Legs need to be very strong, don't they. What's the weight on Mars of the fuel needed to launch direct to Earth?
Would deployable thrust deflectors make sense? Rather than gimballing the engines, just redirect the exhaust away from the vehicle at 30 or 45 degrees. I'm thinking an ablative-coated panel set just below the engines (which I think will be mounded on the sidewalls), that pops out into the exhaust stream.
Quote from: envy887 on 08/29/2016 01:14 pmWould deployable thrust deflectors make sense? Rather than gimballing the engines, just redirect the exhaust away from the vehicle at 30 or 45 degrees. I'm thinking an ablative-coated panel set just below the engines (which I think will be mounded on the sidewalls), that pops out into the exhaust stream.If you mean the deflectors would be attached to the vehicle itself, then at 45°, the vertical thrust would push down of the panels (and hence the ship) exactly as much as it pushes up; resulting in zero vertical net force. The sideways thrust would presumably be balanced in each horizontal direction by ensuring the panels point in different direction, so you wouldn't get any horizontal movement either. Essentially is would be a very elaborate scheme to stop the rockets from doing anything at all. It would just sit there, burning away, until something gave out.If you mean to mount the deflectors on the surface, you are essentially talking about a flame-trench. Which means a large concrete construction, along with some way of suspending the vehicle over it, so a launch platform/gantry of some kind. Which wouldn't be a good place to land (too much stuff in the way), so you'd need a vehicle capable of hoisting and transporting the BFR.... basically you've added an awful lot of hardware.
Quote from: Impaler on 08/29/2016 10:53 amOk lets make that comparison, touch down engines would be as simple as a dozen super draco engines around the nose of the vehicle. [...] None of which helps for the re-launch.
Ok lets make that comparison, touch down engines would be as simple as a dozen super draco engines around the nose of the vehicle. [...]
I assume you mean weight, the mass would be at least 250mT, and much higher if the ship was to carry enough fuel to return directly to LMO or Earth. MCT is a fuel rich architecture, allowing for many mission profiles.
The landing velocity would be close to zero in either scenario.
Who said anything about gimbaling more than a few degrees? The idea is to allow the canting of the entire engine mount in a single degree of freedom. Canting is required anyway by SRP, so there is no extra engineering required for landing.
Making an engine gimbal a huge amount will basically require a completely second thrust structure to keep the inward thrust vectors from crushing the vehicle, thrust structures are usually more massive then the actual engines they hold. Propellant lines need to use bellows to allow them to flex but they are like springs, the bend radius is large so to get a greater deflection the engine needs to be at the end of a longer and wider thrust structure, the mass of all this would quickly become prohibitive.
To launch you can't be canting, you need vertical thrust or your engine efficiency is ruinous.
First the vehicle is not going to be full of propellant at landing, that is nonsense.
Why would a lander want to land with enough propellant to relaunch? Landing prop for earth return is prohibitive, so where would it go with landed prop, and what would it do when it got there?
Quote from: envy887 on 08/30/2016 02:29 pmWhy would a lander want to land with enough propellant to relaunch? Landing prop for earth return is prohibitive, so where would it go with landed prop, and what would it do when it got there?To reduce the time taken to generate enough for relaunch? Use as a raw material?
Quote from: Impaler on 08/30/2016 05:20 amTo launch you can't be canting, you need vertical thrust or your engine efficiency is ruinous.You can't be canting for the entire LMO and TEI burns, but what if it was just for the first few seconds, enough to avoid the risk of ejecta impacting the ship?The heaviest estimate I've seen for BFS dry mass + payload from Mars to Earth is about 160mT. Assuming 7.5 km/s of ΔV is required, that is a takeoff mass of about 1280mT. If there are 5 Raptors, each having around 270mT of thrust, that is 1360mT of thrust at takeoff. For Mars gravity (0.38g), that is a thrust to weight ratio of 1360 / 1280 * 0.38 = 2.8, far more than necessary to achieve liftoff. Allowing a worst case T/W of 1.15, cos^-1(1.15/2. = 65.7°, probably more than enough to mitigate the ejecta risk. After a few seconds of flight, the cant could be reduced to zero, and the flight continue with no further cosine losses to TEI.Quote from: Impaler on 08/30/2016 05:20 amFirst the vehicle is not going to be full of propellant at landing, that is nonsense.As shown above, the ship wouldn't need to be completely full of propellant to relaunch, and it certainly wouldn't be necessary once ISRU is established, but why is it a nonsense? Do you have any supporting calculations?
I think ideally a 1/2 to 1/3 scale Raptor would be ideal for the BFS, but I think SpaceX has indicated they are going to only develop one size of this guy.
Quote from: GORDAP on 08/31/2016 10:55 amI think ideally a 1/2 to 1/3 scale Raptor would be ideal for the BFS, but I think SpaceX has indicated they are going to only develop one size of this guy.Interesting that you should say that. In Noël Bakhtian's thesis 'Drag Augmentation via Supersonic Retropropulsion for Atmospheric Deceleration' she finds that the optimum number of engines for Mars SRP is actually 15 engines in a peripheral array, canted outboard. 15 1/3 scale Raptors would give the same thrust as 5 full scale.
Impaler, I'm drawn to your suggestion re smaller, more responsive engines for Mars landing. But I think Super Dracos aren't the answer for a couple of reasons. First, you still need the Raptors for Mars ascent (unless you are arguing that the Dracos will be able to lift a fully fueled BFS from Mars surface). And the Raptors will have to be canted, so as to avoid backsplash during ascent. This is exactly the same problem as on descent. So if you have to solve it for ascent, why include a separate method for descent?Second, the hypergolic fuel used on the Super Dracos is not easily made via ISRU. That means that the fuel used for Earth landing must be carried from Earth to Mars, then back to Earth again. This would really cut into the available payload landed on Mars (given the dictatorship of the rocket equation). Right?I think ideally a 1/2 to 1/3 scale Raptor would be ideal for the BFS, but I think SpaceX has indicated they are going to only develop one size of this guy.