Author Topic: Many smaller sized Raptor engines on future BFR's?  (Read 14108 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.

Offline 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.

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #20 on: 09/17/2017 07:47 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 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 morI e heavy/higher.
Rocket weight = combined thrust devided by 1,25.

I bet BO will have a more sensible design for NA with fewer large engines while SpX goes for the N-1 approach for their BFR. Hopefully SpX will think twice about putting more than around 20 engines on future larger BFR's.

Offline TomH

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #21 on: 09/17/2017 07:50 PM »
.....I know my rocket........

This is the problem. It is not my rocket, your rocket, or our rocket. It is SpaceX' rocket.

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #22 on: 09/17/2017 07:52 PM »
If I remember correctly, people were saying the same thing about a 9 engine cluster around 2008-2010.
7-9 is the optimum no. of engines for 1st stage giving the best balance between engine out capability and risk of catastrophic engine RUD causing LOM. Highly unlikely SpX will design any BFR with 9 engines as they may be looking at around 20 engines for mini-BFR. 20 is just about acceptable for a 1st stage and not any more so hopefully SpX will keep this no. for future larger BFR's by scaling up Raptor.

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #23 on: 09/17/2017 08:02 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
I would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines =  less plumbing on the bottom of the BFR.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #24 on: 09/17/2017 08:10 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
I would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines =  less plumbing on the bottom of the BFR.


As a Chemical engineer I can say that this is not true. The smaller the plumbing is the less thick the plumbing wall has to be to withstand the same pressure.
A rocket engine applies similar principles as some chemical reactors actually.

http://4wings.com/lib/files/tubing.pdf

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

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #25 on: 09/17/2017 08:10 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
I would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines =  less plumbing on the bottom of the BFR.

No, wall thickness is proportional to diameter. Once you reach a certain minimum size TWR does not change much with thrust.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #26 on: 09/17/2017 08:23 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.

Hi TomH,

You should point to where my technological understanding is flawed, so I can be better informed...  ;)

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #27 on: 09/17/2017 08:24 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
I would have thought that engine TWR would increase with increasing size/thrust although SpX seems to think otherwise. For a given Pc rocket engine the plumbing wall thickness would remain constant so the OD/ID ratio gets smaller as a rocket engine is scaled up so in theory a larger rocket engine should have a higher TWR than a smaller one if all other variables are constant. So a future larger BFR should have the no more than the same no. of engines as mini-BFR. Less engines =  less plumbing on the bottom of the BFR.

No, wall thickness is proportional to diameter. Once you reach a certain minimum size TWR does not change much with thrust.
So SpX can dev. what thrust version of Raptor they like for future larger BFR's if engine TWR does not change much with thrust. We will wait and see what designs SpX go for their future larger BFR's. This could indicate that Raptor's size may be funding constrained as larger engines are more costly to dev. Once SpX sat. constellation is raking in the money then they will be able to afford to dev. scaled up Raptors to keep engine nos. on future larger BFR's from spiraling out of control.

Online AncientU

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #28 on: 09/17/2017 08:50 PM »
They've talked about the engine being scaleable -- 'easily' was the adjective used IIRC -- so they will grow whatever size engine(s) they need from the Raptor family.  My bet is to see second size at this IAC...  sub-scale is likely in or near flight qualification testing.
"If we shared everything [we are working on] people would think we are insane!"
-- SpaceX friend of mlindner

Offline guckyfan

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #29 on: 09/17/2017 09:30 PM »
They've talked about the engine being scaleable -- 'easily' was the adjective used IIRC -- so they will grow whatever size engine(s) they need from the Raptor family.  My bet is to see second size at this IAC...  sub-scale is likely in or near flight qualification testing.

Elon Musk said they optimize for best T/W and the 3MN range is where the overall optimum lies. That optimum value may be quite broad but probably twice that is no longer near optimum.

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #30 on: 09/17/2017 11:57 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John
« Last Edit: 09/18/2017 12:12 AM by livingjw »

Offline Pipcard

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #31 on: 09/18/2017 12:54 AM »
-snip-
Where can I get that "Nozzle Pack" software? Google isn't being particularly helpful in this case.

edit: it's made by OneSpeed and isn't released yet.
« Last Edit: 09/18/2017 05:42 AM by Pipcard »

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #32 on: 09/18/2017 12:54 AM »
Nice logarithmic chart!

T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.
If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size would that not imply a T/W ratio of  an unimaginably good score of around 500-600?


« Last Edit: 09/18/2017 01:26 AM by Peter.Colin »

Offline rakaydos

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #33 on: 09/18/2017 01:07 AM »
Nice logarithmic chart!

T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.
If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size wouldn't that not imply a T/W ratio of around an unimaginably good score of around 500-600?

Benifits of Full Flow Stage Combustion, in that by adding 50% more turbopump machinery (which is by far not the entirity of the mass of the engine) you can get closed cycle efficencies with a chamber pressure higher than most open cycle engines.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #34 on: 09/18/2017 01:35 AM »
Nice logarithmic chart!

T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.
If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size wouldn't that not imply a T/W ratio of around an unimaginably good score of around 500-600?

Benifits of Full Flow Stage Combustion, in that by adding 50% more turbopump machinery (which is by far not the entirity of the mass of the engine) you can get closed cycle efficencies with a chamber pressure higher than most open cycle engines.

Exactly!

Offline Semmel

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #35 on: 09/18/2017 05:14 AM »
Re scalability: I can't remember where but I read there is a principal limit to chemical rocket engine sizes. Something to do with fluid and combustion dynamics. As the limit is approached, the engine becomes more and more complex. Surely the cycle type and fuel type influences the limit, so raptor is hard to compare with existing engines in this regard. Larger might not be better.
« Last Edit: 09/18/2017 06:54 AM by Semmel »

Offline Explorer

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #36 on: 09/18/2017 10:19 AM »
Re scalability: I can't remember where but I read there is a principal limit to chemical rocket engine sizes. Something to do with fluid and combustion dynamics. As the limit is approached, the engine becomes more and more complex. Surely the cycle type and fuel type influences the limit, so raptor is hard to compare with existing engines in this regard. Larger might not be better.

AFAIK the big problem is combustion instability and the resulting oscillation damage. Still the F1 had almost 7000 kN thrust 50 years ago. I think it's reasonable that today 10000 kN if not more is quite feasible and 3000 not that big a deal.
Send orbiters to Uranus and Neptune, dammit.

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #37 on: 09/18/2017 11:10 AM »
Nice logarithmic chart!

T/W for Raptor can't be lower than the 198 for SpaceX own M1D, because otherwise the Raptor wouldn't be the rocket engine with the highest T/W ratio, as it was presented at IAC 2016.
If Elon Musk states Raptor has 3 times as much thrust than M1D at a similar size would that not imply a T/W ratio of  an unimaginably good score of around 500-600?

