What's better, more low thrust engines, or fewer high thrust engines?

[DerekL1963]

10 hours ago, Dusty9261 said:

I think it's worth chipping in that the N1 moon rocket failed 4 times [its only flights too] BECAUSE it used so many engines, which made for complex and fragile plumbing/wiring/etc.

Versus, of course, the Saturn V, and I think we all know how that went.

There's more to it than just "the N1 had too many engines". The Saturn V first stage was test fired as a stage - the N1 was test fired as individual engines which were then assembled into a stage and launched.

 

10 hours ago, Dusty9261 said:

Not saying many small engines are bad, but I feel the need to point out that one can only have so many before it becomes a problem in and of itself.

Given that the N1 was a rush job that skipped an important step, I think that's an unproven assertion.   We simply don't know if there is an upper bound for a properly designed and tested booster.

[shynung]

15 hours ago, Dusty9261 said:

I think it's worth chipping in that the N1 moon rocket failed 4 times [its only flights too] BECAUSE it used so many engines, which made for complex and fragile plumbing/wiring/etc.

Versus, of course, the Saturn V, and I think we all know how that went.

Not saying many small engines are bad, but I feel the need to point out that one can only have so many before it becomes a problem in and of itself.

N1 didn't fail because of its many engines. It did because a stray bolt got into the turbopump.

I'd say it failed because of shoddy engineering. Having many engines aren't a problem by itself if those engines can be properly managed.

[Bill Phil]

1 hour ago, shynung said:

N1 didn't fail because of its many engines. It did because a stray bolt got into the turbopump.

I'd say it failed because of shoddy engineering. Having many engines aren't a problem by itself if those engines can be properly managed.

But the likelihood that one engine will fail increases. And if that one failure is catastrophic...

Also, the complications of multiple engines did lead to at least one of the failures. The guidance system they used couldn't manage that number of engines.

[shynung]

42 minutes ago, Bill Phil said:

But the likelihood that one engine will fail increases. And if that one failure is catastrophic...

Also, the complications of multiple engines did lead to at least one of the failures. The guidance system they used couldn't manage that number of engines.

The point of having many engines is redundancy. That is, the failure of one engine should not cause mission failure.

In 8 October 2012, during the SpaceX CRS-1 mission, the Falcon 9 rocket experienced a single engine failure. The rocket compensates by firing the remaining 8 engines longer than usual, and the spacecraft reaches orbit.

 

[Bill Phil]

1 hour ago, shynung said:

The point of having many engines is redundancy. That is, the failure of one engine should not cause mission failure.

In 8 October 2012, during the SpaceX CRS-1 mission, the Falcon 9 rocket experienced a single engine failure. The rocket compensates by firing the remaining 8 engines longer than usual, and the spacecraft reaches orbit.

 

I know about redundancy. Butbtt more engines there is, the more likely one will fail. And the N-1 would shut down the corresponding engine if that happened, but it happened enough times that the computer just shut down ALL of the engines.

Some people make the argument that it's less reliable when you have more engines, but I don't see their reasoning.

[shynung]

3 hours ago, Bill Phil said:

I know about redundancy. Butbtt more engines there is, the more likely one will fail. And the N-1 would shut down the corresponding engine if that happened, but it happened enough times that the computer just shut down ALL of the engines.

Some people make the argument that it's less reliable when you have more engines, but I don't see their reasoning.

From Wikipedia:

Quote

Complex plumbing was needed to feed fuel and oxidizer into the clustered arrangement of rocket engines. This proved to be extremely fragile, and was a major factor in the design's launch failures. Furthermore, the N1's Baikonur launch complex could not be reached by heavy barge. To allow transport by rail, all the stages had to be broken down and re-assembled. The engines for Block A were only test fired individually and the entire cluster of 30 engines was never static test fired as a unit. Sergei Khrushchev stated that only two out of every batch of six engines were tested.^[24] As a result, the complex and destructive vibrational modes (which ripped apart propellant lines and turbines) as well as exhaust plume and fluid dynamic problems (causing vehicle roll, vacuum cavitation, and other problems) in Block A were not discovered and worked out before flight.^[25] Blocks B and V were static test fired as complete units.

N1 went down because of shoddy engineering. Stuff like pogo oscillation and other vibrations encountered during flight ripped the thing apart. The computer was meant to only shut down the problem engine and the engine directly opposite of it. When those engines were shutdown, sudden turbopump stoppage from them sent a shockwave through the fragile plumbing, and ruined the rest.

[kunok]

6 hours ago, Bill Phil said:

But the likelihood that one engine will fail increases. And if that one failure is catastrophic...

This. A rocket motor is not a light bulb, that you can put a lot together and if one fails it will do nothing to the others.

Putting 9 rocket engines together may increase the redundancy and improve the tolerance to little failures, but rockets motors can explode and other catastrophic failure modes, and that failures can break the adjacent engines or even the full stage.

So putting 9 engines gives redundancies to little failures but multiplies for 9 the catastrophic failure rates. And that's not taking into account the increased complexity gives other structural problems like more resonance modes.

Engineering is always making trade-offs.

[Bill Phil]

3 hours ago, shynung said:

From Wikipedia:

N1 went down because of shoddy engineering. Stuff like pogo oscillation and other vibrations encountered during flight ripped the thing apart. The computer was meant to only shut down the problem engine and the engine directly opposite of it. When those engines were shutdown, sudden turbopump stoppage from them sent a shockwave through the fragile plumbing, and ruined the rest.

N1 was bad for a variety of reasons.

On the first flight the guidance system shut down ALL of the engines on the first stage, and yes, some engines we're seeing problems, but they weren't affecting ALL engines. And the later stages were locked.

[RocketSquid]

It depends. For example:

Say engine A has a thrust of 5 arbitrary thrust units, a mass of 5 arbitrary mass units, a cost of 5 arbitrary currency units, and an ISP of x seconds. Engine B, meanwhile, has a thrust of 1 arbitrary thrust unit. A cluster of 5 Engine B's would be better than a single Engine A if:

1. Engine B costs less than 1-(Cost of fuel system/5) ACUs
2. Engine B weighs less than 1-(mass of fuel system/5) AMUs
3. Engine B has an Isp higher than x seconds
4. The cluster and fuel system are more reliable than Engine A.

Obviously, it would likely not be an all-or-nothing scenario, so you would have to weigh your options. Continuing the example, let's assign the rest of the stats:

Engine A: Failure chance of 0.01%, fuel system masses 2 AMU and costs 3 ACU

Engine B: Failure chance of 0.015%, Mass of 1 AMU, Isp>x, Cost of 0.8 ACU.

Cluster: Fuel system masses 3 AMU . Fuel system costs 2 ACU. Fuel system reliability is figured into the engine failure chance. The cluster can still function with four of the 5 engines.

Final values:

Engine A: Mass: 7; Failure chance: 0.01%; Cost: 8 ACU; ISP=x;

Engine B: Mass: 8; Failure chance: 0.075%; Critical failure chance: 0.0045%; Cost: 6 ACU; ISP>x

 

[Bill Phil]

Having a large cluster also adds a bunch of extra plumbing, but can be beneficial.