Good Scifi Rocket Nozzle Designs... Less vs more nozzles

[Spacescifi]

It is not often discussed that I have seen, but rocket plumes in vacuum can cause all sorts of problems for scifi or real space vessels like:

1. The expanding particles can create backflow on the hull, meaning if any part of your hull is near the plume, the plume WILL push it in the opposite direction. I doubt even if KSP simulates that. This can make attitude adjustments more difficult/sensitive.

2. Thermal damage to the hull the plume backwash hits.

3. If you're using an advanced magnetic nozzle for a fusion or antimatter thermal propellant heated rocket, your exhaust plume will probabaly also toss plenty of charged particles out the sides, if what I read is correct.

I read it from this: https://www.sciencedirect.com/science/article/pii/S1000936112000246

And I quote: There are several accidents caused by plume effects. During the mission of the U.S. “Gemini” manned spacecraft, because no protective covers were used, the observation windows of this spacecraft suffered serious plume sediment contamination. 

 Another accident happened during the service of the Japanese “BSE-1” satellite which was launched by the United States. The plume of this satellite’s attitude control thrusters contaminated the solar cell surface and reduced the power of solar cells. Besides, in the U.S. “Voyager” Spacecraft, several 0.9 N rocket motors were used for orbit adjustments. The flight data showed that the plume impact resulted in a 22% drop in the motor’s average thrust and a 60% drop in torque of the satellite. Although some preventive measures were originally introduced to reduce the plume’s direct contaminations and impingements, the design still could not protect the satellite from the plume impacts. Moreover, the plume contamination also caused a poor performance of the tracking instrument in Mariner 10. Though Mariner 10 recovered its normal function through a series of spacecraft adjustments, the control system consumed incidental fuel and shortened the lifespan.

End of quote.

 

All of thus seems to point to this conclusion, that an optimal rocket ship that reduces negative plume effects in vacuum will have a minimum of nozzles, not a multtitude.

Ideally, I think having one or gimballed two nozzles on the rear of the ship would be ideal.

Is it not true that a spaceship can do all basic maneuvers with just TWO gimbaled rear engines?

Pitch can be done by verical tilting of the nozzles up or down, yaw could be done by sideways tilt left or right, and roll could be done by by pitching one nozzle up while another down.

 

Such a design favors either a broad saucer hull or a long cylinder, although a cylinder would have a slower roll turn rate since it's engines are closer to the center.

 

What do you think?

Edited July 18, 2019 by Spacescifi

[kerbiloid]

https://en.wikipedia.org/wiki/Transtage

 

2 hours ago, Spacescifi said:

Is it not true that a spaceship can do all basic maneuvers with just TWO gimbaled rear engines?

It can't move back.

[Spacescifi]

6 minutes ago, kerbiloid said:

https://en.wikipedia.org/wiki/Transtage

 

It can't move back.

 

True, but that is why a vessel can flip over and retroburn, or just yaw to rotate and turn around and then retroburn.

With a scifi rocket drive that has thrust for several days, and the ability to throttle the thrust up or down, I feel no need for putting anymore nozzles on a vessel than necessary.

[kerbiloid]

7 minutes ago, Spacescifi said:

True, but that is why a vessel can flip over and retroburn, or just yaw to rotate and turn around and then retroburn.

But any RCS vessel can do this staying still in the same orbit.

Edited July 18, 2019 by kerbiloid

[Spacescifi]

33 minutes ago, kerbiloid said:

But any RCS vessel can do this staying still in the same orbit.

 

Yes. How does that relate to the discussion though?

I am not sure what point you are making here?

Orbits are great like that, but for rendezvous, some rocket burns are inevitable.

Otherwise crashing or missing the target altogether is the result.

[StrandedonEarth]

The problem is that any attitude or roll changes will impart forward thrust, requiring a flip’n’retroburn to maintain the same orbit/position, wasting propellants. Maybe not a problem for a sci-fi craft, but also potentially inconvenient or uncomfortable for passengers. It could also result in unwanted translation. 

Might be better off to use “magical “ hi-tech reaction wheels for turning. 

Edited July 18, 2019 by StrandedonEarth

[Spacescifi]

9 minutes ago, StrandedonEarth said:

The problem is that any attitude or roll changes will impart forward thrust, requiring a flip’n’retroburn to maintain the same orbit/position, wasting propellants. Maybe not a problem for a sci-fi craft, but also potentially inconvenient or uncomfortable for passengers. It could also result in unwanted translation. 

Might be better off to use “magical “ hi-tech reaction wheels for turning. 

 

Ha, I do not even worry about propellant in scifi.

Since if that is what a vessel is relying on, even refueling at a gas giant is problematic. Even with a grav-dampener to negate a planet's pull, gas giant skimming would take time and either require burning more fuel, or using an electroplasma jet to reach space. And the electrodes that make the plasma wear out over time, so only so many uses can they have before you either replace them or you just no longer use your electroplasma jet.

 

As for passenger comfort and translation of stored goods moving about the ship I only have one word.

Well two actually. Make it three.

Passengers. On. Airliner.

If you have flown you know what I am talking about.

 

[KSK]

Ship design could probably mitigate most plume impingement issues. If your drive is putting out charged particles to the sides then you’ll want some kind of shadow shield to protect the inhabited parts of the ship, which would also do a good job of protecting sensitive parts of the ship from the drive plume.

Using your main drive nozzles for steering would be fine for maintaining heading during a burn but lousy for any sort of fine maneuvers, for at least the reasons @StrandedonEarth pointed out.

You could try it in KSP - take a 3.75m tank, stick two Mainsails on one end and a capsule on the other. Turn off capsule reaction wheels, put the thrust limiters at 100% on both Mainsails. Now try rendezvousing with something. Then try docking with that something if you’re feeling cocky.

[Nuke]

thrusters that have charged particles often have neutralizers near the engine nozzel. its usually in the form of an electron gun (for positive charged particles) pointed into the plume. the electrons connect up with the charged particles and keep them from coming back. lots of irl ion drives have those. if the particles are negative idk maybe a proton or positron emitter. i dont know if they actually do that though.

Edited July 18, 2019 by Nuke

[Nuke]

6 hours ago, Spacescifi said:

 

True, but that is why a vessel can flip over and retroburn, or just yaw to rotate and turn around and then retroburn.

With a scifi rocket drive that has thrust for several days, and the ability to throttle the thrust up or down, I feel no need for putting anymore nozzles on a vessel than necessary.

sometimes in kerbal i will do rockets which only have gimballed nozzels. often reorienting involves throttling up the engine, maybe about 5-10% and using the gimbals to turn. it wastes fuel and can mess up your trajectory somewhat but gives you 3 axis control. real engines cant deep throttle that well and so its completely impractical irl.

you can do the same thing with only four fixed engines. you arrange them in a square pattern with the upper and lower engines angled apart a few degrees. pitch and yaw can be controlled by operating horizontal and vertical pairs and roll can be accomplished with kitty corner pairs. if your probe is ion drive where you have to run the engines for long periods of time anyway you can control the orientation by pulsing off (pwm) the correct thrusters for short periods of time. if used in conjunction with reaction wheels the ion drives can detorque them during burns and save the reaction wheels for when you are on the float. 

its often easier/more efficient to turn an engine on or off than to throttle it so smaller pwm controlled thrusters could be used, or arrays of smaller thrusters where you can just turn on as many as you need to gain the desired control authority. or use both techniques if you need really fine grained control over a wide range of thrust levels.

Edited July 18, 2019 by Nuke