No, since it can be the same size yet 3 times as heavy.

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #38 on: 09/18/2017 11:22 AM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John

Offline JamesH65

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #39 on: 09/18/2017 12:17 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.

Offline GORDAP

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #40 on: 09/18/2017 01:09 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.

One issue I can think of that argues against a huge number of engines, that I haven't seen mentioned yet, is cost.

I'm pretty sure that an engine that has half the thrust of another one is generally going to be a good bit more than half the cost to produce.  And this before considering all of the extra labor involved in testing and integrating the larger number of engines.

While a 40 engine vehicle might tease out an extra 3 percent of performance than a 20 engine vehicle (of same general size and thrust), I can see it being perfectly reasonable for them to settle on the latter if it costs 20% less to produce.

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #41 on: 09/18/2017 01:24 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.

The argument against many smaller engines hinges on two things:

1) chance of a catastrophic engine failure that brings down the entire vehicle. If the probability of a engine failure causing LOM is extremely low (e.g. 0.01%) then the number of engines isn't a significant driver of mission risk. Some people don't intuitively grasp this because main engine failure has always been a very high probability LOM on past missions.

2) cost of building, assembling, and maintaining more smaller engines. A typical estimate in manufacturing is that doubling the quantity manufactured reduces the cost 10%. For a highly labor intensive item like a rocket engine this might be higher, but this isn't clear. Also, having an engine small enough to use in the upper stage increases the volume of production and reduces the development and capital required to have 2 separate engines in manufacturing.


Offline wannamoonbase

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #42 on: 09/18/2017 01:30 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John

John,

Thank you for the Trust/Weight log chart above. 

There is no shame in a Raptor coming in between the Merlin 1D and NK-33.  Some people seem obsessed with T/W, it's only one measure of an engine's performance. 

If they do want to build a rocket with 28 MLBf it's hard to imagine doing that with 50+ engines.  At some point the complications and added work of multiple engines becomes more of a burden.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #43 on: 09/18/2017 02:38 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John

I think the Raptor can get ~200 TWR, I agree 5-600 is fantasy.

I think having the oxidizer turbine pretty much integrated into the combustion head looks like a huge saver of weight, along with the close co-location of many of the parts. I imagine high pressure piping is probably a large chunk of the weight of a staged combustion engine. You can see that the design took steps to minimize this as much as possible, Just compare the amount of high pressure piping compared to an RD-170/180. This is an area where CAD/3D printing can have a huge effect compared to 40 years ago.

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #44 on: 09/18/2017 02:51 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:

I think the 3MN raptor is probably closer to 1.7-1.8m in diameter. The engine bell would be about the same size as the BE-4.

If you look at the CAD drawings, there were two sizes of SL raptors. The ones on the upper stage were ~1.8m in diameter if you pixel measure(The Vacs were ~4m), and the lower ones were ~1.5-1.6m. However, the sizes match up perfectly in the upper stage for the claimed expansion ratios (200 and 40).

sqrt(40/200) = 0.447

0.447 * 4 = 1.789

Also, if you look at the powerheads in the CAD drawing, they are also different sizes in the US and booster. I think the engines in the Booster were placeholders; they could fit 42 engines, but it  would have required a skirt on the bottom.

I think they will use a vacuum version of the 1MN raptor for the upper stage rated at around 1.4-1.5MN. These could fit 6x in the 9m upper stage with a slight increase in expansion ratio, and thus ISP. I think the Booster will be the 3MN SL version in the orginal presentation, with 19 in the booster and one in the center of the US for landing.

Offline wannamoonbase

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #45 on: 09/18/2017 04:49 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John

I think the Raptor can get ~200 TWR, I agree 5-600 is fantasy.

I think having the oxidizer turbine pretty much integrated into the combustion head looks like a huge saver of weight, along with the close co-location of many of the parts. I imagine high pressure piping is probably a large chunk of the weight of a staged combustion engine. You can see that the design took steps to minimize this as much as possible, Just compare the amount of high pressure piping compared to an RD-170/180. This is an area where CAD/3D printing can have a huge effect compared to 40 years ago.

The side by side with the BE-4 on the previous page is pretty amazing too, that is a whole lot of machinery hanging on the side. 

One can either marvel at the efficiency of the Raptor or the inefficiency of the BE-4.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline rakaydos

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #46 on: 09/18/2017 05:08 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John
I think you're missing exactly what those two preburners and 2 turbines give you. A single preburner, single turbine closed cycle engine is losing performance to back pressure, because the single preburner is trying to push 3 different flows (fuel, oxydizer, and preburner) into the combustion chamber. In full flow staged combustion, each turbine/preburner is only pushing 1 flow (1 and ahalf, really, since each is also a preburner, assuming equal flow), at least doubling the possible chamber pressure without any other advances.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #47 on: 09/18/2017 05:33 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

For anyone interested, I'm willing to make a bet for $50 the Raptor already has at least T/W of above 350.

After attaining a high Isp of a rocket engine, attaining a high T/W is the next logical goal.
Isp is limited by physics to a theoretical maximum.
Nothing in physics is preventing a rocket engine to reach even much higher T/W values.
« Last Edit: 09/18/2017 05:44 PM by Peter.Colin »

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #48 on: 09/18/2017 05:45 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.
7-9 is the optimal no. for a 1st stage which is why NG and F9 have these engine nos. It's when you go beyond about 20 (look what happened to the N-1) on the 1st stage that you are likely to enter problems with increased risk of RUD's causing LOM, higher maintenance costs and increased downtime of maintaining all those engines. If SpaceX ever builds a c.120-130MN thrust booster they should go with a scaled up Raptor for it. Lower risk of LOM coupled with lower maintenance costs and less downtime between missions may outweigh a slight reduction in engine TWR. Just make the booster slightly larger to compensate and use the TWR optimized size Raptors for the ITS ship which needs the highest TWR and performance engines.

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #49 on: 09/18/2017 05:47 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

For anyone interested, I'm willing to make a bet for $50 the Raptor already has at least T/W of above 350.

After attaining a high Isp of a rocket engine, attaining a high T/W is the next logical goal.
Isp is limited by physics to a theoretical maximum.
Nothing in physics is preventing a rocket engine to reach even much higher T/W values.
I think it will be hard to get Raptor's TWR much beyond about 200 due to all the turbomachinery and plumbing required for FFSC.

I think you will lose your $50 bet.
« Last Edit: 09/18/2017 05:49 PM by DJPledger »

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #50 on: 09/18/2017 05:51 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John

I think the Raptor can get ~200 TWR, I agree 5-600 is fantasy.

