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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?

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  • $\begingroup$ Is the thrust control quick enough? I saw some video of hydraulic TVC test and it was very fast. $\endgroup$
    – jkavalik
    Commented Sep 5, 2017 at 16:39
  • $\begingroup$ Even in KSP this one is tricky. Making it profitable (in delta-v) is even harder. $\endgroup$
    – coteyr
    Commented Sep 6, 2017 at 2:50
  • $\begingroup$ @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. $\endgroup$
    – SF.
    Commented Sep 6, 2017 at 8:20
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    $\begingroup$ *throttle controlled avionics $\endgroup$
    – SF.
    Commented Sep 6, 2017 at 9:04
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    $\begingroup$ 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 $\endgroup$
    – Kengineer
    Commented Sep 6, 2017 at 18:01

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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.

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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.

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  • $\begingroup$ It's possible you simply don't need roll control on the bottom stage. Passive fins will dampen any roll. $\endgroup$
    – Joshua
    Commented Sep 5, 2017 at 19:36
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    $\begingroup$ 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. $\endgroup$ Commented Sep 5, 2017 at 19:48
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    $\begingroup$ @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. $\endgroup$
    – SF.
    Commented Sep 5, 2017 at 22:42
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    $\begingroup$ Hence "axially aligned" in my statement. $\endgroup$ Commented Sep 6, 2017 at 0:20
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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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One company in Washington State called Stoke Space is building a two stage rocket called Nova. They are developing the differential throttle in their Hopper vehicle, which is the second stage of the rocket: Stoke Space gets closer to 100% reusable rocket with successful 'Hopper' test flight.

The company webpage haven't reveled the details of the rocket engine, but here is a picture during test posted at Twitter in Jul 26, 2022.

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    $\begingroup$ The Hopper is in fact a prototype of the second stage for their Nova rocket, not a first stage. They don't have a first stage built yet, but they've test-fired an engine for it recently — that engine will probably use gimbaling. $\endgroup$
    – Mqrius
    Commented Jun 12 at 21:49
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There have been various comments on historical uses / future uses of differential throttling, but I thought I would weigh in on why I believe differential throttle is not clearly advantageous over gimballed engines or verniers.

It comes down to the fact that throttling a rocket engine is an involved affair. Even the simplest pressure-fed liquid engines require you take into account the current tank pressure and your valve position, plus possible nonlinear effects due to flex in your fuel manifolds etc changing the propellant. This only becomes more of a problem with high-thrust engines that use one or more turbopumps to feed propellant into the engine, where the fuel flow rate influences the turbine rpm which influences the pump pressure which influences the fuel flow rate, so now you have a system which self-reinforces and will lag your inputs in a complex, nonlinear way.

All of which is to say that getting an engine to immediately provide the exact thrust value you want when you need it, without lag and without blowing itself up, is not always easier than just mounting the engine on a gimbal and swinging it around, while maintaining a constant throttle input. It can be done, but it takes a lot of doing, and if all you want your rocket engine to do is provide a consistent amount of thrust for the duration of its burn time, then it may not be worth the engineering effort.

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