Shooting a rocket "up the arse"

[Zor]

This is one thing i thought about involving science fiction, but have not actually seen directly that much.

Lets say you have a spaceship with a rocket/nuclear salt water/fusion/antimatter/ion drive with big notable end nossil/s (term correction if nessisary would be greatly) that in battle durring acceleration gets fired into it...

1-A Shell (ungided projectile)
2-A missile carrying a nuke
3-A Laser beam

I am going to assume that either of these could easily fuck up the delicate insides of such a craft's engine, but would the fact that it it is spewing out a stream of Plasma into said oncomming laser beam/missile/projectile be enough to stop the oncomming projectile?

Zor

[Ford Prefect]

but would the fact that it it is spewing out a stream of Plasma into said oncomming laser beam/missile/projectile be enough to stop the oncomming projectile?

Yeah, it could. The plasma should be pretty opaque to a laser beam, and depending on how much energy you're pumping out, it could potentially just vapourise an incoming shell or missile, but there's lots of different factors involved hear regarding velocity, composition of the incoming projectile and such.

[Junghalli]

A decently programmed missile would probably try to avoid flying into the hot and destructive gas plume.

[Ryushikaze]

You'd want to shoot for the 'taint' as it were, in essence any engine component just to the side of any of the jet streams but not armored or shielded. A much harder shot, but one much more likely to actually damage the engines and not get obliterated by the exhaust. Even if you just tear open a new exhaust hole, that's a new, possibly uncontrollable exhaust vector that ship now needs to contend with. The feasibility of such a shot, I shall leave for other people to figure out, though I wouldn't expect it to be particularly high, even on larger ships.

[Darth Wong]

You'd want to shoot for the 'taint' as it were, in essence any engine component just to the side of any of the jet streams but not armored or shielded. A much harder shot, but one much more likely to actually damage the engines and not get obliterated by the exhaust. Even if you just tear open a new exhaust hole, that's a new, possibly uncontrollable exhaust vector that ship now needs to contend with. The feasibility of such a shot, I shall leave for other people to figure out, though I wouldn't expect it to be particularly high, even on larger ships.

You're assuming that the gas in the outlet is being directed by the nozzle's physical geometry. If it is a highly collimated beam of some sort, you won't get any leakage from making an extra hole near the exhaust point.

In sci-fi, it's common to have things like ion drives where they shoot out streams of particles at relativistic velocity. Damaging the outlet geometry won't really change anything in that case; it won't leak out the new hole in the side because it has no inclination to do so. It will remain a collimated beam for a long distance after leaving the nozzle anyway.

This reminds me of one of the big problems with underpowered weapon estimates in sci-fi: many fans seem to have no problem with enormously powerful ion-beam thrusters on starships but they turn around and think that high-powered weapons are "unrealistic" or "wanky", even if those weapons are orders of magnitude less powerful than the thrusters. If that were really the case, then why would these ship designers not adapt their thruster technology into weapons?

[Nyrath]

Lets say you have a spaceship with a rocket/nuclear salt water/fusion/antimatter/ion drive with big notable end nossil/s (term correction if nessisary would be greatly) that in battle durring acceleration gets fired into it...

1-A Shell (ungided projectile)
2-A missile carrying a nuke
3-A Laser beam

I am going to assume that either of these could easily fuck up the delicate insides of such a craft's engine, but would the fact that it it is spewing out a stream of Plasma into said oncomming laser beam/missile/projectile be enough to stop the oncomming projectile?

As others have said, it depends upon other variables.

Zubrin's example nuclear salt water rocket has a thrust power of about 430 gigawatts. If a shell was moving slowly enough, it would be vaporized by the exhaust. A missile carrying a nuke would probably be damaged enough that the nuke would fail to detonate. The NSWR's exhaust has a mass flow of about 196 kg/s. I'm not sure how much protection this would provide against a laser beam, but my gut reaction is that the beam would punch right through and seriously damage the NSWR.

