Quote from: envy887 on 04/22/2016 03:30 pmThe other question is how they will want to size the Raptor. At the currently stated 2300kN a single Raptor would be rather underpowered for throwing 100t into LEO, although probably not worse than the Centaur US. A single 2300 kN engine would be ideal for TMI and Mars return though, and dragging extra engines to Mars and back isn't ideal.You're going to need multiple engines to propulsively land BFS back at KSC, Brownsville or wherever, which is absolutely critical for the economics of the architecture. I'm assuming extendable/discardable nozzle extensions on the BFS raptor, which may be a distinctive variant from the initial upper stage raptor vac. Besides, if I was the commander of a 100 individual interplanetary spaceship, I'd either want engines with a ludicrous reliability or a few with a wide gimbal so I have engine out redundancy. Nobody wants to be marooned.They are going to be carting a few unnecessary engines with them, I bet the most edible of my cowskin hats on it.
The other question is how they will want to size the Raptor. At the currently stated 2300kN a single Raptor would be rather underpowered for throwing 100t into LEO, although probably not worse than the Centaur US. A single 2300 kN engine would be ideal for TMI and Mars return though, and dragging extra engines to Mars and back isn't ideal.
Quote from: Impaler on 04/22/2016 04:22 amAnother thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.Questions arise...How long does the SEP tug take to bring the BFS and/or the propellant to EML-1 and return to LEO? How many SEP trips to EML-1 per single BFS launch to Mars and return?How efficient are SEP solar panels after numerous long trips thru the Van Allen belts? How many BFTanker flights to re-fuel the SEP tug itself for its various transits to support one Mars round trip mission? Including bringing up the BFS propellant that the SEP carries all the way to Mars orbit.Besides the added expense, resources and complexity of developing the SEP tug.
Another thing that virtually no one here talks about is MARS ORBITAL RENDEZVOUS.If the BFS can be refueled in LEO then it can likewise be refueled in LMO if propellant can be brought their.This would break the DeltaV budget for Earth return into two legs, assent to LMO and then TEI resulting in a vastly smaller vehicle AND a faster Earth return then would be possible with a single direct launch of even a huge vehicle.To make the vehicle capably of departing for Mars it must start at EML-1 fully fueled which again will allow for a faster transit then a large vehicle starting in LEO.To get the propellants to LMO and the vehicle to EML-1 you use the same solution a SEP tug, fist it moves the BFS to EML-1, picks up and drops off fuel between LEO and EML-1, then makes a fuel run all the way out to mars to rendezvous with the BFS a second time and finally returns to Earth to repeat the cycle.The BFS would only need around 4-5 km/s DeltaV capability in this scenario which makes it hugely smaller and simpler, to launch to LEO the BFR is a 2 stage rocket like F-9 but with reusable 2nd stage which can carry payloads other then the BFS on top.
SpaceX pushes mass fractions to the limit so a 100t empty mass is probably conservative, but lets go with that. However, Saturn V was a 3 stage to orbit system, so the comparisons to a TSTO aren't really accurate.
BFR will likely have mass fractions much closer to Falcon 9 than Saturn V, but with a better ISP than either. Falcon 9 stages at around 2.2 to 2.5 kms, and the upper stage adds about 5.3 kms to get it to LEO. With Raptor's higher ISP, it only takes about 625t of prop to get a 200t ship+payload into LEO.If they can stage at 3.5 kms (with enough prop in the BFR to boostback to reentry at a survivable Mach 6 or 2 kms), then the upper stage/ship only has to add about 4.5 kms to LEO, which happens to be about the same performance it needs to sent 100t through TMI and do a 1 kms (ish) EDL burn at Mars. That only takes about 500t of prop.The other question is how they will want to size the Raptor. At the currently stated 2300kN a single Raptor would be rather underpowered for throwing 100t into LEO, although probably not worse than the Centaur US. A single 2300 kN engine would be ideal for TMI and Mars return though, and dragging extra engines to Mars and back isn't ideal.
Besides, if I was the commander of a 100 individual interplanetary spaceship, I'd either want engines with a ludicrous reliability or a few with a wide gimbal so I have engine out redundancy. Nobody wants to be marooned.They are going to be carting a few unnecessary engines with them, I bet the most edible of my cowskin hats on it.
For fast transits you want to go nuclear. See for example the DRM 5 Addendum 2. In fact I don't think one synod return is possible without EP or refueling in LMO, because the delta-v is SSTO-level or higher. From both options, EP or LMO refueling, I would pick EP because it also saves you a ton of mass in LEO and it doesn't require a huge launch infrastructure on Mars. Note that you would not refuel the BFS in LMO, the EP would bring it back. In fact all in-space propulsion would be done with EP, however the BFS would do aerocapture at Mars.If SpaceX plans to use a reactor on Mars they could use it for NEP too, if it has sufficiently low mass.
Plugging the Saturn IC data in the rocket equation shows that deltaV was just over 3.3 Km/s, it also staged at a pretty low altitude (good for the RTLS thing). Let's assume that the MCT booster does likewise, that leaves about 7 Km/s deltaV to the second stage (aka BFS) which is ok if you plan to refuel and do a TMI burn.
