Landing on fusion exhaust

[SomeGuy123]

Suppose you had a fusion (or a really, really hot plasma stream from gas core fission) exhaust on your spacecraft.  Magnets and stuff keep any of that plasma from touching the walls of your engine, and the outer plasma protects you from the radiant light of the inner plasma.

Anyways, as you come in for a landing, won't the plasma plume reflect off the surface of the world you are landing on and send hot gas and debris against things like your engine mounts and other delicate structures that aren't able to tolerate it?

[Scotius]

I would be more worried about landing your craft into a puddle of molten sand and rock

[harvell64]

This is probably somewhere where a form of RCS or a chemical engine dedicated for landing.

[Shpaget]

If you are landing on a non compact surface, the surface will ablate, carrying away the hot stuff.

[AeroGav]

Well, the temperature of a plasma exhaust is higher than a chemical rocket plume, but it is also less dense. 

I think what matters more is megawatts of heat per square inch,  as to whether you turn the surface to molten glass or not.

[SomeGuy123]

Well, the whole idea of an engine like this is that you have an ISP of something like 1 million.  Today's rocket engines have an ISP 2500 times lower.  So you have 6.25 million!!! times as much energy per unit of thrust in your exhaust plume.

The only way this even works at all is almost all that energy never reaches the walls of your engine, and your engine is absolutely gigantic, with the star-hot core located far from the cooler gas at the edges.  

The whole concept behind Orion drives which apparently pass basic math tests of feasibility says you can do this, orion drives, the fusion bomb detonation temperature is many many times what the pusher plate can take.  

Anyways, 6.25 million is a big number.  I don't think you could go safely near a planet of any sort with a torch beam like that (assuming you actually could build an engine with enough thrust to matter at all)

 

[Rune]

1 hour ago, AeroGav said:

Well, the temperature of a plasma exhaust is higher than a chemical rocket plume, but it is also less dense. 

I think what matters more is megawatts of heat per square inch,  as to whether you turn the surface to molten glass or not.

Yup, but high Isp (like a fusion torch), also means more wattage per newton of thrust, so for a given TWR (>1 if you are landing), the more Isp you have, the more energy your exhaust has. Actually, now that I think about it, at some point  on the Isp curve, you are actually just shooting relativistic particles out the back... which wouldn't couple with the ground so well. Still, we are already in the "ludicrous" area, so I guess the molten-lava-lake-previously-referred-to-as-landing-pad comments do apply.

So yeah, any sufficiently awesome propulsion system goes pretty bad with landings on a planet, much less a planet with atmosphere. Then again, if you have a fusion reactor, nothing is stopping you from using, say, electric fans powered by said reactor as a low-altitude flying system, or thermal turbojets, at least in places where there is an atmosphere to work with.

 

Rune. Never mind what firing such an engine would do to the atmosphere.

[SomeGuy123]

Yeah, but those fans/thermal turbojets are heavy.  You have insanely awesome rocket performance on the one hand and now you have to bolt on some clunky fans that weigh you down.  Not elegant, although if that's what you gotta do.

On the bright side, it makes a videogame - KSP or others - that has this kind of OP engine at least a little more interesting in that the overpowered engine can't be used everywhere.

[magnemoe]

5 hours ago, Rune said:

Yup, but high Isp (like a fusion torch), also means more wattage per newton of thrust, so for a given TWR (>1 if you are landing), the more Isp you have, the more energy your exhaust has. Actually, now that I think about it, at some point  on the Isp curve, you are actually just shooting relativistic particles out the back... which wouldn't couple with the ground so well. Still, we are already in the "ludicrous" area, so I guess the molten-lava-lake-previously-referred-to-as-landing-pad comments do apply.

So yeah, any sufficiently awesome propulsion system goes pretty bad with landings on a planet, much less a planet with atmosphere. Then again, if you have a fusion reactor, nothing is stopping you from using, say, electric fans powered by said reactor as a low-altitude flying system, or thermal turbojets, at least in places where there is an atmosphere to work with.

 

Rune. Never mind what firing such an engine would do to the atmosphere.

Saying you have high power high isp engines the danger close distance depend on how fast the exhaust spread out, also if its radioactive or other issues like polluting samples.
You will switch to other engines for the final part of landing and start of takeoff. 

