If you have thrust vectoring, what's the minimum set of thrusters for an interplanetary spacecraft?

[SomeGuy12]

Suppose you have a high efficiency, thrust vectoring engine. It's NERVA, VASIMR, or fusion. The exhaust coming out can be precisely controlled so it's perfectly aligned with the spacecraft's center of mass. (and you can detect this easily with accelerometers - if you are off center, there's net torque in the opposite direction, and so you can adjust your thrust direction in a control loop)

How many more engines do you need? Your basic mission profile is flying to other planets, docking with landers, and with massive rotating space stations. The engine exhaust is hazardous to space stations and the engines are nuclear (or nuclear electric in the case of VASIMR), and so there is an unshielded high intensity radiation source at the back of your craft.

[sojourner]

The minimum? One.

[grawl]

I'd add RCS thrusters to your design. Practical to turn around your (probably) big interplanetary ship.

Reaction wheels can do the trick as well.

Also a set of redundant backup thrusters might be mandatory. I wouldn't dare to go in an interplanetary mission with only one motor. Imagine it fails !

A "flower" placement of the thrusters gives you the ability, if one should fail, to compense the displacement of the center of thrust by adjusting the power of each engine.

Edited July 28, 2015 by grawl

[KSK]

Two engine systems I would say. Your main drive sounds fine for interplanetary travel, where you can just flip the spacecraft around and use a single drive for accelerating and decelerating. For approaching space stations or other situations where the main drive is inappropriate or dangerous to use you're going to need a separate set of retro-thrusters and probably RCS thrusters for fine corrections.

[Kinglet]

Or you could have two thrusters on the sides of your ship with hemispherical thrust vector ranges. They would give you excellent control with no need for RCS.

[Nuke]

assuming the engines have a 180 degree range of motion, you can get 5 degrees of freedom with just two at opposite sides (left and right in this case) of the cg. this gives you roll, yaw as well as translate vertical, horizontal, and forward/back. you could control pitch while using forward thust, but that is not desirable for a docking manuver. you need a third one for pitch, though you could just use a pair of fixed thrusters for that. the configuration for a 3 thusters would be kind of like a 'T' with the center of the gimbal range aligned with the line segments of the character. the center of thrust for yaw, and horizontal translate modes would need to be at the same point as the cg, though with variable throttling you can gain a little bit of trim control. 4 thrusters in an x config gives you a little redundancy should one fail.

for example

of course the 180 degree gimbal such as this is mechanical in nature and offers the spacecraft yet another point of failure. you got hoses, bearings, servos, all of which can go wrong or get damaged by debris. since you only have the one system you dont have a backup system in case something goes wrong. with a main gimballed engine and a separate rcs system you have one should the other fail.

Edited July 28, 2015 by Nuke

[Kinglet]

assuming the engines have a 180 degree range of motion, you can get 5 degrees of freedom with just two at opposite sides (left and right in this case) of the cg. this gives you roll, yaw as well as translate vertical, horizontal, and forward/back. you could control pitch while using forward thust, but that is not desirable for a docking manuver. you need a third one for pitch, though you could just use a pair of fixed thrusters for that. the configuration for a 3 thusters would be kind of like a 'T' with the center of the gimbal range aligned with the line segments of the character. the center of thrust for yaw, and horizontal translate modes would need to be at the same point as the cg, though with variable throttling you can gain a little bit of trim control. 4 thrusters in an x config gives you a little redundancy should one fail.

for example

of course the 180 degree gimbal such as this is mechanical in nature and offers the spacecraft yet another point of failure. you got hoses, bearings, servos, all of which can go wrong or get damaged by debris. since you only have the one system you dont have a backup system in case something goes wrong. with a main gimballed engine and a separate rcs system you have one should the other fail.

It would still be good for things like cheap and light maintenance drones, as nobody would really care if one of them does not make it back if they are cheap enough.

[Nuke]

it might be more acceptable for manned missions using nuclear-electric propulsion. you can always eva to swap out a thruster with a spare or bring them in for regular maintenance.

[Armchair Rocket Scientist]

It would still be good for things like cheap and light maintenance drones, as nobody would really care if one of them does not make it back if they are cheap enough.

For cheap, light stuff it's generally easier to just use a bunch of non-gimballing thrusters. The hardware to make an engine gimbal 180 degrees or even hemispherically is going to add a lot of weight and cost.

[GeneralVeers]

The minimum? One.

Wrong. The absolute minimum is two. Because you need to have a backup in case something goes wrong with the first one.