And here is another indicator for you, that my estimate is not too far off. The 2016 IAC presentation had a lot more detail about the various masses. In it, the BFS dry mass was 150 tons and the tanker dry mass 90 tons. So the tanker has exactly 60% of the dry mass of the BFS. Applying that to the current BFS with 85 tons dry, I get 51 tons dry mass, which is almost exactly what I had calculated for the cargo version earlier. http://spaceflight101.com/spx/wp-content/uploads/sites/113/2016/09/ITS-022.jpg
Quote from: Elmar Moelzer on 04/13/2018 03:28 amAnd here is another indicator for you, that my estimate is not too far off. The 2016 IAC presentation had a lot more detail about the various masses. In it, the BFS dry mass was 150 tons and the tanker dry mass 90 tons. So the tanker has exactly 60% of the dry mass of the BFS. Applying that to the current BFS with 85 tons dry, I get 51 tons dry mass, which is almost exactly what I had calculated for the cargo version earlier. http://spaceflight101.com/spx/wp-content/uploads/sites/113/2016/09/ITS-022.jpgNote that I made the same observation above... A cargo vehicle would be slightly heavier since it adds a cargo door with associated structure and hardware, and a payload adapter. That should add less than 10% though.
So, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?
Quote from: rakaydos on 04/13/2018 04:29 pmSo, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?I would assume that you can go straight to full throttle since there are many engines that can't throttle lower than 50%?
If they are serious about the point to point market then a gas-turbine based booster stage may start to look tempting. Can probably boost to Mach 1-2 at 20000m, allow high Isp engines on the BFS, save perhaps up to $100k of fuel per flight, and use engines that are quieter and potentially Ok for many 1000's of flights. Downsides: developments costs, high dynamic pressure staging, and probably a billion dollars worth of Jet Engines to launch the BFS.
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.
Quote from: RonM on 05/04/2018 01:42 pmThere's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.
Quote from: Lars-J on 05/04/2018 04:32 pmQuote from: RonM on 05/04/2018 01:42 pmThere's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.F135-PW-100 engine has afterburning T/W of about 11, expected to increase about 10% soon. For a low deltaV booster that is more than good enough. Thrust can be (relatively easily) augmented with water/LOX mass injection MIPCC, which can maintain thrust and good Isp for up to about Mach 4 and 30km altitude with Isp in 1500-2000s range http://www.ssdl.gatech.edu/sites/default/files/papers/mastersProjects/YoungD-8900.pdfI don't expect SpaceX to persue this. But it does have more potential for a high flight rate application like point-to-point ballistic transport than it ever did for low flight rate space-launch.
Quote from: Lars-J on 05/04/2018 04:32 pmQuote from: RonM on 05/04/2018 01:42 pmThere's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.F135-PW-100 engine has afterburning T/W of about 11, expected to increase about 10% soon. For a low deltaV booster that is more than good enough.
Thrust can be (relatively easily) augmented with water/LOX mass injection MIPCC, which can maintain thrust and good Isp for up to about Mach 4 and 30km altitude with Isp in 1500-2000s range http://www.ssdl.gatech.edu/sites/default/files/papers/mastersProjects/YoungD-8900.pdf
I don't expect SpaceX to persue this. But it does have more potential for a high flight rate application like point-to-point ballistic transport than it ever did for low flight rate space-launch.
This thread is about the BFS, which may be marginally SSTO capable, not the F9 upper stage.BFS does not require much saving in delta V or boosted Isp to give it a lot more payload capacity. Eg for 80tonnes in LEO the BFS as an SSTO with mission 'average' Isp of 3500m/s and 9300m/s deltaV you need about 1150tonnes GTOW. But for Mach2-3 and 20-30km altitude staging from a gas turbine booster you can get more like 3700m/s Isp and assuming 8300m/s deltaV need more like 750 tonnes at staging - a 300 tonne rocket fuel saving (assuming 100 tonnes in jet engines) or a 40tonne payload boost - possibly 200 paying passengers. Which could be increasing revenue by 0.5-1 million dollars per flight.
10x 100 reuse booster raptors that cost $3million each would be $300k per flight.60x $10million afterburning jet engines that last 3000 flights would be $200k per flight, and should be saving a lot of fuel expense as well.
Again I am not really advocating this. I am simply pointing out that the economics of gas turbine based boosters are more attractive for this use case than they have ever been for other space-launchers due to the potentially high flight rate, and desire for operating near to major cities.
What would be the point of that? If SSTO is not viable, then just use the BFR first stage with it and it's all good. I think that strap on F9 boosters would be a bad idea for so many reasons. Two different fuels to handle for starters, different launch pad needed and infrastructure needed, etc, etc.
If SSTO BFS is possible but marginal then is it possible that SpaceX might experiment with on orbit refuelling from another vehicle (but not the tanker variant) before BFR and the tanker variant become available?