I think Zubrin just wants to live to see boots on Mars. So anything that can go there NOW rather than in 10 years will have his support. Because he has seen too many of the 10 year estimates always remain 10 years away.Anyway, I think he is wrong and Musk is clearly right. This time 10 years might really be just 10 years away.
Good point. We have had the tools since the 60's to go to Mars. But not the political will to spend the money.
10 MUSK years but those are shorter than NASA years...
Quote from: Lar on 02/22/2018 08:36 pm10 MUSK years but those are shorter than NASA years... It's good to hear it put this way. The whole "time dilation/Elon time" criticism is tiresome and this gives perspective.
Zubrin, along with most of the Space old guard just don’t take BFR/BFS seriously, it’s too outside the paradigm. What SpaceX says it’s doing is mass producing general purpose Spaceships the size of Jumbo airliners that cost about the same to build and operate. They don’t buy that. The BFR/BFS approach doesn’t scale down to what would have seemed reasonable, starting with a few people and small specialized very expensive ships. If it works, it works by entirely transforming the scale and assumptions of the industry. Otherwise it just crashes and burns.
x-33/venturestar is perhaps the best example of this, a vehicle which can only work if three untried at the time technologies had worked well.(metallic TPS, linear aerospike, conformal composite hydrogen tanks).
By non-fragile, I mean not requiring any single technology to work well beyond the state of the art.BFS - at least the 2017 variant is lots less fragile systemically than V*.There are several major risks that might be considered.Graphite structure: Swap to Al/Li, for either stage, or end up at double the target structural massRaptor:If you get no better thrust than has already been announced, and it explodes occasionally.Tanks:End up needing to line the tanks.Heatshield:Ends up only capable to 5km/s.None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.V*, shuttle, ... all had such narrow margins of success that one subsystem underperforming badly would kill the project, meaning 'failure is not an option' and exploding the costs even before questions of efficiency of contractors or cost+ or ... is raised.
Many of his 'aspirational' targets are for things that wouldn't even be on the horizon (or happen at all) if it wasn't for his companies. That's why I'm a bit baffled when people dismiss him entirely for the delays.The worst go from 'This will never happen, how can you take him seriously?' to 'Yeah, it happened, but it took 4 years instead of 2, how can you take him seriously?' Some people like to only see the negative side I guess, even when the positive prevails...
Quote from: speedevil on 02/22/2018 09:09 pmGraphite structure: Swap to Al/Li, for either stage, or end up at double the target structural massRaptor:If you get no better thrust than has already been announced, and it explodes occasionally.Tanks:End up needing to line the tanks.Heatshield:Ends up only capable to 5km/s.None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.BFS's goal is much higher. Reverting back to LiAl will seriously cut the payload. We know the payload loss due to the TPS mass and additional stiffening killed the idea of recovering the F9 US so if that's too heavy BFS payload will go down a lot. Basically SX wouldn't be planning new TPS and new structural materials and new engine cycles and new engine fuels if what worked for F9 or even FH would get the job done. They innovate when necessary.The fact that BFS has so much innovation says it's all needed to fly the mission. Some might under perform and you end up with a lower payload but you don't need much to seriously compromise a design. It's an open question how much you could sacrifice before the design is no longer viable. 1 tonne? 10tonnes? 50tonnes?
Graphite structure: Swap to Al/Li, for either stage, or end up at double the target structural massRaptor:If you get no better thrust than has already been announced, and it explodes occasionally.Tanks:End up needing to line the tanks.Heatshield:Ends up only capable to 5km/s.None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.BFS's goal is much higher.
Reverting back to LiAl will seriously cut the payload. We know the payload loss due to the TPS mass and additional stiffening killed the idea of recovering the F9 US so if that's too heavy BFS payload will go down a lot.
Quote from: dglow on 02/22/2018 03:00 amHis core argument is that we've always been able to do Mars, even with the launchers we had (or could derive from what we had) readily available. Simple vehicles driven by more launches off smaller rockets... vs giant rockets flying very, very complex vehicles.I think you have that backwards. Watching his presentation suggests more "viable (big) rockets regularly launching a minimal architecture with the least possible number of special units."
His core argument is that we've always been able to do Mars, even with the launchers we had (or could derive from what we had) readily available. Simple vehicles driven by more launches off smaller rockets... vs giant rockets flying very, very complex vehicles.
The key single point of failure seemed to be the 100Kw nuclear reactor. Today Kilopwer promises much more "granular" units at 10Kw each (so no single point of failure) with a real shot at being built and flying.
TBH with the description of the BFR/BFS, initial flights using at least 2 BFS's at a time (with a goal to grow the fleet over time) and the possibility of Kilopower reactors going with them, I'd think Zubrin was feeling that Musk and SX are seeing things more his way. Big rockets with a few big payloads used repeatedly. I could see him still being concerned about prioritizing short journey times over a fail-safe free return trajectory, but that could change before first flight.
I think what causes problems is Zubrins willingness to admit that that in fact NASA cannot have everything.". NASA has a finite budget and if you want to actually send people out (rather than talking about sending someone out) you have to make hard (and sometimes very hard) choices. That is a very uncomfortable message for a lot of people to hear.
Remember that NASA did a "Weight scrub" on Shuttle to lose about 15% of its mass because (IIRC) the SSME was 3secs below target Isp and the SRBs were 2secs below target Isp.
