It seems the standard approach to control of rockets during launch is either vernier thrusters, or gimbals on the main engines. Sure that works, and is quite efficient, but I wonder about a simpler solution- with multiple engines, throttle engines on the side you want the rocket lean to, and simply allow the summary thrust vector to wander a little off-center while retaining the same direction.

Was such a solution ever used?

  • Is the thrust control quick enough? I saw some video of hydraulic TVC test and it was very fast. – jkavalik Sep 5 '17 at 16:39
  • Even in KSP this one is tricky. Making it profitable (in delta-v) is even harder. – coteyr Sep 6 '17 at 2:50
  • @coteyr: Separate thrust control in KSP is very tricky. There's a mod, "Thrust Controlled Avionics" that is supposed to be able to do this, but the author went a little overboard with features, adding so many various autopilot options the original idea got a bit lost in the bulk. – SF. Sep 6 '17 at 8:20
  • *throttle controlled avionics – SF. Sep 6 '17 at 9:04
  • Did you mean for your question to be so narrow, or are you trying to ask about non-gimbal methods of attitude control? Because there is also thrust vane control used on the Redstone, and Thrust Injection Control used on some solids. It's not a definitive reference but I found good information here: rocketryforum.com/archive/index.php/t-64305.html – Kengineer Sep 6 '17 at 18:01
up vote 25 down vote accepted

The first stage of the Soviet N-1 moon rocket (Block A) used this type of differential thrust system. It had 30 engines in 2 rings. The outer ring of 24 engines used differential thrust control to control pitch and yaw, and was set up to shut off opposing engines in case of a single engine failure.

Four launches were attempted and all failed in the first stage; the first and third failure were directly due to problems in the electronic control system of the first stage engines. No further launches occurred, so it never really worked.

Differential thrust of a set of axially aligned engines can't provide roll control by itself; either dedicated roll-control thrusters or at least one off-center and movable engine is needed.

Rocket development literature frequently mentions differential throttle as a possibility, but it seems like it hasn't been used in practice very often.

The Surveyor lunar landers had three thrusters for landing, and used differential thrust for pitch and yaw control, but one of the thrusters was movable for roll control.

I believe Dragon 2 was designed to use differential throttle for landing control, but it may wind up not doing propulsive landings at all.

I don't know of any large launchers (apart from the N-1 as described in Josh King's answer) that rely on differential thrust for maneuvering. Attitude control during the high-Q portion of ascent needs to be fast; it may be that throttle control of big engines isn't fast enough to do the job.

  • It's possible you simply don't need roll control on the bottom stage. Passive fins will dampen any roll. – Joshua Sep 5 '17 at 19:36
  • Roll control isn't strictly necessary, but if you've got it you can simplify first-stage guidance to a 2D problem by rolling the vehicle to the correct orientation, which you can't do with fixed fins -- probably less important to modern guidance software than it was in the 1960s. One significant appeal of differential throttle is weight reduction by removing the gimbals/actuators, though, so it would be a shame to add weight back in the form of fins. – Russell Borogove Sep 5 '17 at 19:48
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    @RussellBorogove: Thing is, pitch and yaw are in unstable equilibrium (inverse pendulum problem), roll is neutral, so very little force/torque is needed to maintain it. Plus it could be done through differential throttling too - attaching pairs (or actually groups of 4) of slightly angled engines, that cancel each other's offset when going at full thrust, but provide roll if you throttle one pair, and leave the other going full power. – SF. Sep 5 '17 at 22:42
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    Hence "axially aligned" in my statement. – Russell Borogove Sep 6 '17 at 0:20

Astra is currently developing a small sat rocket with differential thrust, and they even "launched" it. Though it's not clear how far the launch procedure has gotten.

Check out

The part about differential throttling starts at about 5:00, but I recommend watching the whole thing.

I know this question is about past developments, but this project definitely deserves to be mentioned here.

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