How feasible is this futuristic Space Gun?

Factual Questions
1

Ok, so our Space Battlewagon needs some point defense to swat pesky incoming missiles. So mounted on various points on the hull are Space Guns.

These are long, smoothbore barrels, since apparently rifling is to improve a bullet’s aerodynamic stability which doesn’t matter in vacuum.

The projectile are tungsten or DU coated in some type of material to seal against the barrel (maybe lead). Half the mass of the projectile is a solid rocket motor, with an ISP of ~200.

The exhaust from the solid rocket motor can be vectored slightly by tiny vanes controlled by 4 actuators. Embedded in the bullet is a sensor that looks for a pulse pattern on the target generated by a targeting laser on the Space Battlewagon. It also has a laser receiver on the projectile itself so that the space battlewagon can beam it target course information from the Space Battlewagon’s more advanced (and therefore much more massive) sensors.

To save mass, the launch out the gun barrel is done by injecting liquid hydrogen and oxygen into a combustion chamber behind the bullet. Will this work as a launch mechanism? Conventional gunpowder contains a relatively low number of joules/kilogram, while liquid hydrogen is one of the best substances for this, per thischart.

For fine tuning your course, I’ve read about covering the projectile in tiny pits each containing a tiny amount of solid rocket propellant. Each pit has an electrical igniter and is wired to the embedded computer controlling the projectile. You would ignite these tiny pits to make small changes in trajectory, and this mechanism is simpler than using mechanical valves and liquid fuel that would have a tough time tolerating acceleration.

Back of the napkin estimate : if the projectile leaves the barrel at 1000 m/s (explosion velocity of liquid hydrogen is apparently1800-3200 m/s) and the rocket motor has an ISP of 220, the projectile will reach a velocity of 2400 m/s.

Other back of the napkin estimates suggest that this space battlewagon might begin engaging enemy warships at ~2000 kilometers. This particular Space Gun would be for point defense to damage incoming projectiles coming from the enemy space battlewagon. The enemy projectiles might be coming at ~10-20 kilometers/second (launched by Space Railguns or superconducting quench guns).

This space point defense gun would be aimed at each incoming projectile and would fire a smaller projectile that will collide with it to disable it’s guidance system, destroy any fission warheads contained inside, and knock it off course.

Unlike the CWIS system, you wouldn’t use a gatling gun, since with guided bullets in vacuum you won’t miss, so you don’t need a high rate of fire. Unless the bullet is damaged in flight or has a manufacturing defect (this is The Future! where nanotechnology means that manufacturing defects are as quaint as horse driven buggies) it will hit.

2

What’s the question, how feasible is this? Well, it could certainly be done. Whether it’s feasible, though, depends on the goal for it. What does it need to be able to hit? How much damage does it need to be able to do? What are the consequences of a hit or of a miss? What techniques are available for making and launching these guns, bullets, and ships? Answer these questions, and we can say how feasible the idea is.

3

The first problem I see is the perennial one for space weapons - heat. All that friction and exploding hydrogen is going to get that barrel warm in a hurry, and there is a nice insulating vacuum all around to keep it that way. You are going to need coolant systems.

The second (paradoxically) is probably ice. The end result of your initial propellent is water, and some of that will dump heat very rapidly to crystallise into clouds of snow. You might need a serious shot of nitrogen per firing to keep things clear for the next shot - you don’t want your projectile impacting a snowdrift just out of the end of the barrel…

4

I probably wouldn’t opt for laser guidance; any sort of gas cloud or debris could get in the way and potentially cause problems, like say… the ice clouds from your propellant.

Why not use some kind of linear accelerator? It’s reasonable to assume your capital ship would have plenty of power, so just use it to have a propellant-less launch system.

The big question is how fast enemy missiles will be coming in, and what level of maneuvering they’ll be doing- that’ll really determine the feasibility of guided projectiles vs. the “wall of lead”. So will any stealthy features of the missiles themselves.

I can easily imagine a scenario where the technology for guided point defense missiles exists, but that the circumstances of stealthy hypervelocity missiles would mean that the telling thing is how fast a CIWS system could detect the threat, react and get projectiles into the path of the oncoming missiles.

5

If railguns exist in this scenario, why are you using explosive propellant? Not only is the muzzle velocity inferior but you’re expending mass that isn’t easily replaceable. Not only do faster projectiles hit their targets harder, they are less likely to miss.

