2km/s is ridiculously high for EDL unless you're doing a large braking burn.Sandbagging stuff is not helpful and will mislead you.
Quote from: Robotbeat on 06/13/2016 07:08 pm2km/s is ridiculously high for EDL unless you're doing a large braking burn.Sandbagging stuff is not helpful and will mislead you....and reasonable Km/sec budget ranges from LEO to Mars surface are...?
7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission....a big question is Mars to Earth.
Quote from: Robotbeat on 06/13/2016 07:38 pm7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission....a big question is Mars to Earth.If you budget 6.3 km/s (5.0 km/s for TMI and 1.3 km/s for EDL), you will typically be able to make a ~100-day transit with plenty of margin. Fast TMI is typically 4.5 to 4.9 km/s:Edit: fuel loads and payloads can always be tweaked for faster or slower transits, so tank size isn't really a limiting design factor. Design for a standard case, then optimize for other factors.
Quote from: philw1776 on 06/13/2016 07:26 pmQuote from: Robotbeat on 06/13/2016 07:08 pm2km/s is ridiculously high for EDL unless you're doing a large braking burn.Sandbagging stuff is not helpful and will mislead you....and reasonable Km/sec budget ranges from LEO to Mars surface are...?7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission....a big question is Mars to Earth.
Upto now SpaceX modus operandi has been to under deliver initially and over deliver after a few iterations.
Dragon is not a scaled down MCT. So they will probably start with something small to validate it and learn and only later do the huge stuff when everything else closes.
But there's tons of room for mass optimization. How lightweight can things really get? Incredibly lightweight. Think backpacking, but with materials and manufacturing capabilities available in the 2030s.
I think he's talking about starting at EML1, dropping to ~LEO and doing an Oberth burn
Given Musk's suggestions of 2-3MN thrust to weight optimum for SC engines, I think that the design starts to look quite constrained. with perhaps the big choice being 2 rings (7-9 engines, 2000-3000 tonne GTOW) or 3 rings, 21-25 engines and 6-9000 tonne GTOW, and given development costs, launch site costs, and market it will almost certainly be the former.
MCT booster design height:An optimally over-expanded rocket (eg RD193) produces 56 tonnes thrust per square meter of nozzle area at sea level, won't be much difference between various hydrocarbon rockets as all have similar pressures and velocities at nozzle exit.Packing of multiple nozzles will only cover about 60% of the rocket base area at best, probably more like 50% given allowances for gimballing.Need about 1.3g acceleration at liftoff (or more for higher ISP)So 56*.5/1.3= ~20-25 tonnes of mass per square meter of base area that can be supported by a rocket. For LOX/CH4 bulk density of ~800kg/m that means that rockets can only support a column of fuel about 25-30m high in a prismatic form factor.There are of course advantages to being thin for reduced aero losses - so maximum fuel column height within this constraint is likely to be the design chosen.There will of course be substantial height taken by domed tank ends, 1st and 2nd stage engines, and finally cargo bay, but assuming a common bulkhead between LOX and CH4 and a single stick design I think could safely assume that will only be 30m+~1.4 stage diameter (4x domed ends for 2 stacked stages) + engine lengths (perhaps 4m per stage).So overall I think 1st+2nd stage very unlikely to be taller than 50-60m. Though payload might add 30m in some cases. Ie rocket portion is pretty damned similar heights to Falcon9. It will just be substantially bigger diameter.100M+ tall rockets only make sense for LH2.Given Musk's suggestions of 2-3MN thrust to weight optimum for SC engines, I think that the design starts to look quite constrained. with perhaps the big choice being 2 rings (7-9 engines, 2000-3000 tonne GTOW) or 3 rings, 21-25 engines and 6-9000 tonne GTOW, and given development costs, launch site costs, and market it will almost certainly be the former.Assuming 9x2.5MN engines each 4m long , that is about 9m diameter and total 50m tall (without cargo)Assuming 25x2.5MN engines each 4m long, that is about 14m diameter and 58m tall (without cargo)
How about the reusable RaptorVac S2 first, likely 5+ meters, with a later Falcon-Raptor high performance S1 to replace the F9 Merlin cores? That eliminates duplicious ground systems and possibly having 2 launchers.