Author Topic: Many smaller sized Raptor engines on future BFR's?  (Read 11747 times)

Offline Peter.Colin

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Assuming the optimal Raptor engine size is smaller than previously thought by SpaceX. Scaling the Raptor engines up more than their optimal size will add unnecessary weight on the rocket. Because of the larger engine weight to engine trust ratio.

If the optimal engine size has already been reached with Raptor. This will lead to a trend that bigger rockets will have lots of relatively small engines at their base. Instead of a few big ones.

What would the maximum number of these smaller engines on larger future rockets be, and why?
And what would the maximum rocket dimensions be? (I guess rocket height is limited due to the maximum thrust per nozzle area)



« Last Edit: 09/17/2017 08:41 PM by Peter.Colin »

Offline Peter.Colin

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #1 on: 09/16/2017 08:21 AM »
Reaching optimal Raptor engine size, with a scalled down version may be the number one reason for scaling down the BFR while keeping the same engine layouts for both BFR and BFS.

Having an optimally performing rocket engine which is smaller than the competition, is a real strategic advantage!

I think it may lead other rocket companies to reverse course in engine size and number of engines, in a few years.
« Last Edit: 09/16/2017 08:56 AM by Peter.Colin »

Offline TomH

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #2 on: 09/16/2017 08:45 AM »
I don't think so. Scaling the dimensions of a rocket engine as not so simple as the dynamics within the combustion and expansion chambers behave differently. IMHO they are more likely to keep the same design and titrate the chamber pressure and thrust upward, like they did with Merlin.

Offline Peter.Colin

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #3 on: 09/16/2017 09:27 AM »
I don't think so. Scaling the dimensions of a rocket engine as not so simple as the dynamics within the combustion and expansion chambers behave differently. IMHO they are more likely to keep the same design and titrate the chamber pressure and thrust upward, like they did with Merlin.

Predicting the optimal size of an engine before the engine is built is the most difficult part.
Everything has to be taken into account including what you mentioned above.
I think SpaceX got lucky that the optimal engine size came out smaller in real life tests than predicted beforehand.
(E.g. Isp was measured higher than predicted for the small Raptor version)
Scaling down a Raptor is more easy than scaling up.
And of course they will try to increase the Isp to the theoretical maximum, like they did with Merlin.

« Last Edit: 09/16/2017 10:04 AM by Peter.Colin »

Offline RotoSequence

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #4 on: 09/16/2017 10:54 AM »
Predicting the optimal size of an engine before the engine is built is the most difficult part.
Everything has to be taken into account including what you mentioned above.
I think SpaceX got lucky that the optimal engine size came out smaller in real life tests than predicted beforehand.
(E.g. Isp was measured higher than predicted for the small Raptor version)

Link?

Offline Peter.Colin

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #5 on: 09/16/2017 12:25 PM »
Predicting the optimal size of an engine before the engine is built is the most difficult part.
Everything has to be taken into account including what you mentioned above.
I think SpaceX got lucky that the optimal engine size came out smaller in real life tests than predicted beforehand.
(E.g. Isp was measured higher than predicted for the small Raptor version)

Link?

For instance:

http://www.thespaceshow.com/show/22-jun-2017/broadcast-2934-ms.-gwynne-shotwell

(@ 40.08)

Offline livingjw

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Do you believe that the thrust to weight and ISP are heavily dependent on the thrust level? I can assure you they are not. The proper size for a rocket has way more to do with mission requirements, flight rates, development and production costs. If I have misunderstood your position, my apologies.

John
« Last Edit: 09/16/2017 03:18 PM by livingjw »

Offline RedLineTrain

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Re: Maximum number of Raptor engines on future BFR's?
« Reply #7 on: 09/16/2017 11:42 PM »
Predicting the optimal size of an engine before the engine is built is the most difficult part.
Everything has to be taken into account including what you mentioned above.
I think SpaceX got lucky that the optimal engine size came out smaller in real life tests than predicted beforehand.
(E.g. Isp was measured higher than predicted for the small Raptor version)

Link?

For instance:

http://www.thespaceshow.com/show/22-jun-2017/broadcast-2934-ms.-gwynne-shotwell

(@ 40.08)

I assume what was requested was a link to the source for "Isp was measured higher than predicted for the small Raptor version."  Shotwell didn't say anything like this at 40.08.

