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 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
Quote from: Peter.Colin on 09/17/2017 10:09 am... 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:
... 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......
Quote from: livingjw on 09/18/2017 11:22 amThe 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.JohnI 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.
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
Quote from: livingjw on 09/17/2017 11:57 pmI 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.JohnFor 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.
Quote from: ZachF on 09/18/2017 02:38 pmQuote from: livingjw on 09/18/2017 11:22 amThe 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.JohnI 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.
Quote from: Peter.Colin on 09/18/2017 05:33 pmQuote from: livingjw on 09/17/2017 11:57 pmI 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.JohnFor 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.
Quote from: livingjw on 09/18/2017 11:22 amThe 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.JohnI 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.
Quote from: rakaydos on 09/18/2017 05:08 pmQuote from: livingjw on 09/18/2017 11:22 amThe 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.JohnI 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.
Quote from: JamesH65 on 09/18/2017 12:17 pmLots 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.
Quote from: DJPledger on 09/18/2017 05:45 pmQuote from: JamesH65 on 09/18/2017 12:17 pmLots 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 engines2nd stage: 6 vacuum + 3 atmospheric enginesHalve the number of engines:1st stage: 21 atmospheric engines2nd stage: 3 vacuum + 1-2 atmospheric enginesIf 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.
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.
Quote from: DJPledger on 09/18/2017 07:21 pmLooks 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.
Quote from: Pipcard on 09/18/2017 06:44 pmQuote from: DJPledger on 09/18/2017 05:45 pmQuote from: JamesH65 on 09/18/2017 12:17 pmLots 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 engines2nd stage: 6 vacuum + 3 atmospheric enginesHalve the number of engines:1st stage: 21 atmospheric engines2nd stage: 3 vacuum + 1-2 atmospheric enginesIf 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.