I think having the oxidizer turbine pretty much integrated into the combustion head looks like a huge saver of weight, along with the close co-location of many of the parts. I imagine high pressure piping is probably a large chunk of the weight of a staged combustion engine. You can see that the design took steps to minimize this as much as possible, Just compare the amount of high pressure piping compared to an RD-170/180. This is an area where CAD/3D printing can have a huge effect compared to 40 years ago.

The side by side with the BE-4 on the previous page is pretty amazing too, that is a whole lot of machinery hanging on the side. 

One can either marvel at the efficiency of the Raptor or the inefficiency of the BE-4.

As I stated above, I think a better estimate for 3MNSL raptor diameter is ~1.7m (~4m for 3MNVac), the point still stands though, you can see the much reduced plumbing, and smaller turbomachinery.

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #51 on: 09/18/2017 05:56 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

For anyone interested, I'm willing to make a bet for $50 the Raptor already has at least T/W of above 350.

After attaining a high Isp of a rocket engine, attaining a high T/W is the next logical goal.
Isp is limited by physics to a theoretical maximum.
Nothing in physics is preventing a rocket engine to reach even much higher T/W values.
I think it will be hard to get Raptor's TWR much beyond about 200 due to all the turbomachinery and plumbing required for FFSC.

I think you will lose your $50 bet.

Even if it ends up being under 200, a 300bar rocket engine with a TWR like that is already enough to put all other rockets to shame. Reusability and restartability is just icing on the cake.

My own personal guess is somewhere between 150-250 for the SL version (meaning a weight between 1200-2000kg).

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #52 on: 09/18/2017 06:05 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John
I think you're missing exactly what those two preburners and 2 turbines give you. A single preburner, single turbine closed cycle engine is losing performance to back pressure, because the single preburner is trying to push 3 different flows (fuel, oxydizer, and preburner) into the combustion chamber. In full flow staged combustion, each turbine/preburner is only pushing 1 flow (1 and ahalf, really, since each is also a preburner, assuming equal flow), at least doubling the possible chamber pressure without any other advances.

While FFSC was always considered harder than ORSC, I wouldn't be surprised if that is only because it was harder with the engineering tools of a few decades ago, but may no longer be the case. Raptor seems to be going along pretty smoothly while BE-4 and AR-1 seem a bit more stuck.

Simulating the complex startup and flows of FFSC might have been extremely hard with 70s technology, but much easier with today's advanced computers. Meanwhile, the materials problems that arise from ORSC haven't changed much.

I would not be surprised to hear Raptor is progressing better than expected at IAC.

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #53 on: 09/18/2017 06:36 PM »
The NK33 utilizes a closed cycle similar to the Raptor. Raptor has two turbines, 2 pumps and 2 pre-burners. NK33 has one turbine, two pumps and one pre-burner. Raptor has higher pressure. Chamber plumbing and pumps scale directly with volume and pressure. The Raptor has the advantage of better materials, analysis, QA, CNC, and 3D printing so you might expect it to have better thrust to weight than the NK33 despite its higher pressure and complexity. T/W of 500-600 for such a design is shear fantasy.

John
I think you're missing exactly what those two preburners and 2 turbines give you. A single preburner, single turbine closed cycle engine is losing performance to back pressure, because the single preburner is trying to push 3 different flows (fuel, oxydizer, and preburner) into the combustion chamber. In full flow staged combustion, each turbine/preburner is only pushing 1 flow (1 and ahalf, really, since each is also a preburner, assuming equal flow), at least doubling the possible chamber pressure without any other advances.

While FFSC was always considered harder than ORSC, I wouldn't be surprised if that is only because it was harder with the engineering tools of a few decades ago, but may no longer be the case. Raptor seems to be going along pretty smoothly while BE-4 and AR-1 seem a bit more stuck.

Simulating the complex startup and flows of FFSC might have been extremely hard with 70s technology, but much easier with today's advanced computers. Meanwhile, the materials problems that arise from ORSC haven't changed much.

I would not be surprised to hear Raptor is progressing better than expected at IAC.

FFSC still has the same ox-rich problems as ORSC, all it eliminates is the turbine interseal. Musk said the toughest challenge with Raptor was the metallurgy of the oxidizer side turbopump. But they have been testing Raptor powerpacks for 5 years and appear to have licked this problem a long time ago.

Offline Pipcard

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #54 on: 09/18/2017 06:44 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.
7-9 is the optimal no. for a 1st stage which is why NG and F9 have these engine nos. It's when you go beyond about 20 (look what happened to the N-1) on the 1st stage that you are likely to enter problems with increased risk of RUD's causing LOM, higher maintenance costs and increased downtime of maintaining all those engines. If SpaceX ever builds a c.120-130MN thrust booster they should go with a scaled up Raptor for it. Lower risk of LOM coupled with lower maintenance costs and less downtime between missions may outweigh a slight reduction in engine TWR. Just make the booster slightly larger to compensate and use the TWR optimized size Raptors for the ITS ship which needs the highest TWR and performance engines.
7-9 engines on the first stage is only optimal if the first stage is reusable. If you want to have common engines (sea level and vacuum versions) on both stages, you need more than 9 on the first stage.

Original ITS:
1st stage: 42 atmospheric engines
2nd stage: 6 vacuum + 3 atmospheric engines

Halve the number of engines:
1st stage: 21 atmospheric engines
2nd stage: 3 vacuum + 1-2 atmospheric engines

If there were 9 engines on the first stage, you would need a separate engine production line for the second stage in order to have a manageable T/W ratio or throttling capability for landing.
« Last Edit: 09/18/2017 06:58 PM by Pipcard »

Offline DJPledger

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #55 on: 09/18/2017 07:21 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.
7-9 is the optimal no. for a 1st stage which is why NG and F9 have these engine nos. It's when you go beyond about 20 (look what happened to the N-1) on the 1st stage that you are likely to enter problems with increased risk of RUD's causing LOM, higher maintenance costs and increased downtime of maintaining all those engines. If SpaceX ever builds a c.120-130MN thrust booster they should go with a scaled up Raptor for it. Lower risk of LOM coupled with lower maintenance costs and less downtime between missions may outweigh a slight reduction in engine TWR. Just make the booster slightly larger to compensate and use the TWR optimized size Raptors for the ITS ship which needs the highest TWR and performance engines.
7-9 engines on the first stage is only optimal if the first stage is reusable. If you want to have common engines (sea level and vacuum versions) on both stages, you need more than 9 on the first stage.

