Author Topic: multiple combustion chamber Merlin  (Read 19356 times)

Offline krytek

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multiple combustion chamber Merlin
« on: 10/28/2011 07:24 pm »
I had this interesting idea recently while recalling Elon's admiration for Russian tech.

Considering the huge amount of capital needed to design, test and build bigger engines (like the 1 billion figure Musk claimed for Merlin 2), is it possible to use a cluster of existing engines, like the Merlin 1d, with a shared turbopump (e.g RD-170) to solve some of those issues?

My reasoning against a Merlin 2 -

1) High development cost.
2) High amount of new tooling and process.
3) Transportation difficulties.
4) Can not be used for a reusable falcon 9.
5) Lower production rate.
6) ?

Can a Merlin 1d be used in such a configuration?
What are the main changes which will need to be done for such a conversion?
What advantages could such an engine offer against a bigger F-1 class engine?


Couldn't decide if to put this under Advanced concepts or SpaceX,
please move if needed.

Offline iamlucky13

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Re: multiple combustion chamber Merlin
« Reply #1 on: 10/28/2011 07:43 pm »
It really wouldn't be a Merlin 1 anymore if the only common components were the pintle, chamber, and nozzle. The turbopumps be new, although pretty similar for either a single chamber or multi-chamber. Yes, you'd hypothetically save a few new parts and probably some headaches achieving stable combustion in a new chamber, but there are some downsides, like the more complex plumbing.

An F-1 sized engine is transportable as an oversized load, or on some cargo aircraft, but I don't think what you're suggesting would really be much smaller, just easier to break down for moving (remove the nozzles and either one gets a lot more compact).

I imagine what Elon admires about the Russian engines is more the high thrust-to-weight ratio, throttling ability, and the high efficiency than the multi-chamber design. Personally, I'm a little in awe of the fact that they created a successful design that runs oxygen rich combustion products from the preburner through the turbopump. High temperature, high pressure, nearly pure oxygen is an incredibly corrosive fluid - you can burn steel that way. I don't mean melting steel, but actually chemically combusting it.

Offline Downix

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Re: multiple combustion chamber Merlin
« Reply #2 on: 10/28/2011 08:13 pm »
Personally, I'm a little in awe of the fact that they created a successful design that runs oxygen rich combustion products from the preburner through the turbopump. High temperature, high pressure, nearly pure oxygen is an incredibly corrosive fluid - you can burn steel that way. I don't mean melting steel, but actually chemically combusting it.

*pst* it's not oxygen-rich.
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Offline TrueBlueWitt

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Re: multiple combustion chamber Merlin
« Reply #3 on: 10/28/2011 08:23 pm »
Personally, I'm a little in awe of the fact that they created a successful design that runs oxygen rich combustion products from the preburner through the turbopump. High temperature, high pressure, nearly pure oxygen is an incredibly corrosive fluid - you can burn steel that way. I don't mean melting steel, but actually chemically combusting it.

*pst* it's not oxygen-rich.

By "they" I'm guessing he means the Russians.. not SpaceX?

Offline krytek

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Re: multiple combustion chamber Merlin
« Reply #4 on: 10/28/2011 08:29 pm »
Yes, SpaceX does not have oxygen rich engines.

Offline iamlucky13

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Re: multiple combustion chamber Merlin
« Reply #5 on: 10/28/2011 08:45 pm »
Personally, I'm a little in awe of the fact that they created a successful design that runs oxygen rich combustion products from the preburner through the turbopump. High temperature, high pressure, nearly pure oxygen is an incredibly corrosive fluid - you can burn steel that way. I don't mean melting steel, but actually chemically combusting it.

*pst* it's not oxygen-rich.

By "they" I'm guessing he means the Russians.. not SpaceX?

That would be correct.

Offline baldusi

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Re: multiple combustion chamber Merlin
« Reply #6 on: 10/28/2011 08:57 pm »
I think it might make sense to do that with four AJ-500 engines. But not with the Merlin 1. First, it's not big enough. Just compare the 667kN of the 1D, against the around 2MN of the AJ-500.
Second, as has been stated, the Merlin is not a very efficient engine. I personally think they are going to replace it with something new.
And third, the problem is not only the combustion chamber for deep throttling, but also the turbopump. So, it would still complicate the landing of a Returnable Falcon 9.

