Quote from: meekGee on 01/07/2017 02:17 pmA docking spacecraft is in zero g and can take its time with the maneuver. You have all the authority you need, and can abort and try again.An F9 doing a hoverslam is so completely a different thing.It is decelerating at multiple g, in wind, with very limited controls, especially towards the end - mostly main engine gimbaling.Not the same as dockingwrong. Avionics wise it is the same. Landing just has more constraints and external influences. The F9 always knows where it is going to land. Landing a vehicle is not hard (see lunar and mars landers). Landing actually uses less external sensors (altitude radar). Rendezvous and docking require long range (star trackers, radar, etc) and short range (radar, lidar, TV, etc) sensors.
A docking spacecraft is in zero g and can take its time with the maneuver. You have all the authority you need, and can abort and try again.An F9 doing a hoverslam is so completely a different thing.It is decelerating at multiple g, in wind, with very limited controls, especially towards the end - mostly main engine gimbaling.Not the same as docking
Quote from: meekGee on 01/07/2017 05:14 pmThe secret sauce of the reentry burn and the aerodynamic flight segment are unique to SpaceX and already more difficult.Doesn't require any changes to the avionics to perform those tasks. Just a little more programming
The secret sauce of the reentry burn and the aerodynamic flight segment are unique to SpaceX and already more difficult.
Landing can be done independent of the actual time, unlike yaw steering and rendezvous/docking. That is a major change to the avionics/ flight software architecture
Quote from: Robotbeat on 01/07/2017 05:52 pmHow is yaw steering hard? You need more on board memory to store a look up table of launch solutions, but other than that, how is it hard? Particularly, how can it possibly be easier than landing?Landing on a small platform with rockets, in atmosphere, and in Earth gravity is anything but easy.Time reference
How is yaw steering hard? You need more on board memory to store a look up table of launch solutions, but other than that, how is it hard? Particularly, how can it possibly be easier than landing?Landing on a small platform with rockets, in atmosphere, and in Earth gravity is anything but easy.
Well, I'd say if you claim Yaw steering as a higher level navigational problem, and docking as a higher level control problem, then the hoverslam landing is both.
Landing on a spot on the surface however is not as simplified. Even if you are in orbit,
But then you have to deal with all the issues of unstable, under-controlled atmospheric flight, reentry, atmospheric uncertanities and wind from the moment of interface on,
and an advanced navigational problem.
I'd say docking is the easiest of the problemsFollowed by yaw steeringAnd when you have mastered both, you can join a harder league and try reentry and spot-landing in atmosphere.
Quote from: LouScheffer on 01/05/2017 02:06 amBut for SpaceX, it's almost surely software. The first stage is solving convex optimization in real time, a much harder task than yaw steering. Not true. Targeting for yaw steering is harder. Again, the landing pad is a static target and always be in the same place no matter what time it is launched. Launching into a specific orbital plane at anytime within a launch window is much harder.
But for SpaceX, it's almost surely software. The first stage is solving convex optimization in real time, a much harder task than yaw steering.
This paper presents a parametric yaw steering law which has been used to provide closed-loop yaw guidance for the launch of the HEAO (High Energy Astronomy Observatory) satellite mission using the Atlas/Centaur launch vehicle. This bilinear tangent steering law provides near optimal yaw steering for maneuvers requiring insertion into orbits with a specified inclination and node. [...] The flight computer implementation of these laws in a rotating coordinate system using real-time integration of the equations of motion is detailed. Explicit solution of the parametric guidance equations requires the inflight solution of (2x2) two-point boundary value problems in the pitch and yaw planes. Excellent results are obtained even for very large (greater than 50 deg) out-of-plane
At spacecraft insertion, the range between Spacecraft 12 and the Gemini XII GATV was approximately 500 nautical miles, and the out-of-plane velocity error resulting from the spacecraft launch-vehicle ascent yaw steering was about 8 ft/sec.
This mission had a 33 second launch window. So even if you can't update the solution for any time within a launch window, (even though Apollo and HEAO did this), you could make it even easier on the rocket avionics by pre-computing a number of trajectories (say 21 of them, 30 seconds apart, covering +- 10 minutes from nominal), then pick one once you decide where in the window to launch. This reduces the problem to something that was solved by a 1960's flight computer.
Quote from: LouScheffer on 01/08/2017 03:26 pmThis mission had a 33 second launch window. So even if you can't update the solution for any time within a launch window, (even though Apollo and HEAO did this), you could make it even easier on the rocket avionics by pre-computing a number of trajectories (say 21 of them, 30 seconds apart, covering +- 10 minutes from nominal), then pick one once you decide where in the window to launch. This reduces the problem to something that was solved by a 1960's flight computer.Not feasible unless does before terminal count. There needs to be time to load and verify the program.
Given that computers in the 1960s could load and verify at least one trajectory, a modern computer can surely load and verify all 21 (in this case) in the hours before launch.
I believe the reasons for lacking it are bean counters, not technology. Unless you have a mission that needs it to reach some otherwise un-available orbit, or some customer demands it, then the decision boils down to how often a few minute window would help, how much a delay until the next window costs, versus the cost to develop and certify.
Given that computers in the 1960s could load and verify at least one trajectory, a modern computer can surely load and verify all 21 (in this case) in the hours before launch. Then it uses the one that corresponds to the selected launch time.
Quote from: LouScheffer on 01/08/2017 07:52 pmGiven that computers in the 1960s could load and verify at least one trajectory, a modern computer can surely load and verify all 21 (in this case) in the hours before launch. Then it uses the one that corresponds to the selected launch time. What makes you think any rocket (except Falcon 9) today is using a modern guidance computer?
The landing is fixed and known and never changes. There is no computing to determine or find it. ...Again, wrong. The landing site is known and fixed. It stored on board.... The docking target locationis unknown until the first sensor lockon.Yaw steering is not a look up table, it is computed onboard