Author Topic: Many smaller sized Raptor engines on future BFR's?  (Read 11901 times)

Online 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.

Online 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.

Online 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.

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