Original ITS:
1st stage: 42 atmospheric engines
2nd stage: 6 vacuum + 3 atmospheric engines

Halve the number of engines:
1st stage: 21 atmospheric engines
2nd stage: 3 vacuum + 1-2 atmospheric engines

If there were 9 engines on the first stage, you would need a separate engine production line for the second stage in order to have a manageable T/W ratio or throttling capability for landing.
The 1st stage is reusable on both NG and F9 so have optimum engine nos. and have common US engines. Having two separate engine production lines for two sizes of the same fundamental design is not a big deal these days with modern manufacturing methods. Larger Raptors for BFR booster and smaller Raptors for ITS ship to keep engine no. on booster to around 7-9 and the same engine no. on ship. Or make Raptor so deeply throttlable that you can have a single engine on the ship so as to keep optimum engine no. on booster while keeping only one engine production line.

Looks like mini-BFR will have 19-21 engines on booster because SpX are making the Raptor too small for 9 engines to generate sufficient thrust. We will find out soon at IAC2017. The original plan for BFR was for 9 engines in the F-1 class so I don't understand why SpX are going for so many small engines. 19-21 engines on booster may end up being acceptable for all we know but future larger BFR's should not go for any more engines than this. Perhaps BO will be more sensible with the engine nos. than SpX for their future HLV's.
« Last Edit: 09/18/2017 07:49 PM by DJPledger »

Offline rakaydos

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #56 on: 09/18/2017 09:19 PM »
Looks like mini-BFR will have 19-21 engines on booster because SpX are making the Raptor too small for 9 engines to generate sufficient thrust. We will find out soon at IAC2017. The original plan for BFR was for 9 engines in the F-1 class so I don't understand why SpX are going for so many small engines. 19-21 engines on booster may end up being acceptable for all we know but future larger BFR's should not go for any more engines than this. Perhaps BO will be more sensible with the engine nos. than SpX for their future HLV's.
They went for many small engines because they discovered that "optimum number of engines" was a lot more than they originally thought.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #57 on: 09/18/2017 09:30 PM »
Looks like mini-BFR will have 19-21 engines on booster because SpX are making the Raptor too small for 9 engines to generate sufficient thrust. We will find out soon at IAC2017. The original plan for BFR was for 9 engines in the F-1 class so I don't understand why SpX are going for so many small engines. 19-21 engines on booster may end up being acceptable for all we know but future larger BFR's should not go for any more engines than this. Perhaps BO will be more sensible with the engine nos. than SpX for their future HLV's.
They went for many small engines because they discovered that "optimum number of engines" was a lot more than they originally thought.

Interesting Quote from Elon Musk also pointing to many small engines.

"Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them"


http://spacenews.com/elon-musks-ask-me-anything-qa-just-the-space-parts/

« Last Edit: 09/18/2017 09:32 PM by Peter.Colin »

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #58 on: 09/18/2017 09:31 PM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.
7-9 is the optimal no. for a 1st stage which is why NG and F9 have these engine nos. It's when you go beyond about 20 (look what happened to the N-1) on the 1st stage that you are likely to enter problems with increased risk of RUD's causing LOM, higher maintenance costs and increased downtime of maintaining all those engines. If SpaceX ever builds a c.120-130MN thrust booster they should go with a scaled up Raptor for it. Lower risk of LOM coupled with lower maintenance costs and less downtime between missions may outweigh a slight reduction in engine TWR. Just make the booster slightly larger to compensate and use the TWR optimized size Raptors for the ITS ship which needs the highest TWR and performance engines.
7-9 engines on the first stage is only optimal if the first stage is reusable. If you want to have common engines (sea level and vacuum versions) on both stages, you need more than 9 on the first stage.

Original ITS:
1st stage: 42 atmospheric engines
2nd stage: 6 vacuum + 3 atmospheric engines

Halve the number of engines:
1st stage: 21 atmospheric engines
2nd stage: 3 vacuum + 1-2 atmospheric engines

If there were 9 engines on the first stage, you would need a separate engine production line for the second stage in order to have a manageable T/W ratio or throttling capability for landing.
The 1st stage is reusable on both NG and F9 so have optimum engine nos. and have common US engines. Having two separate engine production lines for two sizes of the same fundamental design is not a big deal these days with modern manufacturing methods. Larger Raptors for BFR booster and smaller Raptors for ITS ship to keep engine no. on booster to around 7-9 and the same engine no. on ship. Or make Raptor so deeply throttlable that you can have a single engine on the ship so as to keep optimum engine no. on booster while keeping only one engine production line.

Looks like mini-BFR will have 19-21 engines on booster because SpX are making the Raptor too small for 9 engines to generate sufficient thrust. We will find out soon at IAC2017. The original plan for BFR was for 9 engines in the F-1 class so I don't understand why SpX are going for so many small engines. 19-21 engines on booster may end up being acceptable for all we know but future larger BFR's should not go for any more engines than this. Perhaps BO will be more sensible with the engine nos. than SpX for their future HLV's.

I suspect that development and tooling costs have a lot to do with the decision to have so many engines. They also probably don't want to scale too far beyond their mini Raptor database.

John

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #59 on: 09/18/2017 10:21 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

For anyone interested, I'm willing to make a bet for $50 the Raptor already has at least T/W of above 350.

After attaining a high Isp of a rocket engine, attaining a high T/W is the next logical goal.
Isp is limited by physics to a theoretical maximum.
Nothing in physics is preventing a rocket engine to reach even much higher T/W values.

Peter,

   I agree and Raptor will most likely dethrone the NK33 which is currently the highest thrust to weight staged combustion rocket that exists. I have my doubts, but it may top 200, but I'm sticking with around 160. The chemistry and thermodynamics of the Raptor cycle  mostly known. Material properties and efficient mechanical design will ultimately dictate T/W. Material Properties are a known commodity as well. They have improved since the 1970s but not by so much that you could get anywhere near 350. I can assure you that the Raptor is wringing every bit of strength to weight available out of its materials commensurate with reliability and life requirements. Rocket designers do that.

  This may help: The M1D weights about 1000 lbs and has a chamber pressure of about 10-MPa. The Raptor has a chamber pressure of 30-MPa and pre-burner pressures somewhere around 45-MPa. Pumps, plumbing, valves, pre-burners and the main combustion chamber all scale pretty much directly with pressure, volume and material strength to weight. The M1D is about the same size as the Raptor so lets just scale its weight up by the difference in pressure between the two engines. (I'm ignoring the higher pump and pre-burner pressure!) That alone gives you at least 3000 lbs weight and a thrust to weight of 220. We still haven't accounted for the higher pressure pumps and pre-burners, or the fact that the Raptor has an additional pre-burner and turbine. If we add an additional 400-1000 lbs to account for these weights we are in the 160 = 190 T/W ball park.

In summary, without some new wonder materials, 350 T/W is not in the cards.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #60 on: 09/18/2017 11:21 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

For anyone interested, I'm willing to make a bet for $50 the Raptor already has at least T/W of above 350.

