# single thrust vs. multiple thrust

Does using one 5000 pound thrust engine give the same amount of thrust as five 1000 pound thrust engines? I am asking in terms of effectiveness not efficiency. Do the two different configurations produce the same amount of thrust?

• I appreciate everyone's responses. I thank all of you. Please continue with your discussion which allows me to continue to learn more. – Erudite Philosopher May 28 '20 at 14:30

Yes.

Thrust is additive. If you want more thrust, adding more engines or scaling up the size of your engines are both valid ways of doing it.

There is a balancing act you have to make, however.

At one extreme, single large engines can have unstable exhausts where the combustion products 'stick' to one side of the nozzle, to a first degree of approximation similar to how a shower head when not turned on fully will run in a single stream rather than many small jets. The Rocketdyne F-1 engine used on the Saturn V is often considered the biggest practical size of engine. The Saturn V used 5 such engines, which meant that if one were to fail, there'd be very little in the way of backup.

The other extreme - with many smaller engines - presents different complications. The N-1 rocket, designed to be the Soviet counterpart to the Saturn V in terms of mission and launch capacity, used 30 smaller engines in its first stage. The plumbing alone was a nightmare to solve. The more engines you have, the higher the chances that having one fail catastrophically will result in the destruction of the entire craft. The N-1 rocket flew four times and exploded four times, and it is thought that the complexity required for so many engines is at least partly to blame.

The SpaceX rockets use a large number of smaller engines. The Falcon Heavy uses 27, whereas the upcoming Starship rocket will have somewhere between 31 and 38 engines, depending on what mood Elon Musk is in on Twitter at the time. It seems like they've ironed out whatever problems the N-1 had when it came to plumbing complexity and so forth.

• Thrust is close to but not quite additive. Multiple thrusters have plume-plume interactions that tend to result in total thrust being slightly less than the additive sum of the thrusts from the individual engines. – David Hammen May 27 '20 at 22:23
• Plume-plume interactions seem unlikely to have an effect, since they are between fluid streams traveling away from the vehicle at supersonic speeds. Plume-atmosphere interactions might have a slight effect, reducing base pressure. And each engine of a cluster is pointed through the center of mass, rather than parallel, so there are slight angle losses. – Christopher James Huff May 27 '20 at 23:16
• @DavidHammen It's close enough for the purposes of this answer. The question is at a basic level, and worrying about plume-plume interactions or other such details doesn't really add anything to the answer. – Ingolifs May 27 '20 at 23:54
• On the contrary, I believe the Falcon 9 intentionally uses plume-plume effects to mitigate improper expansion ratios on ascent – Anton Hengst May 28 '20 at 2:46
• @AntonHengst If you can find a source (link) to documentation of that, it would be a great read. – Carl Witthoft May 28 '20 at 14:04

Yes, but...

Ignoring plume interactions the propulsive effect of an engines thrust does depend on the spacecraft's center of mass being along the line of thrust. If for example a single-engine spacecraft vectors its thrust slightly to turn there is a tiny loss of linear acceleration as there is now angular acceleration.

For a single-engine craft is small because the rocket would spin out of control if it wasn't.

Now if there are multiple engines then you can point them in different directions because the torques can cancel. In this case the thrust available for acceleration of the spacecraft is reduced by the factor $$\cos(\theta)$$ and these reductions are usually referred to as "cosine losses"

• 'tis true, yet total magnitude (not vector) of thrust is additive. It's not like we had a complex wavefunction with interference effects :-) . – Carl Witthoft May 28 '20 at 14:05
• @CarlWitthoft in what situation would add magnitudes of thrusts and ignore their vector sum? I can't think of any, except when they are known to be parallel, in which case you've still considered their vectors. – uhoh May 28 '20 at 14:30