Author Topic: Many smaller sized Raptor engines on future BFR's?  (Read 37058 times)

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #80 on: 09/19/2017 07:34 pm »
rsdrvis9,

Close, but not quite:

 - The combustion physics requires a certain dwell time in order to  efficiently mix and burn. This dwell time is captured by the parameter "Characteristic Length", L* = Vc/Ath. RP1/Lox L* = 102-127 cm. CH4/Lox is probably a little higher maybe around 150-160 cm. So knowing your chemistry gives you an L* which allows you to calculate a combustion chamber volume, Vc = L* x Ath; hence, Vc is proportional to Ath. If you look at rockets of different thrust, you will clearly see that lower thrust rockets have proportionally bigger combustion chambers and higher thrust rockets have smaller chambers. The same goes for the pre-burners. Also note that these components handle the highest pressures.

To summarize:

- For a given cycle, a rocket engine weight scales roughly with thrust because:
    - Throat area and mass flow are proportional to thrust.
    - The pre-burners, combustion chamber and plumbing are pressure vessels proportional to throat area.
    - The turbines and pumps are proportional to pressure and mass flow.
    - Most of the expansion nozzle contains low pressure, its weight is dominated by minimal unit wall weight proportional to nozzle area.

- A sphere 2x bigger has 4x the surface area and 8x the volume. At the same pressure,with the same material allowables it is 8x heavier, so the weight per unit volume stays the same. This is classic pressure vessel behavior.

John

John, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.

It is not an analogy. Its engineering. L* does depend on mixing details and may be lower for gas/gas mixing in the main combustion chamber. That does not invalidate the engineering approach.

Online Semmel

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #81 on: 09/19/2017 07:51 pm »
John, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.

It is not an analogy. Its engineering. L* does depend on mixing details and may be lower for gas/gas mixing in the main combustion chamber. That does not invalidate the engineering approach.

Sorry, didnt mean to be emphasize the term 'analogy'. I enjoy your engineering approach (not only here) quite a bit. I however wanted to point out that your point about the length of the cumbustion chamber actually should be reversed. Hence a higher T/W ratio and the balancing factors do not apply here.

Also, as ZachF pointed out, integrating the LOX preburner/turbine onto the head of the injector makes quite a lot of sense. I bet this design saved them quite a lot of grief.

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #82 on: 09/19/2017 08:28 pm »
John, your analogy doesnt work because Raptor has both gaseous Oxygen and gaseous Methane mixing. I bet that the combustion physics of Raptor is unlike any other engine because of this. It may very well be that this leads to a very high T/W ratio. At least more than you would expect in a liquid-liquid engine.

It is not an analogy. Its engineering. L* does depend on mixing details and may be lower for gas/gas mixing in the main combustion chamber. That does not invalidate the engineering approach.

Sorry, didnt mean to be emphasize the term 'analogy'. I enjoy your engineering approach (not only here) quite a bit. I however wanted to point out that your point about the length of the cumbustion chamber actually should be reversed. Hence a higher T/W ratio and the balancing factors do not apply here.

Also, as ZachF pointed out, integrating the LOX preburner/turbine onto the head of the injector makes quite a lot of sense. I bet this design saved them quite a lot of grief.

- My post addressed why T/W tends to stay constant when you scale a rocket engine. I agree that integrating the LOX pre-burner/turbine into the head of the main chamber should increase T/W. I also agree that gas/gas mixing and combustion might have a lower L* than liquid/liquid or liquid/gas, which would also increase T/W. These are the types of details that can get you from 160 - 200 T/W. They won't get you to 350. Either way, the T/W trend with thrust will continue to be relatively flat.

-  BTW, L* is not the length of the combustion chamber, it sets the volume of the combustion chamber:
volume of the combustion chamber is equal to L* times the throat area. L* depends both on the chemistry and the details of the injection and mixing.

John

« Last Edit: 09/19/2017 08:44 pm by livingjw »

Online Semmel

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #83 on: 09/19/2017 09:31 pm »
- My post addressed why T/W tends to stay constant when you scale a rocket engine. I agree that integrating the LOX pre-burner/turbine into the head of the main chamber should increase T/W. I also agree that gas/gas mixing and combustion might have a lower L* than liquid/liquid or liquid/gas, which would also increase T/W. These are the types of details that can get you from 160 - 200 T/W. They won't get you to 350. Either way, the T/W trend with thrust will continue to be relatively flat.

