# Thrust/weight ratio of monopropellant thruster engine

Doing some digging online, but I was unable to come up with an answer to this...

What are ballpark thrust-to-weight ratios for typical hydrazine monopropellant thrusters? I'm looking for a value that would be comparable to the quoted T/W ratios for conventional bipropellant engines (I'm not sure whether those typically factor in the mass of the turbopump or not). Also, what would the value be for a typical hydrogen peroxide monopropellant thruster?

• Monopropellant rockets generally aren't up to the standards of bipropellant types. Even relatively simple designs like the LEM ascent stage were bipropellant I think. The only references I've seen for monoprop are their use as attitude control thrusters, for example. You might get more responses if you can say what kind of application you're curious about. – Andy Feb 16 '16 at 17:46
• If a craft happened to use its monopropellant thrusters for moderate orbital maneuvers instead of just attitude control, it would be good to know the range of thrust/weight for the thrusters. Specific impulse is quoted very often for monoprop thrusters but T/W ratio almost never is. – sevenperforce Feb 16 '16 at 17:57
• Thrust and mass are usually quoted independently; just multiply mass in kg by 9.81 to get weight in newtons, then divide thrust by weight. – Russell Borogove Feb 16 '16 at 19:22

The thrust-to-weight ratio is going to vary greatly for small thrusters (and hydrazine monoprops are generally quite small), with the very smallest having much lower thrust-to-weight ratios; mass doesn't scale down linearly with thrust at the low end.

For example, Airbus makes 1N, 20N, and 400N hydrazine thrusters, all pressure fed with similar operating characteristics:

Thrust (N)    Mass (g)       TWR
1        290       0.35
20        650       3.14
400       3800      10.73


Aerojet Rocketdyne also offers a family of hydrazine monoprops with generally similar TWR trends.

Astronautix doesn't list any hydrazine thrusters above 1kN. The most powerful I could find in my search were the AJR MR-80 used on the Viking Mars lander (3.1kN, 37:1 TWR), and its derivative, the Mars Lander Engine used for Curiosity's skycrane, at 3.6kN.

I can't find much info on peroxide monoprops.

Quoted masses for pump-driven bipropellant engines should include the pump mass, by the way.

• Thanks! This is a lot lower than I thought it would be, tbh. – sevenperforce Feb 16 '16 at 22:53
• You could get a much better TWR in a larger thruster, but if you're making a bigger engine it makes sense to accept the relative complexity of bipropellant engines to get higher specific impulse -- you just won't see many big monoprop engines. Likewise when you get bigger still, you accept the turbopump. In general, TWR isn't that important because engine mass is such a small fraction of overall mass. – Russell Borogove Feb 17 '16 at 2:46