Two objects colliding in space

It would depend on the energy of the smaller in comparison to the mass of the larger. And there is also the possibility that the smaller one would crash through the hull of the larger like a bullet. What do you want to have happen? It would be surprising for the smaller one to bounce back; it is more likely that either or both would be badly damaged and the smaller would end up partly, or completely, inside the larger. Variables include relative masses, velocities, hull strength, and probably something else.

What KN said. In general, when considering collisions in space, you have to take into account relative velocities, materials strength, and elasticity. Two billiard balls would bounce, and you'd probably be able to calculate the resulting velocities and angles pretty easily. Two glass balls would shatter. Two balls of putty would go sploot and become one ball of putty with a net vector.

For spaceships, visualize two large ships (oil tankers, battleships) colliding. Or for that matter, two cars. Chances are, the construction at the point of impact isn't going to be strong enough to withstand the entire mass of the object pushing it into the other one. If they're not equal size (and there aren't structural integrity fields involved) then think Austin Mini vs Hummer.

You can adjust results by changing the amount of armor on each ship. A sturdy smaller ship could conceivably punch through the outer skin of a less sturdy bigger ship with torpedo tubes intact. The problem would be that the torpedoes would probably hit something within a few dozen meters of being fired, so the smaller ship would be caught in the subsequent explosion.

efreysson said:

Well, the smaller ship is making a desperate, last-ditch effort to destroy the larger one, and so smashes into a damaged section of the hull. One idea I had was to have the ramming destroy the armour enough for missiles to be fired into it to a much greater effect, but I decided to check up on the physics before deciding exactly what happens. Can't very well fire missiles if the smaller ship is lodged.

As for energy, the smaller ship is going faster, but is only about half the weight of the other.

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If the smaller ship is going fast enough to any damage, then any people on it will end up as jelly. That maneuver is a suicide mission, but the armor would have to be something amazing to survive the crash. Think about what happens to a small car that smashes into a bulldozer at 60 miles per hour (100 kph). Then multiply the masses of the things by a few million and change the speed from 60 miles per hour to 20,000 miles per hour. Even if both ships are made of remarkably strong materials, there will be a hole into or through the larger one, and the smaller one will be turned into a flattened mess. Even if both have armor that is equivalent to 24" of high grade stainless steel, there still will be a hole and scrap metal.

Think of what an asteroid would do if it hit a battleship. The smaller ship would be very much like an asteroid, and it might vaporize on impact, as asteroids sometimes do.

At 20,000 mph relative velocity difference, both ships will be blown to smithereens. That's in the level of asteroid-impact velocity.

caw

I think the best way to handle this will be to avoid being too specific about speed and thickness of armor. Maybe an offhand comment that the smaller ship is more sturdily built because the larger one is used to having a battle group to protect it. Then hand-wave an impact at a vulnerable spot, maybe one that has already been weakened by explosions.

efreysson said:

I'm writing a space battle scene, and I'm wondering about the physics of the climax. A huge warship rams another, even bigger one. What happens here without gravity? Does the smaller ship push the bigger one, does it get knocked back, what?

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For space navies, ramming speed will make a lot of sense for us earthbound folks. It matches up with what we know worked historically: https://en.wikipedia.org/wiki/Ramming While a gravitational pull does exist between the two objects, it is minor enough of an effect you can safely ignore it. In the end, you have mass vs mass and relative surface area of the point of contact being your biggest factors. Take a look at some videos about car wrecks, especially a semi with a bunch of trailers smushing a compact car. Look at a train sweeping a railroad crossing clear of whoever was foolish enough to park there. When you have an object with a lot of mass smashing into a smaller one, it's generally going to mangle the little one. The equation is Force equals mass times acceleration. The more mass, the bigger the force. The more acceleration, the more Force. When you have a little object hitting a big one, its mass is low so the force is generally small. But, you can accelerate a small mass to much higher speeds which can give you a huge force in the end. At this point, you get to figure out what you want from your space battle scene. Do you want the smaller ship to be very dense (through shield technology and armor and lots of Unobtainium)? Have it accelerate at full throttle and punch right through the big ship, spin around and unleash a barrage of missiles back at the bigger vessel.