After attaining a high Isp of a rocket engine, attaining a high T/W is the next logical goal.
Isp is limited by physics to a theoretical maximum.
Nothing in physics is preventing a rocket engine to reach even much higher T/W values.

Peter,

   I agree and Raptor will most likely dethrone the NK33 which is currently the highest thrust to weight staged combustion rocket that exists. I have my doubts, but it may top 200, but I'm sticking with around 160. The chemistry and thermodynamics of the Raptor cycle  mostly known. Material properties and efficient mechanical design will ultimately dictate T/W. Material Properties are a known commodity as well. They have improved since the 1970s but not by so much that you could get anywhere near 350. I can assure you that the Raptor is wringing every bit of strength to weight available out of its materials commensurate with reliability and life requirements. Rocket designers do that.

  This may help: The M1D weights about 1000 lbs and has a chamber pressure of about 10-MPa. The Raptor has a chamber pressure of 30-MPa and pre-burner pressures somewhere around 45-MPa. Pumps, plumbing, valves, pre-burners and the main combustion chamber all scale pretty much directly with pressure, volume and material strength to weight. The M1D is about the same size as the Raptor so lets just scale its weight up by the difference in pressure between the two engines. (I'm ignoring the higher pump and pre-burner pressure!) That alone gives you at least 3000 lbs weight and a thrust to weight of 220. We still haven't accounted for the higher pressure pumps and pre-burners, or the fact that the Raptor has an additional pre-burner and turbine. If we add an additional 400-1000 lbs to account for these weights we are in the 160 = 190 T/W ball park.

In summary, without some new wonder materials, 350 T/W is not in the cards.

OK agreed to the bet, I hope I'm not wrong for multiple reasons...  ;)

In my view a same size engine doesn't need to become 3 times as heavy as the chamber pressure increases 3 times, also for a Raptor FFSC as shown. But could be to optimistic about that.
« Last Edit: 09/18/2017 11:33 PM by Peter.Colin »

Offline Lars-J

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #61 on: 09/19/2017 05:50 AM »
OK agreed to the bet, I hope I'm not wrong for multiple reasons...  ;)

Pro tip, Peter... Don't go to Vegas. You have already made one bet with hideous odds, why are you itching to lose money by more?  :)

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #62 on: 09/19/2017 06:13 AM »
OK agreed to the bet, I hope I'm not wrong for multiple reasons...  ;)

Pro tip, Peter... Don't go to Vegas. You have already made one bet with hideous odds, why are you itching to lose money by more?  :)

Thanks for the tip, me loosing bets would be a good lesson in humility, but I really tend to win more bets than that I loose them.  :)
« Last Edit: 09/19/2017 06:50 AM by Peter.Colin »

Offline hkultala

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #63 on: 09/19/2017 07:01 AM »
AFAIK the falling T/W of too big engines is actually not about engines but chambers (because of nozzles).

Assuming similar exit pressure, thrust is propotional to nozzle area.

In order to make a nozzle with 4 times the nozzle area and 4 times the thrust, the nozzle needs to have 2 times greater diameter.

But this practically also makes the nozzle longer because nozzle has to have a certain form. Nozzle which has twice the diameter and twice the length does not weight 4 times more, it' closer to 8 times more.

So, the T/W starts decreasing when the chambers get too big.
« Last Edit: 09/19/2017 07:02 AM by hkultala »

Offline JamesH65

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #64 on: 09/19/2017 11:58 AM »
Lots of people saying that many engines is bad. Increased chance of RUD, more complexity. And yet SpaceX have flown the 9 engined Falcon 9 with no failures at all for quite a few years. That's a LOT of flight hours on engines with no failures. More complex? No, just more of them, and smaller, which makes removal and inspection easier, and replacement considerably easier. There is quite a bit of plumbing of course, but is that a real issue?

So I'm not seeing the problem with large numbers of engines on the stage. Can anyone enlighten as to why it is such a 'bad thing'.
7-9 is the optimal no. for a 1st stage which is why NG and F9 have these engine nos. It's when you go beyond about 20 (look what happened to the N-1) on the 1st stage that you are likely to enter problems with increased risk of RUD's causing LOM, higher maintenance costs and increased downtime of maintaining all those engines. If SpaceX ever builds a c.120-130MN thrust booster they should go with a scaled up Raptor for it. Lower risk of LOM coupled with lower maintenance costs and less downtime between missions may outweigh a slight reduction in engine TWR. Just make the booster slightly larger to compensate and use the TWR optimized size Raptors for the ITS ship which needs the highest TWR and performance engines.

Why is 7-9 optimal? It could be optimal for the F9, but as soon as you change the size of the rocket, the optimal number will immediately change.

N1 is NOT a good example. Engine and sensor technology is SOOO much better nowadays that any comparisons to N1 are irrelevant.

Increased chance of RUD seems wrong as well. As I said, there has not been Merlin failure in flight for years, and they have flow a LOT of engines. Some twice....

I also see the convenience of smaller engines being easier to make, install, remove, repair and even just move around the factory to be a massive advantage.




Offline wannamoonbase

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #65 on: 09/19/2017 01:13 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

John,

I'm curious, any idea how subcooling propellants may affect the T/W ratio?

Steven
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #66 on: 09/19/2017 02:10 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

John,

I'm curious, any idea how subcooling propellants may affect the T/W ratio?

Steven

It doesn't, really. The difference between subcooled and boiling is negligible compared to the temperature in the combustion chamber. As long as the turbopumps can maintain the same chamber pressure, thrust will be about the same.

Offline Semmel

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #67 on: 09/19/2017 02:27 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

John,

I'm curious, any idea how subcooling propellants may affect the T/W ratio?

Steven

It doesn't, really. The difference between subcooled and boiling is negligible compared to the temperature in the combustion chamber. As long as the turbopumps can maintain the same chamber pressure, thrust will be about the same.

So it has no influence on Thrust, but does it have on weight? Maybe the turbopump becomes more efficient or the cooling lines around the engine chamber can take more energy so that the chamber can be operated at higher temperatures? I dont know the answer, I am asking.

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #68 on: 09/19/2017 02:51 PM »
I have an old chart from K. D. Wood's spacecraft Design book that shows the general trend for rocket engine T/Ws.
It is a bit dated, but so are most rocket engines. This chart shows that thrust to weights are nearly flat between 50 klbs and 1 mlbs.  I have spotted the M1D and NK33. I would expect the Raptor T/W to be somewhere between these two. Lets guess T/W = 160. I think OneSpeed's thrust guess at 2.5 mN sounds about right. The improvement in SpaceX's T/Ws comes from improved material, analysis, QA, accurate CNC and 3D printing technologies.  I can safely say that the chemistry and thermodynamics have not changed one bit since this chart was made.  There is nothing in the Raptor chemistry or design which would allow it to deviate from normal sizing trends.

