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@livingjwJust out of curiosity. If the Raptor have higher chamber pressure (like for example 315 bar or 330 bar) in the future and the rest of the engine is mostly unchanged. What would be the changes to the thrust and the ISP of the engine?
Quote from: Zed_Noir on 09/29/2018 07:48 pm@livingjwJust out of curiosity. If the Raptor have higher chamber pressure (like for example 315 bar or 330 bar) in the future and the rest of the engine is mostly unchanged. What would be the changes to the thrust and the ISP of the engine?I believe that within a certain small % range of pressure increase, thrust and Isp all go up quite linearly with pressure, that does not whole true for example for a 100% increase.
Quote from: tdperk on 09/29/2018 08:07 pmQuote from: Zed_Noir on 09/29/2018 07:48 pm@livingjwJust out of curiosity. If the Raptor have higher chamber pressure (like for example 315 bar or 330 bar) in the future and the rest of the engine is mostly unchanged. What would be the changes to the thrust and the ISP of the engine?I believe that within a certain small % range of pressure increase, thrust and Isp all go up quite linearly with pressure, that does not whole true for example for a 100% increase.Linearly OK, but at what rate of change, isp/delta-bar? I hope it is 42!
. Vacuum Isp is not affected by chamber pressure.Chamber Pressure250 bar300 bar315bar330barThrust at sea level1,700 kN2,095 kN2,206 kN2,318 kNThrust in vacuum1,834 kN2,229 kN2,340 kN2,452 kNSpecific Impusle at sea level330.0 sec334.6 sec335.6 sec336.5 secSpecific Impusle in vacuum356 sec356 sec356 sec356 sec
Jon, how did you estimate the fuel:oxy mass ratios for the preburners? Your preburner outputs look quite hot, with a full flow design that's an absurd amount of thermal power going into the turbines....
Quote from: MikeM8891 on 09/30/2018 01:30 am. Vacuum Isp is not affected by chamber pressure.Chamber Pressure250 bar300 bar315bar330barThrust at sea level1,700 kN2,095 kN2,206 kN2,318 kNThrust in vacuum1,834 kN2,229 kN2,340 kN2,452 kNSpecific Impusle at sea level330.0 sec334.6 sec335.6 sec336.5 secSpecific Impusle in vacuum356 sec356 sec356 sec356 secWhat about improvements in combustion efficiency when running the engine further still from stoichiometric? Does higher chamber pressure enable running even more methane-rich? Other question: does increased chamber pressure mean decreased throat area? For a theoretical const-mdot engine?
Quote from: IainMcClatchie on 10/02/2018 02:58 amJon, how did you estimate the fuel:oxy mass ratios for the preburners? Your preburner outputs look quite hot, with a full flow design that's an absurd amount of thermal power going into the turbines....I chose 1000 F to minimize the pressure drop across the turbines needed to drive the pumps. The higher the turbine inlet temperature, the lower the pressure drop, the lower the overall pressure rise required of the pumps. Turbine materials are able to handle 100's of degrees higher temperatures, uncooled.John
Here are some rough estimates. I expect these are accurate to within 2%. Basically the vacuum thrust scales linearly with chamber pressure, something like ~7.4 kN/bar. The sea level thrust is going to be 134 kN less than the vacuum thrust; this is based on the 1.3 m nozzle exit diameter. Vacuum Isp is not affected by chamber pressure.Chamber Pressure250 bar300 bar315bar330barThrust at sea level1,700 kN2,095 kN2,206 kN2,318 kNThrust in vacuum1,834 kN2,229 kN2,340 kN2,452 kNSpecific Impusle at sea level330.0 sec334.6 sec335.6 sec336.5 secSpecific Impusle in vacuum356 sec356 sec356 sec356 sec
Isn't 1.7 m too large?
There is obviously a lot of scope for further optimisation of Raptor nozzles for both booster and spaceship. I think the Ø9m booster suffers badly from being too small a diameter.
I think the Ø9m booster suffers badly from being too small a diameter. If not constrained by the tooling they would probably prefer to go for a much larger diameter ~6x the optimised nozzle diameter + spacing pitch (assuming outer ring directly aligned with tank walls) to allow 37 engines, thicker tank walls, a shorter stack, reduced booster surface area, and create more drag on re-entry to lower terminal velocity and reduce landing burn fuel requirements.
Quote from: RobLynn on 10/11/2018 09:53 amI think the Ø9m booster suffers badly from being too small a diameter. If not constrained by the tooling they would probably prefer to go for a much larger diameter ~6x the optimised nozzle diameter + spacing pitch (assuming outer ring directly aligned with tank walls) to allow 37 engines, thicker tank walls, a shorter stack, reduced booster surface area, and create more drag on re-entry to lower terminal velocity and reduce landing burn fuel requirements. I agree on the problems with the smaller diameter. There appears to be advantages to increasing the diameter beyond 9 meters.Yes tooling is expensive, but they are just starting and are making foundational decisions they may live with for decades.Maybe make the 1.0 model of the BFS at 9 meters and get flying. But why not order another tool.That's my 2 cents from my armchair.
Quote from: wannamoonbase on 10/11/2018 04:25 pmQuote from: RobLynn on 10/11/2018 09:53 amI think the Ø9m booster suffers badly from being too small a diameter. If not constrained by the tooling they would probably prefer to go for a much larger diameter ~6x the optimised nozzle diameter + spacing pitch (assuming outer ring directly aligned with tank walls) to allow 37 engines, thicker tank walls, a shorter stack, reduced booster surface area, and create more drag on re-entry to lower terminal velocity and reduce landing burn fuel requirements. I agree on the problems with the smaller diameter. There appears to be advantages to increasing the diameter beyond 9 meters.Yes tooling is expensive, but they are just starting and are making foundational decisions they may live with for decades.Maybe make the 1.0 model of the BFS at 9 meters and get flying. But why not order another tool.That's my 2 cents from my armchair.This is SpaceX. The minute the 9 m flies, they'll be working on the 12, or the 15.Why would it be "decades" if even the first design didn't take even a single decade?
Is the planned expansion ratio still the same as in 2017 version? Or did they make the nozzle of the "sea level optimized" raptor slightly bigger in the 2018 version? (optimizing it for slightly higher than sea level to be better comphromize for vacuum, or optimizing it for higher chamber pressure even on sea level)Increasing the nozzle size might explain the width increase in the base of the rocket?