They could also try it like the russians/sovjets did with the N1
Ah yes.Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?
Quote from: Pete on 01/22/2015 07:30 amAh yes.Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?Weren't N1 failures related to plumbing, not gimballing?
Yes, but it was still a terrible - truly terrible - idea. For every engine you lose, you need to turn off the opposite number.
It's still not a great system. It limits the engine-out options because once again you have only a limited number of steering engines. Also as has already been said, throttle response is slower than actuators.
Quote from: malenfant on 01/22/2015 10:57 amIt's still not a great system. It limits the engine-out options because once again you have only a limited number of steering engines. Also as has already been said, throttle response is slower than actuators. I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.
They must not trust in space assembly and docking much at SpaceX.
Quote from: Hotblack Desiato on 01/22/2015 06:46 amThey could also try it like the russians/sovjets did with the N1Ah yes.Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?Besides, one of the functions of gimballing is to provide roll control. That is a bit difficult to do, with fixed engines with only throttle tweaks.
Quote from: Pete on 01/22/2015 07:30 amQuote from: Hotblack Desiato on 01/22/2015 06:46 amThey could also try it like the russians/sovjets did with the N1Ah yes.Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?Besides, one of the functions of gimballing is to provide roll control. That is a bit difficult to do, with fixed engines with only throttle tweaks.Considering that the Antares rocket and the N-1 used the same engines, I'm gonna agree with you on this.
We've given you lots of reasons, are you ignoring them?
Quote from: Eerie on 01/22/2015 11:57 amQuote from: malenfant on 01/22/2015 10:57 amIt's still not a great system. It limits the engine-out options because once again you have only a limited number of steering engines. Also as has already been said, throttle response is slower than actuators. I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.FWIW the third flight failed when the stage broke up due to excessive roll. Of course everything is fixable and I understand that roll control was increased after that flight.Point is it's not a solution you would choose if you had options. SpaceX have options, the most obvious of which is to increase the stage diameter.My personal opinion is that Raptor will be bigger anyway. Elon Said T/W optimised around this number. He never said they were settled on building it to this number. Trades are clearly ongoing and there are other factors beside T/W.
The most recent comments on BFR indicate that they are optimized at 500klbf per engine, and simply using lots of engines to generate sufficient thrust. I am trying to model this concept for "100 tons useful cargo to the surface of Mars", and reusable return of the lander, which is likely to represent a launch vehicle of roughly 200 tons to LEO, and a bunch of refueling missions.Raptor will be a full-flow staged combustion engine producing 500klbf per engine with (based on older comments that may no longer be reliable) a vacuum Isp of 363 seconds and a sea-level Isp of 321 seconds.What I would like to know, is what are the dimensions we should expect? What I'm having trouble with is actually fitting all those engines into the rear of the rocket; I have no context to understand how closely they should be spaced without triggering cascading failures if one explodes, or how big the engine bells should be. 10-15m seems to be the consensus on fairing diameter, but my intuitive guesses of scale (2m bell diameter with ~3-4m centerline spacing) are clearly wrong, because they indicate 15m is not big enough for this launch vehicle.*Please distinguish between centerline-to-centerline distance, diameter of the bells, and airgap distance between the outer edges of the bells, or this will get confusing.**Please distinguish between two different engines: A sea-level optimized Raptor, and a vacuum-optimized Raptor. My packing problem is with the SL raptor, the VAC raptor I have not begun modelling***Would the fact that this is a tightly packed array of engines allow for smaller nozzles?
The most recent comments on BFR indicate that they are optimized at 500klbf per engine, and simply using lots of engines to generate sufficient thrust. I am trying to model this concept for "100 tons useful cargo to the surface of Mars", and reusable return of the lander, which is likely to represent a launch vehicle of roughly 200 tons to LEO, and a bunch of refueling missions.Raptor will be a full-flow staged combustion engine producing 500klbf per engine with (based on older comments that may no longer be reliable) a vacuum Isp of 363 seconds and a sea-level Isp of 321 seconds.What I would like to know, is what are the dimensions we should expect? What I'm having trouble with is actually fitting all those engines into the rear of the rocket; I have no context to understand how closely they should be spaced without triggering cascading failures if one explodes, or how big the engine bells should be. 10-15m seems to be the consensus on fairing diameter, but my intuitive guesses of scale (2m bell diameter with ~3-4m centerline spacing) are clearly wrong, because they indicate 15m is not big enough for this launch vehicle.
*Please distinguish between centerline-to-centerline distance, diameter of the bells, and airgap distance between the outer edges of the bells, or this will get confusing.**Please distinguish between two different engines: A sea-level optimized Raptor, and a vacuum-optimized Raptor. My packing problem is with the SL raptor, the VAC raptor I have not begun modelling***Would the fact that this is a tightly packed array of engines allow for smaller nozzles?
I think you're thinking about it the wrong way. Not that long ago Elon (or someone else and SpaceX) made a comment about in orbit refueling. That coupled with the recent comments about Raptor only being 500klbs makes me thing SpaceX could be changing paradigm. They may be going away from a ginormous 12-15m LV with 15Mlbs of thrust or more, to something smaller and easier to handle. (By "smaller" I mean more Saturn V size). Engines that are smaller and easier to handle, cores that are smaller and easier to handle, etc. And then they can utilize the volume that helps to make reusability beneficial. More smaller launches with full reusability rather than fewer larger ones. At the end of the day, BFR only needs to be large enough to get a dry MCT to LEO. It can be fueled and crewed in orbit with subsequent BFR launches while prepping for a MArs mission.
...People who argue that only a few engines should gimbal seem to forget the scenario where a gimballing engine shuts down...