Falcon Heavy has too many engines.

[fredinno]

I think the Falcon Heavy has too many engines for it's own good and need to be replaced (along with the Falcon 9 later, for redundency) with a modernized F-1 (The maker, Rocketyne, who has recently also discussed reviving the F-1 as well, but for the SLS boosters). Despite the advantages when it comes to rocket reusability (allowing for better throttling with only part of the engines used for landing). It may also be cheaper, and easier (engineering-wise, one of the reasons the S-IV on Saturn IB became S-IVB), overall, to use a single large engine, rather than many small ones.

Additionally, despite the fact that F-1 has lower ISP than Merlin, a modernized version could probably be made that would provide the extra ~20s of ISP needed to roughly match the Merlin's ISP. What do you guys think?

Edited July 29, 2015 by fredinno

[AngelLestat]

So you are mention that they should forgot of reusability and redundancy?

Also not sure how the pogo is increase by the number of engines.. you have some link to provide?

[SargeRho]

The reason they use 9 small instead of one or two large engines is redundancy. If your only engine fails, your rocket is ....ed. If one of nine engines fail, you might not be able to land the rocket, but it'll still reach orbit.

[Aanker]

The reason they use 9 small instead of one or two large engines is redundancy. If your only engine fails, your rocket is ....ed. If one of nine engines fail, you might not be able to land the rocket, but it'll still reach orbit.

Or you can hit spacebar and hope the second stage makes it on its own! Oh wait...

[fredinno]

So you are mention that they should forgot of reusability and redundancy?

Also not sure how the pogo is increase by the number of engines.. you have some link to provide?

Whoops, what was I thinking. I was writing the forum post rushed. I meant that it becomes more difficult to make each rocket with more engines, and that there are (generally) more vibration problems. The N-1 had 30 engines and had a similar problem. (though it does have 3 more engines).

Generally, the more engines you have, the more pain in the ass it is (though bigger engines also have a this problem, though to a lesser extent).

Source: https://en.wikipedia.org/wiki/N1_%28rocket%29#Development_problems

http://space.stackexchange.com/questions/5400/quality-vs-quantity-for-rocket-engines/5401#5401

Edited July 29, 2015 by fredinno

[Mazon Del]

I'm not an expert admittedly, but I have never heard of a connection between number of engines and pogo-ing. That said though, from what I have heard, the engineering challenges involved with pogo oscillation are not as significant as the challenges in designing what amounts to a Falcon 9 sized rocket engine. There are quite a few advantages to using the 3x3 array of engines that the F9 has. Even though it already has thrust vectoring gimbals, you can increase thrust on the others individually for a variety of purposes. A great example is in the rare event that one of the engines does fail, you can adjust the throttles on the remaining 8 to compensate for the uneven distribution of thrust. This occurred on one of the earlier F9 flights. Additionally, the F9 has baffles/bulkheads (not certain of correct term) separating the engines such that if one were to be damaged through a variety of the most likely sources, the damage is unlikely to cascade into the other engines. In these situations, multiple engines instead of one big one is considered a good thing, even in the face of increased likelihood of a problem from more engines.

Furthermore, the engines on the F9 are quite robust devices. One of the reasons the N1 was unlikely to fly successfully was that all of the 30 NK-15 engines were designed to run "optimally" when riding the edge of part failure. Those engines just couldn't handle too much of a bump in performance before getting damaged. The engines on the F9 have quite a lot more leeway in its design. Don't forget, Apollo had 9 engines as well. It also suffered an engine explosion on one of the flights, but was able to compensate.

Sure, the Falcon heavy will have 27 engines on it, but being effectively 3 F9s strapped together it doesn't seem THAT much more problematic.

Plus, in an amusingly weird way. Depending on how things in the future work for the F-Heavy, some of those engines might be capture-able in orbit. Meaning every launch of something adds 9 more engines that can be strapped to orbitally constructed space ships, probes, stations.

[SargeRho]

Apollo didn't have 9 engines. The Saturn V has 5 engines in the first, 5 in the 2nd and 1 in the third stage. Also the CSM's engine, and the two engines on the LEM.

