Quote from: OneSpeed on 08/27/2016 03:20 amWhilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?A Red Dragon's heat shield and motors don't need to survive past touchdown. Safe for it is no destructive motor failures. Gouging up the heatshield and throwing rocks up into motors is fine as long as it can land once. A vehicle that has to take off again and do another re-entry needs an intact heatshield and motors for Earth return.
Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?
If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?
Approaching 30 days from what (we hope) is the big reveal, I thought it a good time to revisit and post revised BFR/MCT speculation before any info leaks out. [...]Anyone else want to update their speculations?
I would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.
Quote from: OneSpeed on 08/27/2016 04:15 amIf the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?Launching directly back to Earth is the highest demanding part of the mission¹, those first moments of take-off from the Martian surface will have the highest fuel load and hence require maximum thrust. You can't afford cosine losses of canted engines. Otherwise the stage is overpowered for every other part of the missions, and hence wasting mass.²
Shotwell dropped the name Falcon 20 for the BFR. >
Regarding decelerator devices...Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?
Quote from: Oersted on 08/26/2016 11:10 pmRegarding decelerator devices...Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?With HIAD the center of gravity must be sufficiently low and offset to create lift. It's really no different from a capsule in that respect.Some of the concepts here which show a long cylinder on a small inflatable aren't realistic IMO.
Quote from: Impaler on 08/27/2016 04:26 amI would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.Every time a mission critical event in deep space succeeds, it is a (tongue firmly in cheek) 'miracle'. To paraphrase Elon, part of good spaceship design is minimising the number of serialised miracles that need to occur.
Quote from: Oli on 08/27/2016 06:23 amQuote from: Oersted on 08/26/2016 11:10 pmRegarding decelerator devices...Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?With HIAD the center of gravity must be sufficiently low and offset to create lift. It's really no different from a capsule in that respect.Some of the concepts here which show a long cylinder on a small inflatable aren't realistic IMO.For high velocity Mars EDL, yes, I agree. However, for Earth entry, especially from LEO, lift is less of a requirement. For example IRVE-3, which is quite a long cylinder, has been successfully demonstrated.
There must be sufficient distance between center of gravity and center of pressure for stability.
Please bear in mind, I don't expect the engines to be canted for the entire mission.
Quote from: Chris_Pi on 08/27/2016 03:57 amQuote from: OneSpeed on 08/27/2016 03:20 amWhilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?A Red Dragon's heat shield and motors don't need to survive past touchdown. Safe for it is no destructive motor failures. Gouging up the heatshield and throwing rocks up into motors is fine as long as it can land once. A vehicle that has to take off again and do another re-entry needs an intact heatshield and motors for Earth return.Sure, the BFS has to take off again, but why are you so sure rocks would be thrown up into the motors? If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?
Quote from: Hanelyp on 05/08/2015 11:28 pmI'm wondering about a rover with a solar concentrator to melt a thin layer of surface material in place as a solidified layer. Presuming this layer didn't crack too badly it should make a good surface for a landing pad or road.Don't know but intuition says anything less than one m thick under MCT will be flying FOD.###ADDED: Your question deserves a better answer so I looked up what NASA scientists have found in their studies. The following statement is a verbatim copy of a summary that addresses the problem. It was a section of the Mars Design Reference Architecture, Addendum A, published in 2009:"5.9.1 Summary and recommendationsThe predictions and recommendations for a 40-t spacecraft on Mars are described in summary in this section. The next section of the report will then explain in detail how these predictions were obtained.The engine exhaust plume from a 40-t lander on Mars will blow dust, sand, gravel, and even rocks up to about 7 cm in diameter at high velocity. These ejecta will cause significant damage to any hardware that is already placed on the martian surface within the blast radius. However, the blast radius is modest, extending out to approximately 1 km. The largest debris is accelerated by the plume to lower velocities and, thus, falls closer to the landing site; and the smallest particles are attenuated by the martian atmosphere, also falling closer to the landing. Thus, maintaining the distance of about 1 km between the landing site and any existing surface assets will completely solve this problem for all sizes of debris.A second concern arises because the exhaust from the large engines will form deep, narrow craters that are directly beneath each of their nozzles, and these craters will redirect the supersonic jet of gas with sand and rocks up toward the landing spacecraft. This has been demonstrated in large-scale engine tests in sand and clay (Alexander, et al, 1966) 25, small-scale experiments (Metzger, 2007) 26, numerical simulations (Liever, et al, 2007) 27, and soil dynamics analysis (see section 5.10.2.3), so there is no question that this will occur. It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. These variables have been taken into consideration in this report. An example of a small-scale test is provided in figure 5-55. The impact of debris striking the lander will be sufficient to cause damage to the lander, possibly resulting in LOM and LOC, and therefore must be prevented. Of special concern is damage to the engine nozzles, because with a multiple-engine lander the debris that is ejected by one engine will be aimed directly at the other engines. One mitigation approach is to add shielding to the spacecraft to block the debris. This will increase the mass of the lander and, therefore, reduce the mass of the payload by approximately 1 t."
I'm wondering about a rover with a solar concentrator to melt a thin layer of surface material in place as a solidified layer. Presuming this layer didn't crack too badly it should make a good surface for a landing pad or road.
I'm pretty sure there's a link to the article around here somewhere - I do vaguely remember reading it, But I can't find it right now. Very short version is high-thrust motors close to the surface dig really deep holes that throw junk back at the motors. Either have a prepared pad, keep the motors far away, or build something that can take the beating. The Curiosity rover ended up with quite a lot of big gravel all over the top and visible gouges in the ground from the skycrane motors. And those were at the top end of 25 feet of cable and canted somewhat.
... but why are you so sure rocks would be thrown up into the motors? If the Raptors were sufficiently canted, wouldn't they direct ejecta away from the spaceship?
Quote"5.9.1 Summary and recommendations[...] It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. [...] "
"5.9.1 Summary and recommendations[...] It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. [...] "
Staging requires more thrust than any other point in the BFS mission. It's in a bigger gravity well, with a full fuel load and a full payload.
Someone suggested the possibility that MCT/BFS is just a massively scaled-up Crew Dragon. While I'm sure other things are happening than that, it did get me to thinking that arranging BFS's Raptors in the same "covered dugout" configuration that Crew Dragon uses for its Super Draco's might be the winning guess.
Quote from: envy887 on 08/27/2016 04:52 pmStaging requires more thrust than any other point in the BFS mission. It's in a bigger gravity well, with a full fuel load and a full payload.We don't know the final delta-v of the first stage at separation. If it works anything like conventional launches, then the delta-v for a second stage to LEO is much less than the delta-v from Mars surface to direct Earth return.