Want to have two ponderous hulks slugging it out like battleships from WWII? Have one ram into the other and then use its close support weapons to pound the hull of the other ship as it slowly tries to back out from the hole it made.

Want to go super sci-fi? Have the smaller ship dodge and weave on its way to the big ship's hull and hit its warp drive, ripping a hole right out of reality and out of the side of the bigger vessel. This should trash the warp drive on the smaller ship since it would suddenly be trying to accelerate a bunch of extra mass it wasn't designed for as well as require overriding all the safety interlocks that don't allow you to go to warp in the presence of another vessel. Now your heroes in the little ship are out of the battle but in a badly damaged ship that's out of the approved space lanes and what's that up ahead? Oh no! An uncharted planetoid! Brace for impact crew, we're going in hot!

Here are some folks who have a ton of neat resources on spaceships and the physics of space warfare. This particular article talks about a game they've developed based around real physics and would be worth a read.
http://www.projectrho.com/public_html/rocket/spacegunconvent.php

By the by, elastic collisions are a lot of fun. Watching folks demonstrate them with fitness balls is especially funny: https://www.youtube.com/watch?v=5H3sy2-fUpY

My current WIP uses "ship-busters", which are basically an A.I. controller, a drive system, and a thousand-pound ball of steel. Relatively cheap and quick to make, more or less "single use".

If you want to have two ships smash into each other because it works for your story then go ahead. Personally, I'm a fan of space opera and adventure stories rather than getting hung up on the nit-picky physics of a scene. Are you putting your characters in jeopardy? Is this a dark moment for them as they sacrifice their only means of protection in a crazy last-ditch chance to beat the odds? Good! Handwaving away the tech stuff is easy - there are shields and inertial dampeners and gravitic stabilizers that keep them from being turned into mush when the ship maneuvers. Smashing their plucky little vessel into the big bad guys warship Can't Possibly Work but oho dear readers, this is Captain SeatofthePants, the luckiest buccaneer this side of Antares. What you didn't know until now is that the Cap'n heard tell of a legendary maneuver from a battle on Old Earth when ships sailed the high seas. A desperate gambit by a crew that was out of options whereby they turned *into* the gun arc of Her Majesty's frigate, rammed the Warship Imperator and knocked a few of the guns out of order and put themselves out of the firing arc of the rest of the Imperator's cannons. One nailbiting boarding party later and the little ship had conquered Goliath. To be sure, they were now on board a heavily damaged frigate with their old ship taking on water and no certain way to escape their doom but tune in next chapter and we'll see how they made it out of that jam.

Maybe their missile tubes are amidships so getting within the shield bubble allows them to drop a heavy load of Shipbuster Mk IV missiles right up the tailpipe of the bigger ship.

Heck, give the smaller vessel crumple zones - losing your forward sensor arrays and living quarters is a small price to pay if you manage to take out the pride of the alien Jerkaloid's fleet.


However you choose to write this, a little ship smashing into a bigger one should make a mess of the smaller ship - unless you want to explain why it doesn't. Maybe they are using some new shield and armor tech that makes them like a bullet and they just zip right through the bigger vessel. Maybe they just cheat and pop their escape pods at the last moment using their ship as a giant missile. Accelerating a warship into a bigger warship will ruin just about anyone's day.

I wouldn't expect the little ship to be able to push the big one around, unless it was some kind of space tug. Clearly, you'd have to be some kind of madman to think you could take on a fully loaded warship but if you sidled up to that big bad monster, latched on and fire up your pressor fields then maybe you could tear the aft end of the ship off since you're putting a massive off-axis load on an already weakened structure.

It's your universe, you get to make up whatever you like to help move the story along.

Good luck!