[fredinno]

I'm not an expert admittedly, but I have never heard of a connection between number of engines and pogo-ing. That said though, from what I have heard, the engineering challenges involved with pogo oscillation are not as significant as the challenges in designing what amounts to a Falcon 9 sized rocket engine. There are quite a few advantages to using the 3x3 array of engines that the F9 has. Even though it already has thrust vectoring gimbals, you can increase thrust on the others individually for a variety of purposes. A great example is in the rare event that one of the engines does fail, you can adjust the throttles on the remaining 8 to compensate for the uneven distribution of thrust. This occurred on one of the earlier F9 flights. Additionally, the F9 has baffles/bulkheads (not certain of correct term) separating the engines such that if one were to be damaged through a variety of the most likely sources, the damage is unlikely to cascade into the other engines. In these situations, multiple engines instead of one big one is considered a good thing, even in the face of increased likelihood of a problem from more engines.

Furthermore, the engines on the F9 are quite robust devices. One of the reasons the N1 was unlikely to fly successfully was that all of the 30 NK-15 engines were designed to run "optimally" when riding the edge of part failure. Those engines just couldn't handle too much of a bump in performance before getting damaged. The engines on the F9 have quite a lot more leeway in its design. Don't forget, Apollo had 9 engines as well. It also suffered an engine explosion on one of the flights, but was able to compensate.

Sure, the Falcon heavy will have 27 engines on it, but being effectively 3 F9s strapped together it doesn't seem THAT much more problematic.

Plus, in an amusingly weird way. Depending on how things in the future work for the F-Heavy, some of those engines might be capture-able in orbit. Meaning every launch of something adds 9 more engines that can be strapped to orbitally constructed space ships, probes, stations.

Apollo had max. 8 engines firing at once, and on the Saturn IB. The S-IB stages were never clustered together like the Falcon 9 1st stages are on the Falcon Heavy. Still, the engineering challenges to try to keep the Merlins from ripping itself apart due to increasing back pressure and vibration increases are probably problematic.

http://space.stackexchange.com/questions/7841/why-does-the-falcon-9-have-9-engines

[Bill Phil]

Whoops, what was I thinking. I was writing the forum post rushed. I meant that it becomes more difficult to make each rocket with more engines, and that there are (generally) more vibration problems. The N-1 had 30 engines and had a similar problem. (though it does have 3 more engines).

Generally, the more engines you have, the more pain in the ass it is (though bigger engines also have a this problem, though to a lesser extent).

Source: https://en.wikipedia.org/wiki/N1_%28rocket%29#Development_problems

http://space.stackexchange.com/questions/5400/quality-vs-quantity-for-rocket-engines/5401#5401

It can actually be easier to build the rocket, since you can use already built engines and not make new designs.

The N-1 failed for more reasons than the number of engines. For example, the system controlling the engines had a glitch, which ended up shutting off all engines. That's the control system, not the engines themselves.

Correlation does not imply causation.

[Thomassino]

27 Merlins combined have thrust of 17658kN, mass of 12,69 tonnes and ISP of 282s. 3 F-1s have thrust of 20310kN, mass of 25,2 tonnes (almost double that of Merlins) and ISP of 263s. So F-1s would be a little overpowered and inefficient. Also, F-1s are not throttleable and not reignitable (crucial abilities for stage recovery), not mentioning tremendous refurbishment and innovation costs, so no, it won't be viable.

[AngelLestat]

As you mention, one of the biggest reasons is that to land an emptly stage back on earth, you need very little thrust, you can not set the thrust so low with single big engines, from my understanding is not only hard, is super hard.

In this case, only the center core engine ignites to land.

Edited July 29, 2015 by AngelLestat

[magnemoe]

It can actually be easier to build the rocket, since you can use already built engines and not make new designs.

The N-1 failed for more reasons than the number of engines. For example, the system controlling the engines had a glitch, which ended up shutting off all engines. That's the control system, not the engines themselves.

