Rocket Plumes In Space Versus Scifi

[Spacescifi]

 

So I have read several things about real rocket plumes in space:

1. Higher velocity plumes tend to be elongated.

2. The more mass in the plume the more likely it becomes visible in space.

3. Glowing plumes are'nt bad if there is a lot of mass propellant flow. If not... then your nozzle will likely overheat. Unless you are using a magnetic nozzle.

 

Question: Is there ANY reason at all why a plume in space in real life would be pointy? I mean there is no atmosphere to constrict the plume at all... unless they got a +50 tesla magnetic field compressing the plume... I dunno.

Like:

 

Instead of the more core like this:

 

I assume that the physics hold true no matter what rocket propellant we use.

Whether futuristic AM thermal with water or hydrogen, or a more conventional nuclear thermal rocket with hydtogen propellant.

 

You may correct or add to this as you wish.

Have a great day!

Edited April 14, 2020 by Spacescifi

[RoninFrog]

7 minutes ago, Spacescifi said:

Like:

Looks like some kind of Hall Effect thruster.  Those can be very pointy in a vacuum.  I'm pretty sure the Dawn engine is Hall Effect.

[sturmhauke]

That would be the Epstein Drive from The Expanse, described as an advanced fusion drive with a magnetic compression coil. So the plume might look something like that, but it's not an ion drive.

[kerbiloid]

Maybe it's just the central and the hottest part of the plume.
Maybe their actual plume is skirt-looking, but the engine produces such picture of pressure and temperature that the exhaust is pressed to the center and looks hot, but quickly cools and gets invisible to the edges.

(Just thinking out an excuse for the movie makers.)

[DDE]

3 hours ago, Spacescifi said:

Question: Is there ANY reason at all why a plume in space in real life would be pointy?

Yes, if the ejection velocity is really high and the mass flow is really tiny. Ion engines are basically particle accelerators.

[Spacescifi]

13 hours ago, DDE said:

Yes, if the ejection velocity is really high and the mass flow is really tiny. Ion engines are basically particle accelerators.

 

So if the mass flow is more normal (liquid hydrogen propellant) but it looks like a pointy plume.... that's basically death ray exhaust right?

The kind of thing you get when you supercharge your propellant with generous amounts of antimatter no?

Edited April 14, 2020 by Spacescifi

[Guest]

Well, even a particularly powerful nuclear or direct drive fusion will give you that effect. It's all about exhaust velocity. Yes, it does make a workable particle cannon, but rocket exhaust is poorly collimated, so it won't be efficient at doing damage, and aiming by turning the whole ship is hard. The Kzinti Lesson still does apply, just keep in mind that there are better death rays out there.

[magnemoe]

On 4/15/2020 at 1:50 AM, Dragon01 said:

Well, even a particularly powerful nuclear or direct drive fusion will give you that effect. It's all about exhaust velocity. Yes, it does make a workable particle cannon, but rocket exhaust is poorly collimated, so it won't be efficient at doing damage, and aiming by turning the whole ship is hard. The Kzinti Lesson still does apply, just keep in mind that there are better death rays out there.

Yes, it will spread out pretty fast then we talk space term, 30 km is short in space but still long for artillery. 
I think that drive would be an pretty good anti missile weapon as missiles has to get in close, the missiles will have to dogleg around the plume and will come in pretty slow from the side, also as you are running away you could drop short range anti missile probes who could wait for the missiles to come to you. Your only problem with be that the plume would make it hard to aim direct energy weapon rearward so no tail guns. 

[Guest]

Not really, by the time the missiles will have to worry about the plume, it'll be too narrow to be a problem. 30km isn't all that short in space, either. Missiles are best handled by throwing a lot of high velocity mass their way.

[magnemoe]

14 minutes ago, Dragon01 said:

Not really, by the time the missiles will have to worry about the plume, it'll be too narrow to be a problem. 30km isn't all that short in space, either. Missiles are best handled by throwing a lot of high velocity mass their way.

30 KM work for pumped x-ray nuke lasers, probably shaped nukes works  to but less effective. 
Anybody worried about increases radiation please wag their tails
 

[Guest]

Actually, tests did not conclusively show pumped x-ray nuke lasers working at all. I'm of the opinion that they don't. Shaped charge nukes have more merit, but the jet from those will also spread out quickly, might not be good up to 30km, especially against a missile. Same problem as with the engine plume, really. Not quite midrange unless you're fighting with conventional guns or low-power railguns, but not unreasonably close.