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)
Quote from: Peter.Colin on 09/16/2017 09:27 amPredicting 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?
Quote from: RotoSequence on 09/16/2017 10:54 amQuote from: Peter.Colin on 09/16/2017 09:27 amPredicting 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)
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
... 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: 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:
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.
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
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
Peter,.......You also seem to be obsessed with filling the base of the vehicle with rocket exhaust.......