John

John,

I'm curious, any idea how subcooling propellants may affect the T/W ratio?

Steven

It doesn't, really. The difference between subcooled and boiling is negligible compared to the temperature in the combustion chamber. As long as the turbopumps can maintain the same chamber pressure, thrust will be about the same.

So it has no influence on Thrust, but does it have on weight? Maybe the turbopump becomes more efficient or the cooling lines around the engine chamber can take more energy so that the chamber can be operated at higher temperatures? I dont know the answer, I am asking.

Lower volume (per mass) helps both plumbing weight and turbopump efficiency, but higher viscosity hurts both. I'm not sure which is more significant, but they tend to cancel and the net effect shouldn't be more than a few percent.

Regen cooling efficiency should also be a bit higher with subcooled fuel (though again viscosity hurts a little), and might allow a higher chamber pressure if the turbopumps can handle it. Every part of the engine has to be optimized for every other part, so I don't think there's a strict rule of thumb here.

Offline rsdavis9

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #69 on: 09/19/2017 03:44 PM »
So a sphere 2x bigger at the same pressure is 8x heavier?
The way I figure it is 2x the diameter is 2x longer circumference which means 2x greater force which means 2x thicker walls.
Now if the walls of a sphere are 2x thicker than the whole surface area of the sphere goes as x^2 so the mass goes as x^3

So to summarize:
1. the weight of an engine goes as the cube of scaling.
x^3
2x size increase = 8x weight
This is because the combustion chamber, rocket nozzle, and plumbing are all like a pressure vessel and go as the cube.
Since thrust goes as the throat area than the thrust is x^2
So smaller engines win out on weight.

2. If the pressure is doubled then the thrust doubles, and the wall thickness doubles, and the weight doubles.
So no difference on weight per thrust.

Do I have these heuristics right?

All of this is assuming that the major weight of the engine is in the pressure components.
Obviously there are other components and they may not scale the same.




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Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #70 on: 09/19/2017 04:20 PM »
So a sphere 2x bigger at the same pressure is 8x heavier?
The way I figure it is 2x the diameter is 2x longer circumference which means 2x greater force which means 2x thicker walls.
Now if the walls of a sphere are 2x thicker than the whole surface area of the sphere goes as x^2 so the mass goes as x^3

So to summarize:
1. the weight of an engine goes as the cube of scaling.
x^3
2x size increase = 8x weight
This is because the combustion chamber, rocket nozzle, and plumbing are all like a pressure vessel and go as the cube.
Since thrust goes as the throat area than the thrust is x^2
So smaller engines win out on weight.

2. If the pressure is doubled then the thrust doubles, and the wall thickness doubles, and the weight doubles.
So no difference on weight per thrust.

Do I have these heuristics right?

All of this is assuming that the major weight of the engine is in the pressure components.
Obviously there are other components and they may not scale the same.

You are mostly right except:

2. If pressure is doubled then wall thickness is less than doubled so the weight is also less than doubled.
So higher pressure at same engine dimensions also leads to higher T/W ratio as do smaller engines
(provided their Isp remains high if you shrink them).

« Last Edit: 09/19/2017 04:27 PM by Peter.Colin »

Offline GORDAP

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #71 on: 09/19/2017 04:39 PM »
So a sphere 2x bigger at the same pressure is 8x heavier?
The way I figure it is 2x the diameter is 2x longer circumference which means 2x greater force which means 2x thicker walls.
Now if the walls of a sphere are 2x thicker than the whole surface area of the sphere goes as x^2 so the mass goes as x^3

So to summarize:
1. the weight of an engine goes as the cube of scaling.
x^3
2x size increase = 8x weight
This is because the combustion chamber, rocket nozzle, and plumbing are all like a pressure vessel and go as the cube.
Since thrust goes as the throat area than the thrust is x^2
So smaller engines win out on weight.

2. If the pressure is doubled then the thrust doubles, and the wall thickness doubles, and the weight doubles.
So no difference on weight per thrust.

Do I have these heuristics right?

All of this is assuming that the major weight of the engine is in the pressure components.
Obviously there are other components and they may not scale the same.

You are mostly right except:

2. If pressure is doubled then wall thickness is less than doubled so the weight is also less than doubled.
So higher pressure at same engine dimensions also leads to higher T/W ratio as do smaller engines
(provided their Isp remains high if you shrink them).



Source?   I believe wall thickness scales linearly with pressure in a pressure vessel.

Offline rsdavis9

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #72 on: 09/19/2017 04:40 PM »
You are mostly right except:

2. If pressure is doubled then wall thickness is less than doubled so the weight is also less than doubled.
So higher pressure at same engine dimensions also leads to higher T/W ratio as do smaller engines
(provided their Isp remains high if you shrink them).

Ok I give up. Why is wall thickness less than doubled?
Under pressure(tension) it should be exact.
Under compression you have buckling and other things.
 
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Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #73 on: 09/19/2017 06:16 PM »
You are mostly right except:

2. If pressure is doubled then wall thickness is less than doubled so the weight is also less than doubled.
So higher pressure at same engine dimensions also leads to higher T/W ratio as do smaller engines
(provided their Isp remains high if you shrink them).

Ok I give up. Why is wall thickness less than doubled?
Under pressure(tension) it should be exact.
Under compression you have buckling and other things.

A double chamber pressure does not lead to double pressure at the end of the expansion nozzle cone.
So the end of the cone does not have to be twice as thick only the beginning of the cone.

For the tubing the Barlow equation states that wall thickness scales linearly with pressure only when the outside diameter of the tubing remains the same. For similar flow at higher pressure you want your inner diameter to remain the same.
That doesn't scale linearly with wall thickness as you can calculate yourself with this online calculator:

http://www.engineersedge.com/calculators/pipe_bust_calc.htm

Because the cone has the largest contribution to the weight I suspect higher chamber pressure is usually positive for T/W.
« Last Edit: 09/19/2017 06:26 PM by Peter.Colin »

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #74 on: 09/19/2017 06:47 PM »
...
A double chamber pressure does not lead to double pressure at the end of the expansion nozzle cone.
So the end of the cone does not have to be twice as thick only the beginning of the cone.
Most nozzles are not, strictly speaking, cones. They are bell-shapes.

Your statement is only true of the net pressure when operating in the atmosphere; but all rocket engines operate in vacuum eventually, where the pressure drop across the nozzle is nearly linear with chamber pressure.

Quote
Because the cone has the largest contribution to the weight I suspect higher chamber pressure is usually positive for T/W.