Thank you for giving that estimate. I guess we will find out when Elon makes his speech in a little over one week. But I would expect it to be better than Merlin 1D FT B5. Out of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.


Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #84 on: 09/19/2017 10:00 pm »
This was posted in the BE-4 thread by Yokem55, an engineering thought experiment on changing the RS-25 from FRSC to FFSC:

http://www.eaglehill.us/programs/journals/spaevo/2015a1/

I found it interesting and it's seems somewhat related to what we are talking about
« Last Edit: 09/19/2017 10:02 pm by ZachF »
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Offline MikeAtkinson

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #85 on: 09/19/2017 10:14 pm »
Out of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.

Rocket Propulsion Elements - Sutton

My copy is about 20 years old, but the 9th edition is Feb 2017.

Offline spacenut

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #86 on: 09/20/2017 12:11 am »
I think SpaceX will get a full scale Raptor at the 600k + lbs thrust engine.  Once they have this I say about 19 will keep them under 12 million lbs thrust the flame trench can handle at the Cape.  It will be Nova size in power, but being reusable, able to deliver 100-150 tons to LEO.  Expendible probably 200+ tons, but that will not happen. 

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #87 on: 09/20/2017 02:05 am »
Out of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.

Rocket Propulsion Elements - Sutton

My copy is about 20 years old, but the 9th edition is Feb 2017.

NASA SP-125 is still one of the most complete. I have attached the PDF (Don't you love the internet!). It covers theory to practical design details with lots of drawings and graphs. Couple this with NASA's online CEA program and you have a very good start. SP-125 mostly covers gas generator cycles. Your education won't be complete until you dig into the Russian staged combustion engines. They are truly phenomenal. In SP-125 a staged combustion cycle is referred to as dual combustion cycle. I had Sutton's book for an undergraduate course in 1971. It was OK, but not as much detail as in SP-125. SP-125 helped get us to the Moon.

Offline AncientU

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #88 on: 09/20/2017 01:30 pm »
Out of interest, can you suggest a good book on rocket engine engineering? I am not afraid of math.

Rocket Propulsion Elements - Sutton

My copy is about 20 years old, but the 9th edition is Feb 2017.

NASA SP-125 is still one of the most complete. I have attached the PDF (Don't you love the internet!). It covers theory to practical design details with lots of drawings and graphs. Couple this with NASA's online CEA program and you have a very good start. SP-125 mostly covers gas generator cycles. Your education won't be complete until you dig into the Russian staged combustion engines. They are truly phenomenal. In SP-125 a staged combustion cycle is referred to as dual combustion cycle. I had Sutton's book for an undergraduate course in 1971. It was OK, but not as much detail as in SP-125. SP-125 helped get us to the Moon.

A 1967 reference with a foreward by Wernher von Braun... telling.
« Last Edit: 09/20/2017 01:31 pm by AncientU »
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Offline guckyfan

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #89 on: 09/20/2017 04:43 pm »
I had the impresssion occasionally that developing rocket engines was a lost art in the US, reinvented only when SpaceX and BO appeared on the scene.

So many threads on potential new launch vehicles over the years and they always looked which engines are available off the shelf, not developing a new engine tailormade for the needs.

Offline AncientU

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #90 on: 09/20/2017 05:50 pm »
I had the impresssion occasionally that developing rocket engines was a lost art in the US, reinvented only when SpaceX and BO appeared on the scene.

So many threads on potential new launch vehicles over the years and they always looked which engines are available off the shelf, not developing a new engine tailormade for the needs.

Does seem to have been a very long time since US contributed to advancing the state-of-the-art in rocket propulsion.  Certainly Merlin's record-setting T/W and Raptor FFSC, T/W, fuel selection, reusable, air-startable, etc. (triggering the world-wide movement to Meth/LOX, reusable engines) seem to be contributions.  The pair of ORSC engines that are to competing to replace RD-180 appear to only be trying to recreate mature RD-180 technology (or less aggressive in Blue's case).
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Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #91 on: 09/20/2017 08:13 pm »
About the “chemistry” being a given for all engines with similar fuel system.
Burning methane is actually a few hundreds of different chemical reactions combined.
SpaceX is simulating the chemistry combined with the combustion physics in great detail and likely optimizing it also.
Optimizing could for instance be more complete combustion for higher Isp or minimizing dwell time for higher T/W.
This is something which was not possible before.


