Wooo, your landing mass is more like 200 mT, not 300 and your going to use Raptor engines for all your deceleration and bring yourself to a hover at 100 m from the surface. The touchdown engines are not slamming your into the surface they are just countering the gravity on mars and giving you fine control to maneuver around any boulders.The thrust total your projecting is more then an entire Raptor engine would produce, if we needed or wanted that much we wouldn't bother with these vernier engines so I think you've mixed up some where (several some wheres?), for example you make each engine weight 1 mT when you were adding them to the vehicles mass.A 200 mT lander on mars has a weight of 744 kN meaning you need just 10 Super Draco engine equivalents which means 1 mT and no significant impact to the landers dry mass. Their would be some cosine loss too but I actually expect an improved engine running on Methane to be employed improving the thrust as well as a touchdown mass of just 175 mT. The propellant for a few seconds of touchdown (20 seconds at mars gravity would be 75 m/s) is not going to be significant propellant fraction, though obviously a significant portion was burned prior to this by the Raptor engine, that is a separate calculation unaffected by our choice of touchdown systems.On Earth were going to be lighter by 75 mT and we would be landing on a concrete pad so we can have a little higher speed at touch down and we might very briefly throttle up right at touchdown too. I've doubtful that Raptor can safely fire in atmosphere but if I'm wrong then it would certainly be used here when their is no danger of debris from the surface.
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines. Scaling these engines up and fueling them with methlox should provide the fine control needed for landing. Could be used to lift-off the surface, too, before the large, centerline engine(s) are started. This technology/approach could be useful for a reusable lander that explores undeveloped sites.
Quote from: AncientU on 11/01/2015 12:03 pmThe set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines. Scaling these engines up and fueling them with methlox should provide the fine control needed for landing. Could be used to lift-off the surface, too, before the large, centerline engine(s) are started. This technology/approach could be useful for a reusable lander that explores undeveloped sites.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.
Quote from: Burninate on 11/01/2015 12:13 pmQuote from: AncientU on 11/01/2015 12:03 pmCould be used to lift-off the surface, too, before the large, centerline engine(s) are started.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.
Quote from: AncientU on 11/01/2015 12:03 pmCould be used to lift-off the surface, too, before the large, centerline engine(s) are started.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.
Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.
Quote from: Burninate on 11/01/2015 12:13 pmQuote from: AncientU on 11/01/2015 12:03 pmThe set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines. Scaling these engines up and fueling them with methlox should provide the fine control needed for landing. Could be used to lift-off the surface, too, before the large, centerline engine(s) are started. This technology/approach could be useful for a reusable lander that explores undeveloped sites.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing. Why are lift-off thrust requirements 'a large multiple' of landing? You only need to clear the ground by 100m (and maybe move laterally a bit)...
Quote from: AncientU on 11/01/2015 12:25 pmQuote from: Burninate on 11/01/2015 12:13 pmQuote from: AncientU on 11/01/2015 12:03 pmThe set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines. Scaling these engines up and fueling them with methlox should provide the fine control needed for landing. Could be used to lift-off the surface, too, before the large, centerline engine(s) are started. This technology/approach could be useful for a reusable lander that explores undeveloped sites.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing. Why are lift-off thrust requirements 'a large multiple' of landing? You only need to clear the ground by 100m (and maybe move laterally a bit)...Because of all that methalox! At landing the vehicle is near the penultimate dry mass. At liftoff it's at around 3.35x the dry mass (in the case of 380s Isp & LMO refueling at ~4.5km/s dV), or 6.55x the dry mass (in the case of 380s Isp & no LMO refueling with Hohmann transfer home at ~7km/s dV) or more (in fast transit cases without LMO refueling).This translates directly into proportionately higher thrust. This higher thrust figure is achievable, but the sheer weight of the engines required adds quite a bit to the vehicle.Admittedly, you need lower acceleration at liftoff than at landing; I need to do further math on this.
Quote from: AncientU on 11/01/2015 12:25 pmQuote from: Burninate on 11/01/2015 12:13 pmQuote from: AncientU on 11/01/2015 12:03 pmCould be used to lift-off the surface, too, before the large, centerline engine(s) are started.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.To reach TEI from Mars surface, you're looking at around 400 tonnes of fuel (depending on dry-mass). Additionally, launch needs to be at 2-3 g to reduce gravity losses. So minimum 2*9.8*500 = 9.3MN. So over 130 Super-Dracos just to reach 100m. (Hell, 25 just to hover.)
But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.
Musk is borrowing somewhat from Mars Direct (or is it Semi-Direct?) where an already-fueled ascent vehicle is fueled up on the surface. I don't see a good reason not to have a fueled up vehicle ready when they arrive.
Quote from: Lars-J on 11/01/2015 03:47 amPerhaps, but that kind of coverage does not exist. Yes. And even when it does, it tells you nothing about the relative strength of the surface. It could be the Martian equivalent of quicksand for all we know.But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.You should have watched the NASA workshop for selecting Mars landing sites. They do know a lot. They have identified landing spots with a hard surface that allow for safe landing. They do have a lot of coverage for different kinds of observation already and the teams can request more observations for each of the proposed 40 landing sites. They can do very thorough orbital survey for multiple data once they have narrowed down to few potential sites.
Perhaps, but that kind of coverage does not exist. Yes. And even when it does, it tells you nothing about the relative strength of the surface. It could be the Martian equivalent of quicksand for all we know.But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.
MCT will certainly not need to be designed for landing on any not suitable off target landing sites so don't try to include such requirements into your mass budget.