As per word of Musk, the ship will be capable of putting 100 t of useful mass on Mars, so let's make it another 100 t for structure (engines, TPS, legs, you name it). So that leaves a 200 t dry mass vehicle, capable of 7 km/s deltaV, with engines giving a Isp of 380 s you need a whopping 1150 t of propellant and a total BFS mass at lift off of about 1350 t.
RB: You completely inverted the meaning of 'Battlestar Galactica'. That was Zubrin's pejorative term for a single huge vehicle that carries all propellant from Earth which was the NASA plan in the 90 day report.Zubrin's argument was two fold, use insitu propellant on mars to reduce outbound propellant needs (which we all agree Musk is doing) AND using a number of smaller modular elements that serve specific roles and provide redundancy. The Battlestart Galactica term was specifically about not making a single all inclusive budget busting vehicle designed to perform multiple tasks.
The BFR/BFS is only big because SpaceX's cargo requirement is hugely ambitious (100t to Mars surface), it wouldn't be so big if the cargo requirement was reduced to be inline with what Zubrin or NASA imagined.
Quote from: su27k on 04/24/2016 05:04 amThe BFR/BFS is only big because SpaceX's cargo requirement is hugely ambitious (100t to Mars surface), it wouldn't be so big if the cargo requirement was reduced to be inline with what Zubrin or NASA imagined.I'd say NASA is currently downsizing its Mars plans, not in terms of payload landed on Mars (still ~80t), but in terms of the size of the individual elements. It's now considering 27t or even 18t landers (instead of 40t), chemical in-space stages with masses around 40t, SEPs with a few hundred kw. Nothing monstrous. In fact the only thing monstrous left is SLS. All that I suppose in an effort to save cost. SpaceX does exactly the opposite, it's going very big (of course SpaceX attempts to colonize Mars instead of sending 4 people every few years).That's why I fear that SpaceX will present a paper at IAC but since it's so diametrically opposed to NASA's evolvable Mars campaign it won't have any impact. In fact it might even give congress more legitimation to push forward with SLS (Look! SpaceX wants to build an ever bigger rocket!).
(Note that all Musk has ever said about the Moon is, paraphrasing, "it's on the way, I guess we should look at how to get there, but it's not anything like a goal of the architecture to be able to go to the Moon".)I, for one, would like to see a new interplanetary transportation architecture be designed to support more than one possible destination, I guess...
Quote from: the_other_Doug on 04/24/2016 05:40 pm(Note that all Musk has ever said about the Moon is, paraphrasing, "it's on the way, I guess we should look at how to get there, but it's not anything like a goal of the architecture to be able to go to the Moon".)I, for one, would like to see a new interplanetary transportation architecture be designed to support more than one possible destination, I guess... Just because going to the Moon is not a goal of the MCT architecture doesn't mean the system won't be capable of being used to go to the Moon. I'm sure people will come up with all sorts of non-Mars uses for the MCT architecture once it's in existence (or even before).
Quote from: CuddlyRocket on 04/24/2016 08:39 pmQuote from: the_other_Doug on 04/24/2016 05:40 pm(Note that all Musk has ever said about the Moon is, paraphrasing, "it's on the way, I guess we should look at how to get there, but it's not anything like a goal of the architecture to be able to go to the Moon".)I, for one, would like to see a new interplanetary transportation architecture be designed to support more than one possible destination, I guess... Just because going to the Moon is not a goal of the MCT architecture doesn't mean the system won't be capable of being used to go to the Moon. I'm sure people will come up with all sorts of non-Mars uses for the MCT architecture once it's in existence (or even before).Imagine Hubble in the Smithonian.
Imagine Hubble keeps operating nearly indefinitely. Much better, IMHO...
That's why I fear that SpaceX will present a paper at IAC but since it's so diametrically opposed to NASA's evolvable Mars campaign it won't have any impact. In fact it might even give congress more legitimation to push forward with SLS (Look! SpaceX wants to build an ever bigger rocket!).
Quote from: Robotbeat on 04/27/2016 01:34 amImagine Hubble keeps operating nearly indefinitely. Much better, IMHO... Nah, we all love it but it's old. Build a one significantly newer and 4x the size. Hell, with that kind of capacity you could have a dedicated manned observatory station, or even throw a hubble-esque telescope into a solar orbit.
Quote from: The Amazing Catstronaut on 04/27/2016 08:38 amQuote from: Robotbeat on 04/27/2016 01:34 amImagine Hubble keeps operating nearly indefinitely. Much better, IMHO... Nah, we all love it but it's old. Build a one significantly newer and 4x the size. Hell, with that kind of capacity you could have a dedicated manned observatory station, or even throw a hubble-esque telescope into a solar orbit.You wouldn't want a manned observatory station - too much vibration and other interference. Better to have an unmanned observatory which can reached and upgraded quickly and cheaply, as needed. Which is exactly what a "cislunar" BFS could provide.