« Last Edit: 09/20/2017 09:29 pm by Peter.Colin »

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #92 on: 09/20/2017 09:37 pm »
Peter,

Yes, they are doing very good work. More to the point, SpaceX is modeling the mixing and combustion chemistry. Combustion is mostly limited by mixing, and mixing as the video shows is fractal crazy! This kind of work goes back decades, but is now advancing rapidly with the advent of affordable massively parallel computers. The combustion reactions themselves go back even further and are well characterized. I just now went to NASA's old Chemical Equilibrium Analysis, CEA sight and ran a Methane-Oxygen case. See attached. The chemical equilibrium assumption works very well for rocket chambers, not so well for nozzles, but we can approximate non-equilibrium effects well enough. I doubt we will see any surprises, just lots of great detail to guide the design and development.

John
« Last Edit: 09/20/2017 09:41 pm by livingjw »

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #93 on: 09/20/2017 10:06 pm »
Peter,

Yes, they are doing very good work. More to the point, SpaceX is modeling the mixing and combustion chemistry. Combustion is mostly limited by mixing, and mixing as the video shows is fractal crazy! This kind of work goes back decades, but is now advancing rapidly with the advent of affordable massively parallel computers. The combustion reactions themselves go back even further and are well characterized. I just now went to NASA's old Chemical Equilibrium Analysis, CEA sight and ran a Methane-Oxygen case. See attached. The chemical equilibrium assumption works very well for rocket chambers, not so well for nozzles, but we can approximate non-equilibrium effects well enough. I doubt we will see any surprises, just lots of great detail to guide the design and development.

John


Hi John,

I really enjoy and I’m amazed by the depth and diversity of your posts.
Rocket science really is; rocket science !
Thanks for sharing your expert view on things, really appreciate it!

Peter
« Last Edit: 09/20/2017 10:34 pm by Peter.Colin »

Offline Robotbeat

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #94 on: 09/20/2017 11:01 pm »
I had the impresssion occasionally that developing rocket engines was a lost art in the US, reinvented only when SpaceX and BO appeared on the scene.

So many threads on potential new launch vehicles over the years and they always looked which engines are available off the shelf, not developing a new engine tailormade for the needs.
Nah, not a lost art. Lots of funded development work on engines, but it was slow work that was extremely expensive without a firm requirement to actually be useful on an actual launch vehicle.

I mean, the Russians made cheap engines, we had good engines for Delta II and Delta IV, why bother developing more?

Also, there's a tendency of armchair spaceflight fans to piece together rockets from other vehicles because it's intellectually easier.
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Offline spacenut

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #95 on: 09/20/2017 11:13 pm »
Yeah, Atlas V could have used the H1 (RS-27) from the Delta II and made a 5 or 7 engine Atlas V instead of the Russian engines.  The RD-180 was cheap though.  Mueller even designed the TR-106 pintile injected engine when he was with another company, but it was shelved.  He used his knowledge to help SpaceX with Merlin.  Now Raptor.  The first test with the subscale Raptor was successful, so it shouldn't take long to get the larger one going. 

Offline TrevorMonty

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #96 on: 09/21/2017 09:30 am »
If SpaceX use tested if not flight ready subscale Raptor they could start designing and building ITSy now. In case of full scale Raptor everything is on hold till it is operational. This is new engine even if based heavily on subscale version, there are no guarantees development will go smoothly.


Offline jpo234

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #97 on: 09/21/2017 09:36 am »
Things can be done in parallel. You don't have to wait for the engines to pass qualification before you design the rest of the booster.
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #98 on: 09/21/2017 10:52 am »
They may choose to build BFS first since they have the subscale engine. They don't need BFR to fly the BFS.

John

Offline philw1776

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #99 on: 09/21/2017 11:20 am »
 Last year's plan at the I a C conference was build the BFS first
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