Offline Peter.Colin

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I don't have any info on that it's just an educated guess/ speculation.

She said they don't exactly know the optimal Raptor size yet. But it will be closer than 2 times the thrust than 3 times the thrust as the small scale 100 ton (=1000kN) version.
The sea-level thrust Raptor anounced at IAC2016 had an envisioned thrust of 3050kN.

So a new optimal 2000KN version is most likely smaller than the envisioned 3050kN version.
What happened?, did't they think it through enough?, was the modeling too worst case ? I can only suspect the 1000KN performed more optimal with higher ISP than expected.
So that a 3050kN version had become sub-optimal.

If that is the case you can either put more 2000kN engines on the 12meter ITS version.
Or scale the rocket down proportionally and keep the same nummer of engines with the same engine layout.

In this case I really do believe optimal engine size and optimal engine layout precede rocket diameter, in the design-choice process. And not the other way around.

Probably a 2000KN Raptor is the same size or even smaller than a Merlin 1D engine.
An around 12 meter diameter BFR rocket will have around 70, 2000kN engines...
An 18 meter Diameter rocket towards 200 engines?

We will know in 2 weeks at the IAC 2017.
I'm hoping to see a surprise 😃


« Last Edit: 09/17/2017 09:49 PM by Peter.Colin »

Offline Peter.Colin

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Do you believe that the thrust to weight and ISP are heavily dependent on the thrust level? I can assure you they are not. The proper size for a rocket has way more to do with mission requirements, flight rates, development and production costs. If I have misunderstood your position, my apologies.

John

Hi John,

No apologies needed!

I do believe that, advanced rockets engines with good Isp can best be small and many:
They have the best engine weight to thrust ratio.


This guy explains it quite nicely (@1.43)



« Last Edit: 09/17/2017 12:03 PM by Peter.Colin »

Offline OneSpeed

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #10 on: 09/17/2017 12:30 PM »
... Probably a 2000KN Raptor is the same size or even smaller than a Merlin 1D engine.
An around 12 meter diameter BFR rocket will have around 75, 2000kN engines...
...

Gwynne actually said 'by a factor of 2, up to a factor of 3' times the 1000kN Raptor is optimal.
Splitting the difference, let's say 2.5 times is optimal.

The 1000kN Raptor is about the same diameter as a Merlin 1D, i.e. 0.89m.
The 3050kN Raptor is 1.51m.

Thrust is proportional to nozzle area, so for 2500kN, the diameter would be about 1.37m, much larger than Merlin 1D.
A 0.75 (9m) scale model of BFR would have 128MN * 0.422 = 54MN thrust.
54MN / 2500kN = 21.6 engines, let's round it down to 21.

For the 12m BFR it would be 128MN / 2500kN = 51.2, say 48 engines.

Both configurations provide excellent packing geometry, and could look something like this:

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #11 on: 09/17/2017 01:00 PM »
... Probably a 2000KN Raptor is the same size or even smaller than a Merlin 1D engine.
An around 12 meter diameter BFR rocket will have around 75, 2000kN engines...
...

Gwynne actually said 'by a factor of 2, up to a factor of 3' times the 1000kN Raptor is optimal.
Splitting the difference, let's say 2.5 times is optimal.

The 1000kN Raptor is about the same diameter as a Merlin 1D, i.e. 0.89m.
The 3050kN Raptor is 1.51m.

Thrust is proportional to nozzle area, so for 2500kN, the diameter would be about 1.37m, much larger than Merlin 1D.
A 0.75 (9m) scale model of BFR would have 128MN * 0.422 = 54MN thrust.
54MN / 2500kN = 21.6 engines, let's round it down to 21.

For the 12m BFR it would be 128MN / 2500kN = 51.2, say 48 engines.

Both configurations provide excellent packing geometry, and could look something like this:


Why would a scale model only have 54MN of thrust ?
You assume the height of the rocket will be 0,75 times less tall also? (128MN *0,5625= 72MN)
If more engines can be packed at the base it's more logical to make the rocket higher and have more payload.
The packing geometry is really nice! But the packing density is worse than the original.
There is also no gimballing center cluster like the original, which isn't needed because in the first 21 engine layout probably all engines can gimbal.