I did watch many of those workshops. And I also know that observations from the above is not as comprehensive as one might think, unless one also has ground observations to validate them. And if you have any reference of how they can judge the hardness of a surface (brittle and compressible vs hard as granite), then please link to the presentation where they show that.
If you are willing to claim that no off-nominal landing will ever happen, nor be planned for, then go ahead. I'm not part of the crowd that cries for a useless abort system, but I do think a sturdier landing gear with some extra margin is mass well spent. Extra margin to allow landing on marginal sites during an emergency/off-nominal, NOT any site on Mars.
Quote from: Burninate on 11/01/2015 01:49 pmQuote from: AncientU on 11/01/2015 12:25 pmQuote from: Burninate on 11/01/2015 12:13 pmQuote from: AncientU on 11/01/2015 12:03 pmThe set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines. Scaling these engines up and fueling them with methlox should provide the fine control needed for landing. Could be used to lift-off the surface, too, before the large, centerline engine(s) are started. This technology/approach could be useful for a reusable lander that explores undeveloped sites.I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them. Liftoff thrust requirements are a large multiple of landing thrust requirements.If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing. Why are lift-off thrust requirements 'a large multiple' of landing? You only need to clear the ground by 100m (and maybe move laterally a bit)...Because of all that methalox! At landing the vehicle is near the penultimate dry mass. At liftoff it's at around 3.35x the dry mass (in the case of 380s Isp & LMO refueling at ~4.5km/s dV), or 6.55x the dry mass (in the case of 380s Isp & no LMO refueling with Hohmann transfer home at ~7km/s dV) or more (in fast transit cases without LMO refueling).This translates directly into proportionately higher thrust. This higher thrust figure is achievable, but the sheer weight of the engines required adds quite a bit to the vehicle.Admittedly, you need lower acceleration at liftoff than at landing; I need to do further math on this.Not true... a 100mT payload plus the vehicle dry weight (~25mT?) at landing -- at lift-off, 25mT plus 4x propellant gives same mass as at landing. (Sorry for the gross approximations.) This gets you back to LMO where you fuel-n-go for Hohmann transfer home. If vehicle is a lander, it refuels in LMO and prepares for another descent to the surface.The 'sheer weight' of a set of 8 superdracos is less than 1mT if I recall correctly.
Ahh, I knew that sounded like a lot. Thanks for spotting the error - 10 times too much mass allocated to engines. That should bring the extra mass margin down below +10%.If you're trying to avoid unprepared landing site excavation issues, you need space for the exhaust plume to spread out a bit. I picked ~1km for a very rough & arbitrary figure. At 100m, the exhaust plume is barely larger than the vehicle fairing diameter. Shut the main engines down at 1km AGL after doing the full job of entry & descent, and they won't destroy the landing pad. Then drop for a bit (now's a nice time to correct to vertical and unfurl the legs), & start controlled thrusting on the sideways-canted canard engines; Any terrain damage they do will be well away from the place the legs impact the ground. They still need plenty of thrust, however. If they were only capable of precisely Mars gravity acceleration, they could hover at this 1km point, but not descend (because they could never correct for that additional velocity); Pure gravity loss. To minimize gravity loss they need substantially larger thrust in order to accomplish an efficient suicide burn.EDIT: To clear up your confusion, a supplementary set of engines towards the top of the vehicle ("Canard engines") are intended to solve the problems raised in posts like this http://forum.nasaspaceflight.com/index.php?topic=37466.msg1372150#msg1372150 . As a secondary point, they might be used if a nested MAV design turns out to be needed because of low ISRU mass payoffs.
... I expect SpaceX to sell normal commercial flights on BFR for a good long time before it is use for mars ...... As NASA is the only conceivable customer for a first mission they need to be courted to create a mission utilizing SpaceX as the primary contractor....
Quote from: Burninate on 11/01/2015 09:57 amAhh, I knew that sounded like a lot. Thanks for spotting the error - 10 times too much mass allocated to engines. That should bring the extra mass margin down below +10%.If you're trying to avoid unprepared landing site excavation issues, you need space for the exhaust plume to spread out a bit. I picked ~1km for a very rough & arbitrary figure. At 100m, the exhaust plume is barely larger than the vehicle fairing diameter. Shut the main engines down at 1km AGL after doing the full job of entry & descent, and they won't destroy the landing pad. Then drop for a bit (now's a nice time to correct to vertical and unfurl the legs), & start controlled thrusting on the sideways-canted canard engines; Any terrain damage they do will be well away from the place the legs impact the ground. They still need plenty of thrust, however. If they were only capable of precisely Mars gravity acceleration, they could hover at this 1km point, but not descend (because they could never correct for that additional velocity); Pure gravity loss. To minimize gravity loss they need substantially larger thrust in order to accomplish an efficient suicide burn.EDIT: To clear up your confusion, a supplementary set of engines towards the top of the vehicle ("Canard engines") are intended to solve the problems raised in posts like this http://forum.nasaspaceflight.com/index.php?topic=37466.msg1372150#msg1372150 . As a secondary point, they might be used if a nested MAV design turns out to be needed because of low ISRU mass payoffs.I think 1 km is far too high an elevation to start worrying about plumes impinging the ground. A large rocket lifting off the ground is not still bathing the launch pad in flames when it is 1 km up, rather it looks to be mostly over within 1-2 times the height of the launch tower. Remember our goal as you point out is to avoid making craters in the ground and making dangerous ejecta which might impact the vehicle, at 100 m height their should be no danger to the vehicle even if some sand and dust are being swept up on the surface. Under the near vacuum conditions of mars the plume will also spread MUCH wider then the vehicles base, just as the plume of a rocket expands markedly as it rises, it will never resemble the 'welding torch' look of a rocket at liftoff on Earth.