Correlation does not imply causation.

Yes and control systems are a bit more advanced today than in 1970 Soviet

[Nothalogh]

If you really hate the Merlin, just wait until they get the Raptor engine flying, it's in ground testing right now

[fredinno]

I don't hate the Merlin. I'm concerned about the consequences of using 27x Merlins on a Falcon Heavy.

[Kryten]

It isn't likely to fly all that much anyway, so failures aren't too likely. With current sat designs and SpaceX's unwillingness to do Ariane-style dual launch, it's very expensive for what it would be putting up. If you take a look at the actual current FH manifest, most of the payloads could have gone on Proton for a good amount less

[Frozen_Heart]

Not going to happen. The reason for each core having 9 engines is for engine out capability and for landing the rocket. Also using 10 of the same engine allows mass production which brings price down.

Having a single large engine on the core would get rid of all of these advantages.

[AngelLestat]

I personally hate 9 engines by core stage, because in my KSP falcon heavy imitation, implies a lot of lag XD

[Nibb31]

There are three advantages of having 9 engines on each core:

- Engine out capability. The Falcon 9 can make orbit with one or two engine failures.

- Deep throttlability. Large rocket engines can't throttle down to 10%, so instead you only burn with 1 engine out of 9.

- Economies of scale. It's cheaper to mass produce a single engine for all stages rather than two different engines for first and upper stage.

- - - Updated - - -

There are three advantages of having 9 engines on each core:

- Engine out capability. The Falcon 9 can make orbit with one or two engine failures.

- Deep throttlability. Large rocket engines can't throttle down to 10%, so instead you only burn with 1 engine out of 9.

- Economies of scale. It's cheaper to mass produce a single engine for all stages rather than two different engines for first and upper stage.

The OP mentions the N1 rocket, but none of the failures of the N1 were due to having a large number of engines.

[Kibble]

The OP mentions the N1 rocket, but none of the failures of the N1 were due to having a large number of engines.

At least the first failure was caused by the difficulty controlling so many rocket engines.

Also there's a fourth advantage to having several motors - each launch you get much more flight time and data to evaluate performance.

Edited July 29, 2015 by Kibble

[Dkmdlb]

It's difficult for someone not spending the money to make a persuasive case on cost against the people who are spending the money.

Do you think they have looked into using few larger engines? If yes, what do you think they found? If no, why don't you think they have investigated it?

[CptRichardson]

I think the Falcon Heavy has too many engines for it's own good and need to be replaced (along with the Falcon 9 later, for redundency) with a modernized F-1 (The maker, Rocketyne, who has recently also discussed reviving the F-1 as well, but for the SLS boosters). Despite the advantages when it comes to rocket reusability (allowing for better throttling with only part of the engines used for landing). It may also be cheaper, and easier (engineering-wise, one of the reasons the S-IV on Saturn IB became S-IVB), overall, to use a single large engine, rather than many small ones.

Additionally, despite the fact that F-1 has lower ISP than Merlin, a modernized version could probably be made that would provide the extra ~20s of ISP needed to roughly match the Merlin's ISP. What do you guys think?

Uh, they're PLANNING on reducing the number of engines once they get the Raptor engine online.

[fredinno]

Uh, they're PLANNING on reducing the number of engines once they get the Raptor engine online.

But the Raptor runs on CH4, not RP-1. Falcon rockets would pretty much have to be redesigned, no?

[SargeRho]

As far as I remember (I'll see if I can find the source later), there were plans for a Raptor upper stage, but I don't remember anything about Raptor first stages for anything but the MCT/BFR.

[Nothalogh]

But the Raptor runs on CH4, not RP-1. Falcon rockets would pretty much have to be redesigned, no?

Yes, Falcon Heavy is mostly a stopgap solution while the Raptor and its airframe are developed

[AngelLestat]

Uh, they're PLANNING on reducing the number of engines once they get the Raptor engine online.

Where do you hear that?

The raptor engine is for the new type of massive launcher that is in current design.

Not for the falcon9 or falcon heavy. neither the second stage.