So the target ship would be damaged if the shell was moving fast enough, the missile was armored enough, or if a laser was used. As far as I can tell.

[Nyrath]

This reminds me of one of the big problems with underpowered weapon estimates in sci-fi: many fans seem to have no problem with enormously powerful ion-beam thrusters on starships but they turn around and think that high-powered weapons are "unrealistic" or "wanky", even if those weapons are orders of magnitude less powerful than the thrusters. If that were really the case, then why would these ship designers not adapt their thruster technology into weapons?

Offhand I'd say it's because said fans have no idea that such propulsion systems have exhausts measured in gigawatts.

This is known as John's Law for SF authors: "Any interesting space drive is a weapon of mass destruction. It only matters how long you want to wait for maximum damage. Interesting is equal to 'whatever keeps the readers from getting bored'".

Your second comment is of course the famous Kzinti Lesson: "a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive."

This is generally not used, since a drive optimized to be an efficient propulsion system will be a less than perfect weapon, and vice versa. A drive that can switch modes will suffer from the "Swiss Army Knife" syndrome, where it will be capable of performing both functions but will do both poorly.

[Cykeisme]

This reminds me of one of the big problems with underpowered weapon estimates in sci-fi: many fans seem to have no problem with enormously powerful ion-beam thrusters on starships but they turn around and think that high-powered weapons are "unrealistic" or "wanky", even if those weapons are orders of magnitude less powerful than the thrusters. If that were really the case, then why would these ship designers not adapt their thruster technology into weapons?

This is generally not used, since a drive optimized to be an efficient propulsion system will be a less than perfect weapon, and vice versa. A drive that can switch modes will suffer from the "Swiss Army Knife" syndrome, where it will be capable of performing both functions but will do both poorly.

What is being said is that when a given civilization with a given level of technology can generate that that amount of power for propulsion, it damn well should be able to arm its ships with weapons with similar power levels.

Nobody is talking about a single device being engineered to serve both purposes.

[GrandMasterTerwynn]

What is being said is that when a given civilization with a given level of technology can generate that that amount of power for propulsion, it damn well should be able to arm its ships with weapons with similar power levels.

This doesn't compute. To take a real-world example, a space shuttle launch has a power output of several gigawatts, with a total energy expenditure in the hundreds of terajoules. However, the only weaponry we have that can deliver that much energy are nuclear devices. Weapon power does not necessarily scale with propulsive power. Furthermore, it's possible to hit a point of diminishing returns for increasing weapon power. Barring unobtanium hulls and handwavium physics, a spaceship is going to be easy to kill with weapons not exceeding a few megajoules of energy delivery. Even if this spaceship has engines which can generate terajoules or exajoules of energy with power outputs of hundreds of gigawatts.

[Sea Skimmer]

Missiles have flown right up jet pipes in real life, which tends to be rather bad. The Israelis gave the A-4 Skyhawk a greatly extended jet pipe in part to migrate the threat of an SA-7 exploding inside it, the warhead was tiny enough for a couple extra feet to make a big difference in if the missile blew off the whole tail or not. It thus does not seem unreasonable that a future missile could be tough enough to survive the presumably much hotter and higher velocity exhaust of an advanced space engine. However if the engine is powerful enough, it might just stop the missile without destroying it, and blow it back away.

[Darth Wong]

What is being said is that when a given civilization with a given level of technology can generate that that amount of power for propulsion, it damn well should be able to arm its ships with weapons with similar power levels.

This doesn't compute. To take a real-world example, a space shuttle launch has a power output of several gigawatts, with a total energy expenditure in the hundreds of terajoules. However, the only weaponry we have that can deliver that much energy are nuclear devices. Weapon power does not necessarily scale with propulsive power.

That is not the logic underlying the argument. Read my post. If people are going to claim that their engine exhaust systems output relativistic accelerated particle streams, the weapons application is obvious.