Author Topic: Many smaller sized Raptor engines on future BFR's?  (Read 14095 times)

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 »

Online 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!"
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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.

Online 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'.

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