The effect you are basing this on is flawed, and also, the nozzle is actually a small part of the weight. For instance, the SSME had a large nozzle for it's thrust (70:1 expansion, full LH2 regen cooling), but it was still only about 12% of the total engine mass.

Most of the mass is in the main combustion chamber, injector head, turbopumps, and high pressure plumbing.

Offline wannamoonbase

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #75 on: 09/19/2017 07:02 PM »
...Most of the mass is in the main combustion chamber, injector head, turbopumps, and high pressure plumbing.

I think some people are dismissing the pressure.  300 bar, is no joke.

I've worked with carbon steel and stainless steel in 5000-7000 psi range for heat exchanges and piping.  It's no joke.

When the first Raptor info came out last year stating 300 bar I thought that was the most impressive number. 

Holding everything together at 4350 psi with cryogenics coming in one end and rocket exhaust going out the other is no minor task.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #76 on: 09/19/2017 07:15 PM »
...
A double chamber pressure does not lead to double pressure at the end of the expansion nozzle cone.
So the end of the cone does not have to be twice as thick only the beginning of the cone.
Most nozzles are not, strictly speaking, cones. They are bell-shapes.

Your statement is only true of the net pressure when operating in the atmosphere; but all rocket engines operate in vacuum eventually, where the pressure drop across the nozzle is nearly linear with chamber pressure.

Quote
Because the cone has the largest contribution to the weight I suspect higher chamber pressure is usually positive for T/W.

The effect you are basing this on is flawed, and also, the nozzle is actually a small part of the weight. For instance, the SSME had a large nozzle for it's thrust (70:1 expansion, full LH2 regen cooling), but it was still only about 12% of the total engine mass.

Most of the mass is in the main combustion chamber, injector head, turbopumps, and high pressure plumbing.

I think the big trick to Raptor getting a high TWR will be the integration of the oxidizer turbine into the combustion head. Having ~78% of the propellant by mass going through almost zero piping at pressure will save a lot of weight.

Look at the RD-180, the Oxidizer rich flow probably travels  over 2 meters (at 260+bar) before getting into the combustion chamber. Those pipes are probably a big chunk of the RD-180's weight.
« Last Edit: 09/19/2017 07:24 PM by ZachF »

Offline ZachF

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #77 on: 09/19/2017 07:18 PM »
...Most of the mass is in the main combustion chamber, injector head, turbopumps, and high pressure plumbing.

I think some people are dismissing the pressure.  300 bar, is no joke.

I've worked with carbon steel and stainless steel in 5000-7000 psi range for heat exchanges and piping.  It's no joke.

When the first Raptor info came out last year stating 300 bar I thought that was the most impressive number. 

Holding everything together at 4350 psi with cryogenics coming in one end and rocket exhaust going out the other is no minor task.

Pressure will be even higher between the compressor and turbine, probably ~450bar

Offline livingjw

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #78 on: 09/19/2017 07:20 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 relative to their throat area. 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

« Last Edit: 09/19/2017 07:26 PM by livingjw »

Offline Semmel

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #79 on: 09/19/2017 07:25 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.
« Last Edit: 09/19/2017 07:26 PM by Semmel »

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.

Offline 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 »

Offline 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 »

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.

Online 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.

Online 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).
"If we shared everything [we are working on] people would think we are insane!"
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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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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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
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline OneSpeed

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #100 on: 09/21/2017 01:23 PM »
Where can I get that "Nozzle Pack" software? Google isn't being particularly helpful in this case.

NozzlePack is just a C# tool I wrote for myself, but if anyone else finds it useful, that's great. I've recently updated it in an attempt to make it more user friendly. The attached zipfile contains the program, a config file, and two directories containing .xml configuration files for rockets and engines. E.g. here are the file contents for the Falcon S1:

F9S1.xml:
<?xml version="1.0" encoding="utf-8" ?>
<danceFloor>
  <diameter>3.66</diameter>
  <engines>
    <circle>
      <name>Merlin1D</name>
      <radius>0</radius>
      <count>1</count>
      <offset>0.125</offset>
    </circle>
    <circle>
      <name>Merlin1D</name>
      <radius>1.35</radius>
      <count>8</count>
      <offset>0.125</offset>
    </circle>
  </engines>
</danceFloor>

Merlin1D.xml:
<?xml version="1.0" encoding="utf-8" ?>
<diameters>
  <bell>0.89</bell>
  <chamber>0.3</chamber>
  <throat>0.178</throat>
</diameters>

When you run the program, go to File/Load and select the rocket configuration file you are interested in.
I've include F9S1, F9S2, BFR, BFS and New Glenn as starters, but you can copy them and create your own.
You'll need MS Windows with the .NET Framework 4.5.2, which is a free download here: https://www.microsoft.com/en-au/download/details.aspx?id=42643

Offline rsdavis9

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #101 on: 09/21/2017 02:47 PM »
Has anybody considered that instead of individual TVC for each engine that the whole rocket could be steered with differential thrust?
So for example with the falcon 9 get rid of TVC and use differential thrust on different engines around the ring of engines on the outside of the rocket.
Still might have a problem with roll but that could be solved by making the outer ring slightly pointed right then left as you go around the ring and just applying different thrust to the odd or even engines.
For landing you probably want one engine with TVC.

It could save a lot of weight.
With lots of engines I would expect finer control of the vector.
Should be a greater vector with larger diameter rockets.
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline TrevorMonty

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #102 on: 09/21/2017 05:42 PM »
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.
True but the engine needs to have completed some test fires. In case of full scale Raptor we've not heard of any being tested.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #103 on: 09/21/2017 05:51 PM »
Thinking about the proportionality of rockets and rocket engines made me wonder about the following:
 
Would it have been easier to colonize mars if the human species was smaller than it is now, or bigger than it is now?
Or are we ourselves as a humans at 1,80 meter coincidently at the optimal size to build similar sized rockets engines and become a space fairing civilization?

If we where smaller we might have built smaller rocket engines with higher T/W or build the exact same “optimal” engine size as we do now, but less of them where needed, to get 100 people to Mars. Or if we would have been bigger than now, we would be able to build large engines more easily, but maybe get in trouble with material properties not being sufficient. The only option then would be to get many small engines on a large rocket.

And what size would an alien space fairing civilization be, if compared to our own size?

« Last Edit: 09/21/2017 06:34 PM by Peter.Colin »

Offline envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #104 on: 09/21/2017 06:35 PM »
Has anybody considered that instead of individual TVC for each engine that the whole rocket could be steered with differential thrust?
So for example with the falcon 9 get rid of TVC and use differential thrust on different engines around the ring of engines on the outside of the rocket.
Still might have a problem with roll but that could be solved by making the outer ring slightly pointed right then left as you go around the ring and just applying different thrust to the odd or even engines.
For landing you probably want one engine with TVC.

