If you were to have an abort system on MCT, it'd have to work for Mars ascent (as well as Earth), and probably even terminal landing as well. No, this is not impossible. Hard, but not impossible.Or just not have an abort system.
On the issue of an abort capsule I agree with guckyfan and Robotbeat, it is impractical and offers very little use at Mars while on Earth a whole vehicle abort is more practical.
Shotwell mentioned about BFR a few months ago at the South Summit 2015 (Oct 7-9), in Madrid, " [Falcon Heavy] This one is about 4M pounds of thrust, and the mock... the vehicle that takes us to Mars will be three or four times that size"(original video, mostly Spanish-language conference proceeding, but Shotwell's voice still appears beneath a title graphic for the first ten minutes, though not her face. The video I linked above seems to have been created a while after this one was promoted, and does a proper cut to her presentation alone)I also vaguely remember her mentioning offhand that they were developing a 180-210t to LEO superheavy launcher. I've been trying to find the interview, but can't turn anything up.
Quote from: Burninate on 01/20/2016 02:24 amShotwell mentioned about BFR a few months ago at the South Summit 2015 (Oct 7-9), in Madrid, " [Falcon Heavy] This one is about 4M pounds of thrust, and the mock... the vehicle that takes us to Mars will be three or four times that size"(original video, mostly Spanish-language conference proceeding, but Shotwell's voice still appears beneath a title graphic for the first ten minutes, though not her face. The video I linked above seems to have been created a while after this one was promoted, and does a proper cut to her presentation alone)I also vaguely remember her mentioning offhand that they were developing a 180-210t to LEO superheavy launcher. I've been trying to find the interview, but can't turn anything up.Above Quote from the Update thread3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.
Quote from: philw1776 on 01/20/2016 01:21 pmAbove Quote from the Update thread3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.15m wide is pretty reasonable, but the main utility of that is to prevent it from being anywhere near 300m tall. I don't expect it to be anywhere near the fineness ratio of F9. A cylindrical rocket doesn't have infinite room to scale up; The base grows as n^2 while mass grows as n^3, and eventually the base's space to fit rocket motors limits further growth. Additionally, we have raised issues with eg the launch facilities here.I suspect you can't build it much smaller than 12m and still have realistic height.If you try to build it larger than a number somewhere in the 15m-18m range, then you run into issues where bridges need to be rebuilt to get the parts in place, and propellant tanks lose their cylindrical character.
Above Quote from the Update thread3 to 4 times FH thrust fits well with the 12.7-14.7 million pounds thrust BFR models I've built that conform to prior SX statements. It contradicts the ludicrous 15m wide 300+m tall !!! behemoths posted on Reddit which would require over 400 Raptors to lift off if the posters had simply calculated the propellant volume and subsequent mass.
With ~30 engines you probably don't need gimbaling, or at least not on most of the engines. Some variable thrust and a few engines along the edge that gimbal on just one axis should be sufficient and is the Russian way of doing it.
Quote from: Impaler on 01/21/2016 02:35 amWith ~30 engines you probably don't need gimbaling, or at least not on most of the engines. Some variable thrust and a few engines along the edge that gimbal on just one axis should be sufficient and is the Russian way of doing it.That worked sooo great for the N1. I know that was failure of testing, but using variable thrust for control is a truly *terrible* idea. That's when you need more power, not less of it.A few central engines may be fixed, but most should fully gimbal for general control authority and engine out capability. Having all the engines be identical (including their gimbal setups) also simplifies mass production and testing procedures, as SpaceX illustrate whenever their F9 rocket launches. (it seems to be a VERY common misconception that some F9 engines are fixed, they are NOT)
With that said, I don't have any good reason to think non-gimballing engines are going to be a thing on BFR. With MCT, on the other hand, there may be value in fixing the combustion chambers as well as the massive engine bells rigidly to the spacecraft; Those gimbals are a source of potential failure with a few years of vacuum between uses, and whether the odds of that are greater than the odds of some other steering system failing, is a bit more of an open question than with BFR.
~30 engines (+/-50%) sounds about right to me. Around the same as Falcon Heavy, but in a single core (which makes engine-out management a little simpler).15m, too....by the way, we can know for a fact that if it's only 15m in diameter, it is going to be limited to around, say, 50 engines of about 500klbf each if they are squished together as close as they can go and have an exit pressure of around near sea level and a nice high chamber pressure of about 2000-3000psi. You cannot physically fit in more engines. For a lift-off T/W of 1.1 or more, that puts a hard upper bound of around 10,000 tons. So if 15m is the diameter (it probably is), then the lift-off mass cannot be more than 10,000 tons. And that leaves ZERO room for gimballing.A more sane packing arrangement would leave them at around 6000 tons or less.
Re the gimbal / variable-thrust steering discussion for the BFR (booster, not MCT). Given ~30 Raptors what would be the approximate weight saving of variable-thrust?
I still think the BFR/MCT diameter will not exceed 10m. There are many reasons for this. I also think this version would be more economical in the long run. A 3 core heavy for MCT launch. Say 5.5m-8m for a single core for launching deep space probes, filling a fuel depot and other cash making activities. The single core would be able to launch 80-100 tons. A 3 core heavy version 250 tons. A fully reusable 80 ton launcher might eliminate Falcon Heavy. Say it had 9 engines at 550k lbs thrust or slightly more for a 5 million lb thrust reusable rocket. This rocket can put up a lot of stuff to make money from the Air Force, NASA or others to help pay for MCT. A single core could also be adapted to have two Falcon 9 cores attached for boosting slightly over 100 tons to LEO and could still launch from the cape.