In atmosphere you have other issues like air getting into the vacuum chamber or simply hitting air with the exhaust who move at relativistic speed and can create radioactivity. 
 

[K^2]

15 hours ago, SomeGuy123 said:

Anyways, as you come in for a landing, won't the plasma plume reflect off the surface of the world you are landing on and send hot gas and debris against things like your engine mounts and other delicate structures that aren't able to tolerate it?

At energies where things are going to start getting dangerous, the flow won't really "reflect" from the surface anymore. At the energy levels of typical chemical rocket exhaust, the exhaust particles still interact with the lattice as if it's a solid. But you are already at the boundaries of it with something like a NERVA. As you get into even higher energies, the binding energy in solids becomes lower than kinetic energy of incoming exhaust. At that point, exhaust doesn't interact with a solid, but rather with collection of particles that make up a solid. In other words, fluid dynamics takes over. You'll probably be getting some splash-back, but at much lower energies. You might be getting equivalent of a sand-blasting, but that's true with pretty much any engine, and hopefully, you'll cut duration to a minimum.

I wouldn't worry about landing into puddle of molten rock, though. You'll cut a hole in it, but nothing too drastic. As you increase the exhaust velocity, you also reduce the amount of reaction mass by the same fraction. So by the time you're doing serious damage to the surface you're landing on, you also have a pretty low mass flow, so damage will be highly localized.

[Streetwind]

There comes indeed a point where your super-powerful interstellar spacecraft engine is no longer the best landing thruster.

 

For some amusing reading related to the topic, try ProjectRho's exotic space weaponry site, chapter "Propulsion Systems". It explains the Kzinti Lesson (A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive) as well as the more general Jon's Law of Writing Science Fiction Books (Any interesting space drive is a weapon of mass destruction, where interesting is equal to "whatever keeps the readers from getting bored.") and illustrates a few applicable examples from fiction (including some numbers).

 

Edited December 27, 2015 by Streetwind

[magnemoe]

3 hours ago, Streetwind said:

There comes indeed a point where your super-powerful interstellar spacecraft engine is no longer the best landing thruster.

 

For some amusing reading related to the topic, try ProjectRho's exotic space weaponry site, chapter "Propulsion Systems". It explains the Kzinti Lesson (A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive) as well as the more general Jon's Law of Writing Science Fiction Books (Any interesting space drive is a weapon of mass destruction, where interesting is equal to "whatever keeps the readers from getting bored.") and illustrates a few applicable examples from fiction (including some numbers).

 

The engine would not be designed to have an effect at distance so the Kzinti would probably be very safe unless they tried to board. 
However the engine would be an very nice close in weapon to deal with incoming missiles. 

In real world small heat seeking anti air missiles have problems against larger jet engines as they aim for the rear of the engine and the exhaust stop them from enter the engine so they explode behind. 

[steve_v]

8 hours ago, Streetwind said:

For some amusing reading related to the topic, try ProjectRho's exotic space weaponry site, chapter "Propulsion Systems".

Arghhh, damn you for mentioning that accursed website. Every time I go near the thing, it becomes a multi-hour dive down the proverbial rabbit hole... amusing reading indeed, perhaps too amusing.  

Edited December 27, 2015 by steve_v

[Streetwind]

3 hours ago, steve_v said:

Arghhh, damn you for mentioning that accursed website. Every time I go near the thing, it becomes a multi-hour dive down the proverbial rabbit hole... amusing reading indeed, perhaps too amusing.  

Pssst, the article there also links to tvtropes.org...

Been nice knowing you

 

Edited December 27, 2015 by Streetwind

[Shamus6200]

I can confirm, I lost two hours reading through the pages of that site, and I never even made it to tvtropes.org.

[kerbiloid]

You can land in a lake, so the exhaust will vaporize water, while the solid surface below would stay undamaged.

[Scotius]

Then you'd have to very quickly dam every river and stream flowing into said lake. And pray for sunny, dry weather. And pump out ground water seeping back into lakebed.

[RainDreamer]

This sounds like a situation where you would better design a lander specifily to avoid landing using that engine. 

[shynung]

Or, if the ship with the engine really needs to land, an auxiliary solid-core thermal thruster as a landing engine.

Edited December 31, 2015 by shynung