Offline Lars_J

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Re: multiple combustion chamber Merlin
« Reply #7 on: 10/28/2011 09:26 pm »
The Soviets only started doing multiple combustion chambers because they had problems with larger combustion chambers.

But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

With modern experience and computational tools, I don't think larger combustion chambers are as much of a problem as it has been. That path is a dead end, IMO.
« Last Edit: 10/28/2011 09:32 pm by Lars_J »

Offline Lars_J

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Re: multiple combustion chamber Merlin
« Reply #8 on: 10/28/2011 09:29 pm »
I think it might make sense to do that with four AJ-500 engines. But not with the Merlin 1. First, it's not big enough. Just compare the 667kN of the 1D, against the around 2MN of the AJ-500.
Second, as has been stated, the Merlin is not a very efficient engine. I personally think they are going to replace it with something new.
And third, the problem is not only the combustion chamber for deep throttling, but also the turbopump. So, it would still complicate the landing of a Returnable Falcon 9.

Yeah, we get it, you don't like Merlin. (Quite obvious from your post history)  ;) But it is primarily a first stage engine, where its brute force (in numbers) and cost outweighs its lack of efficiency. At some point they will make a larger Merlin engine for 1st stages - but don't expect it to be a radically different design. The only question IMO is if it will be "Merlin 2"-sized, or smaller.
« Last Edit: 10/28/2011 09:30 pm by Lars_J »

Offline sdsds

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Re: multiple combustion chamber Merlin
« Reply #9 on: 10/28/2011 09:38 pm »
The Soviets only started doing multiple combustion chambers because they had problems with larger combustion chambers.

But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

With modern experience and computational tools, I don't think larger combustion chambers are as much of a problem as it has been. That path is a dead end, IMO.

Even with the most excellent computational fluid dynamics simulations I haven't seen proof that stable combustion can be achieved in arbitrarily large combustion chambers.  Has anyone?
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Offline Jim

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Re: multiple combustion chamber Merlin
« Reply #10 on: 10/28/2011 09:44 pm »
The Soviets only started doing multiple combustion chambers because they had problems with larger combustion chambers.

But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

With modern experience and computational tools, I don't think larger combustion chambers are as much of a problem as it has been. That path is a dead end, IMO.

Not true, each of those engines have equivilent reduction (1/2) in thrust also.

Offline iamlucky13

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Re: multiple combustion chamber Merlin
« Reply #11 on: 10/28/2011 10:15 pm »
The Soviets only started doing multiple combustion chambers because they had problems with larger combustion chambers.

But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

With modern experience and computational tools, I don't think larger combustion chambers are as much of a problem as it has been. That path is a dead end, IMO.

Even with the most excellent computational fluid dynamics simulations I haven't seen proof that stable combustion can be achieved in arbitrarily large combustion chambers.  Has anyone?

I tend not to think it matters. We're not seriously talking about arbitrarily large combustion chambers. We're talking about a particular concept that is loosely defined but notionally about the same size as the largest single-chamber engine that's successfully flow so far. We know the F-1 worked.

So assuming combustion stability is reasonably achievable for SpaceX, we're still basically looking at manufacturing difficulty of big chambers, injectors, and bells versus dealing with more plumbing and parts.

Offline Lars_J

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Re: multiple combustion chamber Merlin
« Reply #12 on: 10/28/2011 10:34 pm »
But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

Not true, each of those engines have equivilent reduction (1/2) in thrust also.

Yes, but their new LV's (Angara) are moving towards the use of clustered boosters with RD-191's instead of fewer RD-180 or RD-17X. There are several reasons for that - my point is simply that the Russians are gradually moving away from the use multiple combustion chambers in their in-development or proposed launchers. (Rus with RD-180 was cancelled)
« Last Edit: 10/28/2011 10:35 pm by Lars_J »

Offline pippin

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Re: multiple combustion chamber Merlin
« Reply #13 on: 10/28/2011 11:05 pm »
We know the F-1 worked.

Yes. But we also know that that particular engine was a hell of a mess to make work exactly for combustion stability issues. I'm not sure you want to/can affort to go through the trial-and-error cycles they went through to make the F1 work and you can't reliably say it wasn't sheer luck in  the end...

OK, you know much more about fluid dynamics these days and you can do simulations but on the other hand the F1 was a pretty conservative engine design and not really efficient, it was pretty much just huge.

