricks lacking even basic wings (think Star Trek shuttlecraft) and land vertically under raw power. Yet in an emergency they mysteriously glide.

Since this is tagged science-based, we'll ignore these magical flying bricks. Real flying bricks which can only land safely under raw power would never be rated safe for humans.

Let's say you have a sensible space plane which is capable of gliding. Its been slowed by the atmosphere, but finds itself still going way too fast. Power is out. What's the pilot to do?!

Deploy their parachutes.

What if there are no parachutes?

It's a glider, so it can trade velocity for altitude. Keeping the nose up retains altitude while shedding velocity. This gives the pilot more time to declare an emergency and reach a runway or highway to land on its landing gear which has wheels.

If no suitable runway is available, they look for a flat, open space. This could be a field or water. In a field they may decide to land with their landing gear down, but in water they will land with wheels up.

They will dump any remaining fuel and other flammable material to reduce the chance of fire.

"Skipping" will not help. Sci-fi spacecraft made of unobtanium make great gouges in the landscape as they bounce which would tear a spacecraft apart. Instead, the pilot will endeavour to land gently with as low a sink rate as possible. They will keep the nose up to avoid it digging into the ground and flipping the craft over.

If all goes well, the craft will gently touch the ground. It might bounce a few times. Eventually it will skid to a halt. Hopefully no fires will start as a result.

• They will dump any remaining fuel and other flammable material to reduce the chance of fire. Righto, because energetic chemical propulsion is fit for a sci-fi spaceship (yeah, I'm only nitpicking here, but still... one would hope their space fighter is not designed by Boeing as a derivative from B373-MAX) – Adrian Colomitchi Mar 11 '20 at 7:29
• @AdrianColomitchi Yes, it's probably not flying on unleaded. Unless it's steampunk. If they have something highly reactive they should dump it. Like the hand grenades stored in their control consoles. – Schwern Mar 11 '20 at 7:34

If you are in the unfortunate situation that you have to lithobrake a spacecraft which wasn't designed for that, then you want to make sure you have as little kinetic energy as possible on impact and that you lose that remaining energy as slowly as possible.

First, you want to minimize the speed with which you hit the ground. This can be achieved by entering the atmosphere in a very shallow angle. That means you spend more time in the atmosphere and lose more kinetic energy to atmospheric friction. You also lose that energy in a much more gentle way, so the risk that your spacecraft gets torn apart during atmospheric reentry is reduced.

The planet does have an atmosphere allowing you to aerobrake a bit, does it? If not, you are even more screwed. But it doesn't change anything regarding wanting a shallow angle.

When you impact the ground (or the surface of the ocean, which behaves pretty similar at such speeds), you really don't want to lose all your acceleration at once in one big splash. It would create an enormous sudden acceleration force on the ship, which would be pretty destructive for the craft and for any of the unlucky lifeforms in it.

"Skipping" means that each "skip" only loses a small amount of kinetic energy. Dividing one large crash into many smaller crashes means that each of these crashes is far more survivable.

Even better if you can somehow get such a shallow approach that you can "slide". "Sliding" means that you lose your kinetic energy relatively slowly though friction. Also, that friction will be concentrated on the surface you are sliding on, and that force will act parallel to it. Most of the impact energy will be used on shaving off the lower hull and the lower decks of your spacecraft. Your upper decks might survive relatively unscathed...

...might...

Good luck, you are going to need it.

Moses maneuver

As your ship nears the surface, they open fire with everything they have. The ocean surface is blasted into steam and droplets as the ship plunges in. Weapons blazing they carve a path through the water, turning the water ahead of them into steam and droplets. The ship is slowed by the steam and droplets and frank flames (superheated water vapor) rushing past them but not slowed with a sudden shock as would be the case for an impact with water. The tunnel through the water that they make collapses behind them. By the time they reach the bottom they are barely moving.

Space captains: despite historical gravitas the Moses maneuver remains strictly theoretical.

• Spacecraft have to be airtight and able to handle a pressure difference of at least 1 atmosphere. They generally have pretty good heat-shields as well. Depending on the exact characteristics of your ship, (especially shield/hull strength vs pressure) It might make a workable submarine. If your weapons are too effective, the impact on the bottom will kill you, but adding more aerobraking time before impact might save the crew, (presuming you can survive the depth pressure you end up at, and can either maneuver the ship back up, or transport the crew to the surface some other way.) – Brian Heward Mar 11 '20 at 19:20
• Note that this would require your weapons to be extremely potent. Even if the ship is small (10 sq. m. surface area) and it's landing slowly (10 m/s) that would require the weapons to have around 25 giga-watts of power output (10m^2 * 10m/s * 1000kg/m^3 * 2.5MJ/kg = 25GW; the 2.5MJ is about what it takes to vaporize a kilo of water at room temperature based on a cursory google search) – SamYonnou Mar 11 '20 at 21:15
• @BrianHeward they're specifically designed to handle exactly 1 atmosphere difference in one direction. In fairness, the strength of the hull material is probably the same, but too much pressure on the outside is likely to start crushing it like a tin can. – Will Crawford Mar 12 '20 at 12:31
• @WillCrawford - Maybe a ship designed to enter atmospheres would be robust to > 1 earth atm. There are planets with very much higher pressures than that. – Willk Mar 12 '20 at 12:37
• @Willk can this be my old friend Jo Leidenfrost?¹ ... ¹ – Will Crawford Mar 12 '20 at 12:38

Short version: skipping turns your crew into paste, simply smashing the water has the best chance of success (not much though).

Just imagine you have a perfect rock to skip across the water. To skip it you have to throw it forwards across the water. Upon skipping it'll decelerate its vertical speed and accelerate upwards again, maintaining its horizontal speed much longer.

Now throw the stone with a lot of vertical velocity as well. You could still skip it but it would require more velocity horizontally as well... which would do nothing to save the crew. In fact it would kill them faster!

If the ship simply smashes the water and sinks deep down it'll decelerate over a longer time than if it smashes the water and skips. Now imagine crashing a space ship at mach something velocities into the water. The ship will take a larger hit if it skips than if it simply drills itself into the water, and that is ignoring the fact that you now need to decelerate a much higher horizontal velocity as well.

Now if you can convert the vertical velocity into horizontal velocity it works, as then you have a much longer time to decelerate the same velocity. I hope you've got wings and the time to change your angle!

I highly doubt that a spaceship would be designed to withstand a touchdown with water and nearly terminal velocity.

Already for airplanes that's not a design consideration/validation.

And skipping on water would be even more harsh on the structure.

If you are extremely lucky the spaceship won't crash on water in the attempt to skip, but I don't think it will be usable after the event without major repairs.

• There is a saying among pilots: "A good landing is one you can walk away from. A great landing is one where you can use the craft again." – Philipp Mar 11 '20 at 15:49

Skipping means all the downward impact energy of splashdown is redirected upward. This means MORE stress on your ship causing more damage than it would have taken if it had broken the surface immediately. However it still might be a good idea to add some horizontal speed if you can use it to have more air push your ship up before the impact with water or land, (IE if you have wings to gradually convert downward to horizontal force, or you can add enough horizontal speed that the curvature of the planet drops the ground away from you.)

Another reason to try to skip your ship could be that it's effectively invulnerable, and has inertial dampeners so the extra impact force is less important than not sinking before the crew can get out.