It could save a lot of weight.
With lots of engines I would expect finer control of the vector.
Should be a greater vector with larger diameter rockets.

Last year's ITS booster was presented with rings of differential thrusting non-gimballing engines surrounding a TVC gimballed central cluster. Dragon 2 uses differential thrust steering for the abort motors. And N-1's booster used differential thrust for steering.
« Last Edit: 09/21/2017 06:35 PM by envy887 »

Offline rsdavis9

Re: Many smaller sized Raptor engines on future BFR's?
« Reply #105 on: 09/21/2017 06:56 PM »
Has anybody considered that instead of individual TVC for each engine that the whole rocket could be steered with differential thrust?
So for example with the falcon 9 get rid of TVC and use differential thrust on different engines around the ring of engines on the outside of the rocket.
Still might have a problem with roll but that could be solved by making the outer ring slightly pointed right then left as you go around the ring and just applying different thrust to the odd or even engines.
For landing you probably want one engine with TVC.

It could save a lot of weight.
With lots of engines I would expect finer control of the vector.
Should be a greater vector with larger diameter rockets.

Last year's ITS booster was presented with rings of differential thrusting non-gimballing engines surrounding a TVC gimballed central cluster. Dragon 2 uses differential thrust steering for the abort motors. And N-1's booster used differential thrust for steering.

So why even bother with the center TVC gimballing? Only for landing?
If you could get rid if the TVC system completely that would eliminate some weight...
 
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline yokem55

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #106 on: 09/21/2017 07:02 PM »
Thinking about the proportionality of rockets and rocket engines made me wonder about the following:
 
Would it have been easier to colonize mars if the human species was smaller than it is now, or bigger than it is now?
Or are we ourselves as a humans at 1,80 meter coincidently at the optimal size to build similar sized rockets engines and become a space fairing civilization?

If we where smaller we might have built smaller rocket engines with higher T/W or build the exact same “optimal” engine size as we do now, but less of them where needed, to get 100 people to Mars. Or if we would have been bigger than now, we would be able to build large engines more easily, but maybe get in trouble with material properties not being sufficient. The only option then would be to get many small engines on a large rocket.

And what size would an alien space fairing civilization be, if compared to our own size?
Throttle levels are much slower to respond and less precise than hydraulic TVC. Whether differential throttling could be fast  or precise enough I think is unlikely.

Offline jpo234

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #107 on: 09/21/2017 07:28 PM »
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.
True but the engine needs to have completed some test fires. In case of full scale Raptor we've not heard of any being tested.

Why? You actually need a working exemplar is, when you assemble the first booster. You can and have to do a lot of design work before that happens.
Best example is Blue: The New Glenn design is obviously fairly advanced and yet a complete BE-4 hasn't even been test fired on the stand.
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 envy887

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #108 on: 09/21/2017 07:52 PM »
Thinking about the proportionality of rockets and rocket engines made me wonder about the following:
 
Would it have been easier to colonize mars if the human species was smaller than it is now, or bigger than it is now?
Or are we ourselves as a humans at 1,80 meter coincidently at the optimal size to build similar sized rockets engines and become a space fairing civilization?

If we where smaller we might have built smaller rocket engines with higher T/W or build the exact same “optimal” engine size as we do now, but less of them where needed, to get 100 people to Mars. Or if we would have been bigger than now, we would be able to build large engines more easily, but maybe get in trouble with material properties not being sufficient. The only option then would be to get many small engines on a large rocket.

And what size would an alien space fairing civilization be, if compared to our own size?
Throttle levels are much slower to respond and less precise than hydraulic TVC. Whether differential throttling could be fast  or precise enough I think is unlikely.
This, and it has less control torque than a typical gimbal setup.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #109 on: 09/29/2017 10:33 PM »
After seeing the presentation today, I feel like the new size and new design of the BFR makes it a perfect multi utility workhorse.
It could be in service for a long time before the next BFR will be realized.
What size would make this one seem like a rowboat, as Musk said the 12 meter would seem in the future.
And is a bigger size even nescerry?
« Last Edit: 09/29/2017 10:33 PM by Peter.Colin »

Offline wannamoonbase

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #110 on: 09/29/2017 10:36 PM »
After seeing the presentation today, I feel like the new size and new design of the BFR makes it a perfect multi utility workhorse.
It could be in service for a long time before the next BFR will be realized.
What size would make this one seem like a rowboat, as Musk said the 12 meter would seem in the future.
And is a bigger size even nescerry?

I think that if this BFR actually gets built and flies that it's the larger BFR doesn't get built. 

A 12 million Lbf rocket, is not a small rocket, this could do a ton of work.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline rakaydos

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #111 on: 09/30/2017 12:22 AM »
After seeing the presentation today, I feel like the new size and new design of the BFR makes it a perfect multi utility workhorse.
It could be in service for a long time before the next BFR will be realized.
What size would make this one seem like a rowboat, as Musk said the 12 meter would seem in the future.
And is a bigger size even nescerry?

I think that if this BFR actually gets built and flies that it's the larger BFR doesn't get built. 

A 12 million Lbf rocket, is not a small rocket, this could do a ton of work.
I disagree. Once we start developing Ceres and beyond, a Nova-class rocket isnt going to cut it- we'll need a (non-nuclear) Orion class for the really big space construction.

But that might not be an EARTH lift vehical, if we can manufacture it on a lighter celestial body, like a moon base.

Offline Peter.Colin

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Re: Many smaller sized Raptor engines on future BFR's?
« Reply #112 on: 09/30/2017 07:19 AM »
We might draw parallels on the current Earth transport system.
If the “Falcon 31” is like the semi truck and it could contain one stackable container.
We could have a really big container ship for plannet to plannet transport, maybe with a different kind of propulsion. This container ship stays in space (stays in the sea), and the containers are offloaded again by Falcon 31 Spaceships on for instance Mars.

However not everything fits in a container, heavy mining equipment, bulldozers, parts of chemical plants, large spaceship bulkheads, huge vessels etc etc.
There could be a real need for a bigger “Falcon X” (Bigger Falcon Rocket version x,x presented at IAC 20xx)
Also Elon Musk explaned yesterday that the living conditions are quite suitable for this first BFS, implying there will be a next probably bigger one.

I do think there is no real physics determined size limit to the large cargo spaceship that stays in orbit.
But for the Falcon X there probably are some physics determined size limits, like rocket height.
I do believe physics would allow the rocket diameter to be at least 10 times bigger.
But maximum height maybe only twice as high, since one engine can only lift a certain collumn of rocket wheigt above it.



« Last Edit: 09/30/2017 09:57 AM by Peter.Colin »

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