As for “guided bullets in vacuum <> won’t miss”, if only. If the enemy has the same maneuvering technology, then incoming projectiles are jinking randomly to try to dodge your defensive fire. And even computer-controlled aiming/guidance isn’t perfect.

Probably a spacewar fought with foreseeable technology would go something like this: Main ships of the line face each other at the outer range of their heavy beam weapons, while launching missile buses. The buses are smaller and more maneuverable and thus harder to hit then the main ships and have a chance at getting closer. At the optimum range the buses fire their projectiles which are smaller and more maneuverable still; and at final range the missiles eject multiple independent warheads. Presuming that beam weapons impose a waste heat penalty, the main ships’ firing capacity is not limitless and so they have to balance their total attack/defense firepower in order to not be worn down by an enemy ship.

This site: Space War Intro - Atomic Rockets -has a pretty intensive analysis of different aspects of spacewar, and is a fun read as well.

6

As I understand it, rifling solves the issue of the bullet bouncing back and forth on either side of the nozzle as it’s fired, causing it to leave the nozzle at an angle, not to correct against air effects.

7

If the enemy fires a rocket motor to “jink”, you fire a rocket motor to “jank”. Assuming your interceptor round has more acceleration, it won’t miss.

Why does the interceptor round have more acceleration? Because it has more delta-V to spare. The incoming projectile must hit your ship. Therefore any “jinking” it does has to be countered by a corresponding “jank” to remove the velocity added by the jinking, and a reverse jink to get back on the original track for the intercept.

The interceptor projectile doesn’t need to move randomly, in fact, it doesn’t even need to chase a projectile that has made a maneuver that will cause it to miss the ship entirely.

This space gun is intended as a last line of defense. It’s trading off mass efficiency for the ability to engage more targets simultaneously (since these guns are a lot lighter than railguns and coilguns, you can have a lot of them)

8

What would prevent this projectile from maneuvering again to target your ship?

9

While there may not be any atmosphere to deal with, there could still be space crap to deal with – therefor any gyroscopic stability added by spinning the projectile could be beneficial (unless it has some sort of active guidance). I’d keep the rifling.

10

Nothing, but when it does so, you can intercept it with less delta v expended than the projectile expended.

11

I was actually compelled to register here just to explain where your understanding went awry. Primarily, this is an issue of not seeking out good information or attempting to find corroboration for your own theories as to why something might be. If this was how an external source explained it to you, then you need to treat all information from that source with extreme skepticism; and if you simply thought this up on your own you really need to hold yourself to higher standards especially when you are correcting someone.

The purpose and nature of rifling is extremely well understood and has been for hundreds of years, and has only been further confirmed by every discovery relating to it since then. There is no gray area where the light of science has not yet reached or even a fringe element insisting there is controversy with regards to this matter. I can legitimately see how, in the absence of any ability to read information which actually describes the nature of guns and rifling someone might come to the understanding you did, but since you obviously have access to the internet or at least a messageboard where other people have access, it would have been trivial to try and discover the truth.

Even fairly primitive firearms quickly developed means to avoid the effect you are describing without rifling entering the picture. A simple wad of cloth combined with a projectile that is not obviously mismatched to the barrel prevents any meaningful bouncing against the sides of the barrel. The use of matched molds and barrels, or wooden sabots, meant that smoothbore barrels wouldn’t experience turbulence within the barrel or the equally deleterious escaping of gases past the projectile which would rob the projectile of significant amounts of speed, given that guns are on a fundamental level pnuematic pistons.

The sole purpose of rifling in a firearm is to impart spin. Spin is extremely important to accuracy because of gyroscopic stabilization, a spinning bullet resists changes in its orientation forwards to the target in the most aerodynamically stable flight profile. Modern bullets, either of the older round nose profile or the more recent pointed spitzer, are streamlined to reduce drag but this only works correctly if it is going straight at the target. Without gyroscopic stabilization, any interference from eddies or particles in the air could easily knock the fast but light bullets out of alignment and cause them to no longer fly straight. Because of gyroscopic precession, a fascinating but admittedly unintuitive property of spinning objects, rather than turning away the bullet will tend to go into a wobble which often stabilizes but at the very least prevents it from going entirely sideways.