I don't believe the 1000KN scale model was 0,89 m it's more towards 0,55 meter.
Elon Musk refered at IAC 2016 to the Raptor engines being same size as Merlin 1D but having 3 times the thrust.
The scalled model has similar thrust as Merlin 1D so a third of the nozzle area.

How many raptor engines would you put on a Falcon 9, if 9 would fit?
I wouldn't calculate what number is needed for the current specs, but aim for higher payloads.
For the a larger 10,5-12 meter BFR I'm guessing more like 70 engines with following layout:





« Last Edit: 09/17/2017 07:14 PM by Peter.Colin »

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #12 on: 09/17/2017 02:46 PM »
Engine nos. getting crazy here. Far too many highly stressed parts that could potentially RUD. Also even if no engines fail during a mission, the maintenance costs between flights will be higher with all those engines to check. Anything more than around 20 engines is really pushing it and hopefully EM will keep the engine no. the same from mini-BFR to the future larger versions by dev. larger versions of Raptor. Don't need the absolute best TWR from a booster engine since it's job is to just get the launch stack out of Earth's atm. It's the US engines that need the best possible TWR.

Online docmordrid

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #13 on: 09/17/2017 03:02 PM »
If I remember correctly, people were saying the same thing about a 9 engine cluster around 2008-2010.
DM

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #14 on: 09/17/2017 03:06 PM »
Engine nos. getting crazy here. Far too many highly stressed parts that could potentially RUD. Also even if no engines fail during a mission, the maintenance costs between flights will be higher with all those engines to check. Anything more than around 20 engines is really pushing it and hopefully EM will keep the engine no. the same from mini-BFR to the future larger versions by dev. larger versions of Raptor. Don't need the absolute best TWR from a booster engine since it's job is to just get the launch stack out of Earth's atm. It's the US engines that need the best possible TWR.

BFR is designed so that multiple engines can fail.
70 engines is not that many/unrealistic if they currently can produce 300 Merlin 1D engines a year.


Here is a speculative BFR rocket with 105 engines, this is the one that could come after the 70 engine version by adding another ring. I think either this version or the next should have a bigger gimballing center cluster to remain steerable.
I'll try if a more closely packed engine layout is possible for bigger BFR's.
« Last Edit: 09/17/2017 10:45 PM by Peter.Colin »

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #15 on: 09/17/2017 07:26 PM »
Peter,

-  Engine T/W does not go up as thrust goes down. It just doesn't. Engine T/W tends to be about constant from 100K lbs to 1M lbs thrust class.   ISP isn't much effected by thrust either.

- You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.

John
« Last Edit: 09/17/2017 07:38 PM by livingjw »

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #16 on: 09/17/2017 07:33 PM »
I believe that Space X will develop a larger engine for the full scale, 12M, BFR. FWIW, 48 engines just seems way to many.

John

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #17 on: 09/17/2017 07:43 PM »
Peter,

-  Engine T/W does not go up as thrust goes down. It just doesn't. T/W tends to be about constant from 100K lbs to 1M lbs thrust class.   ISP isn't much effected by thrust either.

- You also seem to be obsessed with filling the base of the vehicle with rocket exhaust. You want to find the optimum vehicle thrust to weight (usually around 1.25). Filling the base is not that important.

John




Hi John,

- For other rockets engines you're probably right, for the Raptor I think it's different, and to my opinion an advantage that will leave other rocket companies like Blue Origin behind in the dust.

- I'm obsessed with filling the base yes, because I know my rocket can be more heavy/higher.
Rocket weight = combined thrust devided by 1,25.

« Last Edit: 09/17/2017 07:49 PM by Peter.Colin »

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #18 on: 09/17/2017 07:43 PM »
I believe that Space X will develop a larger engine for the full scale, 12M, BFR. FWIW, 48 engines just seems way to many.

John
I fully agree with you in that SpaceX will need a scaled up Raptor for future 12m dia. or whatever BFR's to keep the engine no. to not more than around 20 which mini-BFR is likely to have. Use mini-BFR design as a template for future larger BFR's and scale up engines accordingly. Servicing 40+ engines after each mission will be a nightmare and will increase time between missions.

Offline TomH

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #19 on: 09/17/2017 07:45 PM »
Peter,.......You also seem to be obsessed with filling the base of the vehicle with rocket exhaust.......

Obsessed is right. Too much armchair amateur rocket designing based on personal belief rather than on science, engineering, and technological understanding.

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