The reason the RD-18x and RD-19x engines use fewer combustion chambers is simply the fact that the RD-17x has way too much thrust for the modular LV designs you want to do today. If you need the thrust of the RD-17x/RD-18x engines they are generally a good choice because you can even get roll control out of a single engine.

The RD-17x and derivatives are probably the best (from a performance/efficiency perspective) kerolox engines ever to be built in that size class and part of why they are is probably the choice to NOT go with a huge combustion chamber but several smaller ones. I'm not sure you could build an engine like the RD-17x with a single combustion chamber.

Oh. And let's not forget that multiple-cobustion-chamber engines can be much smaller as well.
« Last Edit: 10/28/2011 11:10 pm by pippin »

Offline strangequark

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Re: multiple combustion chamber Merlin
« Reply #14 on: 10/28/2011 11:24 pm »
The Soviets only started doing multiple combustion chambers because they had problems with larger combustion chambers.

But the recent trend for the Russians has been fewer combustion chambers. See the RD-170 (4) -> RD-180 (2) -> RD-191 (1) evolution of the same engine family.

With modern experience and computational tools, I don't think larger combustion chambers are as much of a problem as it has been. That path is a dead end, IMO.

Even with the most excellent computational fluid dynamics simulations I haven't seen proof that stable combustion can be achieved in arbitrarily large combustion chambers.  Has anyone?

No, and I did a project for my M.S. on thermoacoustics a few months back. The complexity involved is truly astonishing. Lots of variables, and the potential for a lot of sensitivity to minor variations in those variables. We're better than we were in the 50-60s when FIRST was initiated for the F-1, but it's still very uncertain territory. Computational thermoacoustics is a young branch of computational acoustics, which is a young offshoot of CFD (which itself is barely old enough to collect Social Security).

There are some solid arguments for multi-chamber engines once you exceed a certain thrust level. If you can have a vastly simplified test program, the sacrifices can be well worth it.
« Last Edit: 10/28/2011 11:25 pm by strangequark »

Offline kevin-rf

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Re: multiple combustion chamber Merlin
« Reply #15 on: 10/28/2011 11:49 pm »
Of course the elephant in the room that is not being talked about is a larger turbo-pump is not a "simple" thing either...which seems to be the premise of this thread.
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Offline krytek

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Re: multiple combustion chamber Merlin
« Reply #16 on: 10/29/2011 12:26 am »
I agree a rocket turbopump is no simple matter (especially the art pieces they call turbopumps SpaceX uses). But you would need to design a new one anyway if you're building a new bigger engine.


The premise of this thread was more about the economic and technical feasibility of developing a larger engine why maximizing commonality with existing hardware and current possible uses.


Offline sdsds

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Re: multiple combustion chamber Merlin
« Reply #17 on: 10/29/2011 01:22 am »
The premise of this thread was more about the economic and technical feasibility of developing a larger engine why maximizing commonality with existing hardware and current possible uses.

Your initial intuition seems good.  In particular, consider the engine developer that has considerable experience with the behavior of a particular combustion chamber at different inlet conditions and thus has considerable confidence about how it will behave under novel conditions.  A cluster of those chambers would seemingly present well defined inlet condition target ranges for the designers of the new turbomachinery.

(As for myself, my fears about multi-chamber turbomachinery can be summed up in a single word.  "Ducts."  Since Monty Python is frequently mentioned here, I think a reference to the Terry Gilliam-directed movie Brazil is not totally out of line.)
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Offline Lars_J

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Re: multiple combustion chamber Merlin
« Reply #18 on: 10/29/2011 01:43 am »
Realistically, though, if a multi-chamber rocket design is pursued - SpaceX would design a larger thrust chamber anyway.

Unless you propose a 9 chamber design with the current Merlin 1D thrust chamber... :) Which would be pointless, since we know the 9 engines do work together.

And since SpaceX was confident enough to put 9 turbo pumps in a stage, I don't see why they would ever feel the urgent need to reduce to only one turbo pump. It's just not going to happen, with good reason.

Offline Jim

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Re: multiple combustion chamber Merlin
« Reply #19 on: 10/29/2011 01:48 am »
I agree a rocket turbopump is no simple matter (especially the art pieces they call turbopumps SpaceX uses).


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