Sideways bullets, which are said to be keyholing because if the long thin hole they make upon hitting a target, have actually become more of a pressing concern now because of how fast and light many current bullets are- the requirements in spin rate and barrel length required to completely stabilize bullets like 5.56 and 5.45 mm as used by current militaries are much stricter, and it is very easy when experimenting with combinations of the two to find one which routinely sends bullets end over end instead of pointing forwards as their engineers intended.

Anyways, I’m going back to lurking and I think you’d be well served by joining me in doing so.

12

Great info, and thanks for registering to share it. I hope you stick around awhile to join in other discussions as the interest strikes you. Welcome aboard.

13

Much as I enjoyed reading your essay on rifling, which was clear, factual and to the point, I hope that the first clause of this sentence is untrue and I feel the second was a little harsh for a simple error of fact. I never scold my pupils for making mistakes; that’s how we learn.

14

One issue with the proposed system is reaction time for firing. If you’re using liquid fuel and liquid oxygen, you’ve got to inject them into the barrel behind the projectile, mix, then ignite. This can be very quick, but for a point defence system you want something that is as close to instantaneous as possible. Remember that your ship is moving, and the targets are unpredictable, so you want to be able to fire at exactly the right time.

Also, this system requires you to keep a laser pointed at a moving target 2000 km away. If your laser system is this good, any chance you can dump more energy through it and avert the need for a projectile? It only needs to boil off a bit of the missile to seriously mess with its ability to target your battlewagon.

15

What my reasoning was is the following :

Lasers have to impart enough beamed energy onto the target to damage it sufficiently. This means they have to supply enough energy to vaporize enough layers off the target that it no longer functions.

A kinetic impact does deliver kinetic energy, which at high velocities does lead to melted materials in some way analogous to laser damage. However, a high density core projectile (like tungsten or DU) essentially creates incredible pressure at the point of impact. The armor material fails and the penetrator physically forces itself through the target.

As I understand it, you need a lot less energy to do this. (although you do have to trade off irreplaceable mass)

It’s not just the energy difference, of course. A sufficiently powerful laser array has to be very heavy. Semiconductor diode lasers are apparently quite heavy (I found spec sheets mentioning 70 kg for 2 kilowatts of laser power), and free electron lasers have a minimum effective size.

I suspect a rapid firing gun, using conventional propellants, would be capable of swatting more missiles per second per kg worth of gun than lasers.

Your space battlewagon would have huge free electron lasers, of course, but it would keep those aimed at the enemy ship instead.

16

The biggest problem you might have is that your enemy may be using kinetic weapons against you. So what happens when you hit an oncoming missile? You just get a cloud of very high speed shrapnel heading towards your spaceship. Is your spaceship highly armored? How about all those laser sensors and heat exchangers and antennae?

17

Your ship is under thrust. I was assuming 1 G of peak acceleration during a battle. This is accomplished with a nuclear salt water engine. You keep changing your acceleration vector during the battle.

Kinetic weapons are targeted on where you are anticipated to be by the time the projectile arrives. This means the computer takes into account the target’s current velocity and current acceleration to arrive at an intercept solution.

But if your ship changes acceleration vector (or just turns it’s engine off), the intercept solution is no longer valid. The projectiles have to have engines of their own to compensate.

If the projectiles are damaged enough that they no longer have working engines, they will probably miss. Probably by a quite a lot at a 2000 kilometer range.

18

I had intended my post as a question (hence the use of, “As I understand it”), but I agree that I didn’t make that very clear. You can take my usage of that phrase as being an implicit question, based on the presumption that no one would allow such hazy wording to stand on this board.

Though I’m not sure that I’m fully convinced by your explanation. By which I mean that I agree that all of the effects that you mention are true and necessary but, as I understand it, the number of degrees off straight that a smooth bore gun (without cloth or some other plug-like technology) is purported to achieve seems like more than can be explained by air eddies or a tumbling bullet. It would seem to indicate that the bullet did not leave the barrel straight. And according to the Wikipedia page on rifling, it also states that bullets still do not leave the barrel straight but rifling makes it consistent which direction the bullets leave the barrel, so that one can adjust the sights accordingly.

To me that would seem to indicate that the bullet, being slightly smaller than the barrel, has room to adjust angle within the barrel. But if you’re certain that all lack of perfectly straight movement is due to tumbling and eddies which affect the bullet after leaving the barrel, then I’m willing to trust that I understood the issue incorrectly.