Quote from: Coastal Ron on 08/25/2017 05:53 amIn a tweet talking about how well the Falcon 9 1st stage landed the barge, Elon Musk gave these figures:Touchdown:Vertical Velocity (m/s): -1.47 Lateral Velocity (m/s): -0.15 Tilt (deg): 0.40 Lateral position: 0.7m from target centerThen somebody asked:What's the ITS landing mount lateral position constraints in m?And Elon Musk replied:Probably 2m or soI guess they have their target defined.The 2m spec would be effectively a margin of error that is 20% the diameter of the entire ITSy. That sounds extremely implausible without the use of some sort of active recovery mechanism, not that I want to open up that can of worms again...Either an active mechanism to grab the core in an approximately 12-15m wide mount, or a sort of passive funnel that would require the stage to go through all new kinds of stresses from the centering process.
In a tweet talking about how well the Falcon 9 1st stage landed the barge, Elon Musk gave these figures:Touchdown:Vertical Velocity (m/s): -1.47 Lateral Velocity (m/s): -0.15 Tilt (deg): 0.40 Lateral position: 0.7m from target centerThen somebody asked:What's the ITS landing mount lateral position constraints in m?And Elon Musk replied:Probably 2m or soI guess they have their target defined.
Think about how the new docking mechanism for the ISS works. It uses a set of leaves to perform final fine alignment. The use of the same sort of method would allow 2m long leaves 4 each on the Booster and ground mount would perform this final alignment and have the mechanisms for a soft capture and also the ability to absorb just like the docking mechanism any vertical landing velocity reaming. It would be a heavy duty docking adapter.
Quote from: oldAtlas_Eguy on 08/25/2017 03:30 pmThink about how the new docking mechanism for the ISS works. It uses a set of leaves to perform final fine alignment. The use of the same sort of method would allow 2m long leaves 4 each on the Booster and ground mount would perform this final alignment and have the mechanisms for a soft capture and also the ability to absorb just like the docking mechanism any vertical landing velocity reaming. It would be a heavy duty docking adapter.OK, but with spacecraft docking with the ISS the spacecraft is physically moved to align with the ISS - and docking speeds are relatively slow.Instead we're talking about a massive spacecraft doing what it can to stop it's fall from space just as it touches the landing platform. I have trouble envisioning how such a system would move an entire ITS 1-2m sideways in less than a second. That would have to be some pretty beefy areas on the ITS, which sounds like a lot of weight.Wouldn't it be more likely that the landing platform is designed so that the ITS can land off-center and still be OK?
SpaceX is trying to save the mass of the landing legs; thus the complexity
If you're landing near the launch pad and a crane can move the S1 onto the pad, I don't see that a problem exists. OTOH, if you're trying to land with millimeter precision in a cradle on the launch pad, that's a different story. Not only do you have unknown winds, you also have rocket exhaust and unpredictable eddy swirls. Any attempt to grapple, capture, dock or whatever of the bottom of the stage must be coordinated with some method of vectored force to the top of the stage. This is not as simple as hitting (0,0) on one Cartesian grid. You have two planar Cartesian grids that have to reach (0,0) at approximately the same time: the bottom of the stage as well as the top of the stage. If the vehicle comes in close to vertical, but 3m off on the lower planar (x,y) coordinates, then some solid or fluid mechanism then pushes the bottom into position, it also pushes the fuselage away from being vertically plumb. Now you have a potential tipping problem. If you want to make the entire vehicle move sideways while remaining close to plumb, it needs to be pushed or pulled from both top and bottom by approximately equal forces.In regards to docking leaves, this situation is dissimilar to micro-G vacuum maneuvering. That is done at a V differential of what, a mm/sec.? While inertia is the only force about which to be concerned. With the landing, we have a vehicle whose prop mass is constantly decreasing (and rapidly approaching zero-meaning no margin for error), a rocket engine whose thrust is not as constant as the inertia of a craft on orbit, unpredictable swirling of winds and engine exhaust, as well as a changing temperature of said leaves as the engine exhaust hits them. What will the heat do to them? Then there is the incredibly complex aerodynamics of the engine exhaust swirling around and between these leaves.Maybe this can be done, maybe not, but it seems to involve a lot of complexity that could be avoided by landing on a flat slab then moving the vehicle a short distance with a crane.
It occurs to me that a 12m ITS tanker that could refill an ITSy 9m spaceship in one trip would replace 3 trips by a 9m tanker. That might be an initial economic argument for building the 12m tanker.
Quote from: Ionmars on 08/25/2017 10:04 pmIt occurs to me that a 12m ITS tanker that could refill an ITSy 9m spaceship in one trip would replace 3 trips by a 9m tanker. That might be an initial economic argument for building the 12m tanker.But it would also need a 12m booster BFR. Once you have that then you have no reason to keep using the 9m ITSy but to use the 12m ITS.
EM said 2m precision was goal... not 2mm.
Quote from: AncientU on 08/25/2017 07:48 pmEM said 2m precision was goal... not 2mm.I was not responding to Elon, but to other posters here. Please explain how you are going to land this booster without landing legs, in a cradle, while being two meters off target?That may as well be an oncological neurosurgeon excising a brain tumor, while being two meters off target.
Quote from: AncientU on 08/25/2017 07:48 pmEM said 2m precision was goal... not 2mm.>That may as well be an oncological neurosurgeon excising a brain tumor, while being two meters off target.
Quote from: AncientU on 08/25/2017 07:48 pmEM said 2m precision was goal... not 2mm.I was not responding to Elon, but to other posters here. Please explain how you are going to land this booster without landing legs, in a cradle, while being two meters off target?
What's the ITS landing mount lateral position constraints in m?
Probably 2m or so
Quote from: Robotbeat on 07/25/2017 02:12 pmThis is like saying it's "nonsensical" to get a car gas tank that can fit more than 20 miles of gas because most of your trips fit under that limit.That's making a mockery of language.I'm not interested in silly analogies. A fully reusable 10mt launcher would cost a small fraction of ITS and actually achieve a decent flight rate. If you see any promising markets for reusable super-heavy lift feel free to name them.
This is like saying it's "nonsensical" to get a car gas tank that can fit more than 20 miles of gas because most of your trips fit under that limit.That's making a mockery of language.