Quote from: john smith 19 on 02/22/2018 11:14 pmRemember that NASA did a "Weight scrub" on Shuttle to lose about 15% of its mass because (IIRC) the SSME was 3secs below target Isp and the SRBs were 2secs below target Isp.A more worked through example with underperforming engines.Taking a BFR with the nominal published liftoff mass of 4400 tons, and assuming that is a 150 ton payload, leads to a BFR with a fuelled mass of some 3045 tons, and assuming the same dry mass ratio as BFS leads to 255 tons dry.700m/s of landing delta-v comes to 314 tons including landing fuel.At staging, the stack weight is this + 1355 (150 ton payload, 85t structure, 20t landing fuel)So, 4400->1669, with ISP of 330 for SL raptors gives 3134m/s.Second stage comes out to 255t in LEO.1355 -> 255, with ISP of 375 = 6138m/s.Or 9272m/s.In order to 'only' have a 50 ton payload to the same delta-v, the second stage vacuum raptors can be replaced with sea-level merlins, with a little margin.Use vacuum merlins, and it's up at 120 tons.Replacing the booster raptors with merlins drops the delta-v from 3134 to 2678, 456m/s.Redoing the sums comes out with the thing still launching 59 tons (79 if you don't need to land)The most important part here is that much, or most of what's going to Mars is not unitary payloads. Even if the raptors underperform so they are no better than flying merlins, Mars is not much affected, as you can launch 79 tons of payload to orbit in one lump.I think it's going to be quite hard to find a design where you don't have 71 tons of movable stores, tank contents, ... in the nominal 150 ton payload.Yes, it's more annoying, and you now need to launch about 18 tankers, not 6 to fill BFS, and you do need an extra two flights to launch cargo, and you are going to have to man (or robot) handle it over.And yes, if you do not fix this by the time you get crew on Mars, it about doubles your ISRU needs as you need two BFS to get back to earth (one retanks the other in orbit and relands).Without the ability to on-orbit refuel, and in this case, transship cargo, any tiny mass growth balloons into inevitable loss of capability really fast.With it, and without a specific unitary payload that absolutely must be launched, there is flexibility for really quite large underperformance.This doesn't of course work at all if the 12 extra launches cost $100M each.In addition to any built in margin on the above structures, the 20t of landing fuel lets 20t of weight growth happen, or 20s underperformance of the s2 engines, or 300m/s lower booster speed, with no penalty at all to the nominal unitary launch performance.You just need to refuel.Almost anything else other than the ability to be able to refuel, and rapidly turn around the vehicle can be forgiven. (I am not above suggesting kerosene is a suitable propellant for BFR, especially to BEO, it would raise all sorts of issues, just a raptor underperforming that badly)
You are assuming the tanker has the same dry mass and payload to orbit as the crew ship? That's unnecessarily conservative.The booster does need more than 700 m/s to land, but it also won't have the same dry mass fraction as the ship. The F9 booster is already almost 40% better mass fraction than the BFS.I really like this type of sensitivity analysis. It's very useful to find the weak points in the design. A spreadsheet that calculates everything for a given under performance would be neat.
I hear you and take your point. Mine was more longitudinal: proposing Saturn V could take us to Mars when conventional wisdom said we needed Nova. Or that we could move faster via a Shuttle derivation than waiting for something newer and bigger like NLS.
My money is on the first BFR ISRU plant using nuclear, possibly supplemented by solar. Elon will only publicly admit to solar for the foreseeable future, simply to avoid the public shitstorm education that we'll all endure before the first reactors are launched.
QuoteI think what causes problems is Zubrins willingness to admit that that in fact NASA cannot have everything.". NASA has a finite budget and if you want to actually send people out (rather than talking about sending someone out) you have to make hard (and sometimes very hard) choices. That is a very uncomfortable message for a lot of people to hear. Indeed. Zubrin is every bit the hot-blooded, wild-eyed visionary. Makes Elon look like a diplomat by comparison.
Quote from: envy887 on 02/23/2018 04:45 amYou are assuming the tanker has the same dry mass and payload to orbit as the crew ship? That's unnecessarily conservative.The booster does need more than 700 m/s to land, but it also won't have the same dry mass fraction as the ship. The F9 booster is already almost 40% better mass fraction than the BFS.I really like this type of sensitivity analysis. It's very useful to find the weak points in the design. A spreadsheet that calculates everything for a given under performance would be neat.I was mostly basing the booster landing on estimates of F9 landing delta-v, and using the IAC mars as conservative.I think the mars 700m/s or so delta-v is likely overkill on earth for the top stage.Why do you think the booster is likely to need more delta-v?Part of the reason for not assuming lower masses for parts is to allow for it to be rugged enough for lots of launches.The (poorly titled) "BFS, how bad can it be" https://forum.nasaspaceflight.com/index.php?topic=45000 could really do with a spreadsheet - I'm not awake enough to have a stab at it at the moment.I'm pretty sure it comes out to the above outline though - if you can get it to reliably and rapidly be reused without explosions, a very great amount can be forgiven in the other technical aspects that might be 'failure' with a vehicle designed to launch specific payloads.Perversely, the very lack of a clue about what to put on Mars actually helps.If you're not very deep down the Martian equipment rabbit-hole when it becomes clear what the actual maximum mass you can launch in one lump is, then it becomes much less of an issue than if you then have the rocket people and the payload people screaming at each other, and them both screaming at the finance people if it's less than planned.
But the booster needs ~3000 m/s for RTLS (roughly 1800 for boostback, 500 for entry, 700 for landing). Though it could probably be adapted for downrange landing with less than 1,500 m/s. This is obviously already feasible with Merlin and Al-Li tanks, since F9 does it. Raptor and CRFP should be a significant improvement.