It's been a while since I published on precision landing, but here's a new article:tinyurl.com/h6kp5n7 Big thanks to @NAE_DC + @SpaceX.
Real-time terrain relative navigation test results from a relevant environment for Mars landingJohnson, Andrew E.; Cheng, Yang; Montgomery, James; Trawny, Nikolas; Tweddle, Brent; Zheng, JasonURI: http://hdl.handle.net/2014/45631Date: 2015-01-05Keywords: EDL; Lander Vision System; GNC; Pin-point Landing; Mars 2020Publisher: Pasadena, CA : Jet Propulsion Laboratory, National Aeronautics and Space Administration, 2015Citation: AIAA Scitech 2015, Kissimee, Florida, January 5-9, 2015Abstract:Terrain Relative Navigation (TRN) is an on-board GN&C function that generates a position estimate of a spacecraft relative to a map of a planetary surface. When coupled with a divert, the position estimate enables access to more challenging landing sites through pin-point landing or large hazard avoidance. The Lander Vision System (LVS) is a smart sensor system that performs terrain relative navigation by matching descent camera imagery to a map of the landing site and then fusing this with inertial measurements to obtain high rate map relative position, velocity and attitude estimates. A prototype of the LVS was recently tested in a helicopter field test over Mars analog terrain at altitudes representative of Mars Entry Descent and Landing conditions. TRN ran in real-time on the LVS during the flights without human intervention or tuning. The system was able to compute estimates accurate to 40m (3 sigma) in 10 seconds on a flight like processing system. This paper describes the Mars operational test space definition, how the field test was designed to cover that operational envelope, the resulting TRN performance across the envelope and an assessment of test space coverage.
Great paper by Lars Blackmore (principal rocket landing engineer at SpaceX) on the challenges of precision landing:
Nice of him to be gracious enough to give credit to how others have been advancing on the same problem... particularly Blue Origin. I don't blame him for not giving away too many particulars as to how SpaceX does it.
Landing on a barge is much harder though. You have one shot and no change of abort and redo.
Hey, unlike certain people around here, I don't consider rendezvous and landing as problems of similar magnitude. Not even close. If it were close, the problem would probably have been mastered long ago.
Wrong. It wasn't controls or guidance that prevented it from long ago. It was the incorporation of Supersonic retropropulsion and engine throttling, and the use of many smaller engines that allow them to be used for landing an empty stage. Autonomous rendezvous with a non cooperative target is harder.
(Ah - moved to a better thread, thanks Chris.)This was in response to a discussion started on one of the mission threads, comparing landing an F9 with orbital docking of say, visiting vehicles to the ISS.-----The problem is not programming. It's the inherent control problem that's so crazy difficult.In a docking situation, there are more control inputs than there are degrees of freedom. Just count the number of RCS thrusters on any visiting vehicle.So if you want to, for example, translate in Y, you fire off a matching pair of thrusters, drift a little bit, then stop.So if you consider paired thrusters, you can also say that the degrees of freedom are decoupled.The docking procedure dictates that the vehicles be aligned from an angular point of view, then made collinear, by translation, then just control the main axis coast until captured.All of this makes the control problem trivial - plus, there are hardly any disturbing forces.OTOH, on a landing:- You're trying to zero 12 degrees of freedom (6 position/rotation of rigid body in space, plus all first derivatives)- You have only 2 strong inputs (tip/tilt of engine plume), a sluggish throttle control, and 3 (x,y,theta) controls from the grid fins, which are losing effectiveness rapidly as you're slowing down.- The 2 strong inputs don't go through the C.M, so everything is coupled. If you want to translate, you need to induce a rotation. Then you need to stop the rotation, reverse it, stop the translation, and undo the rotation again.- The landing procedure includes intentional maneuvering (divert) in multiple axes- Meanwhile, the wind is very significant, variable, and unknown except for its effects.So good luck "just doing a little bit of programming" on a docking vehicle's avionics and teaching it how to land an F9 on a barge.
Quote from: Jim on 01/08/2017 02:08 pmWrong. It wasn't controls or guidance that prevented it from long ago. It was the incorporation of Supersonic retropropulsion and engine throttling, and the use of many smaller engines that allow them to be used for landing an empty stage. Autonomous rendezvous with a non cooperative target is harder.What about nobody wanted it? As in, why bother landing the empty stage and reusing it when to do so negatively affects the payload capacity and we don't have the budget or time to develop a reuseable system anyway? That was certainly the case for Apollo. Pre-Apollo we were trying just to get into space at all. Post-Apollo there was the Shuttle reuse experiment. Now there's SpaceX.I've gotta agree with Jim that it wasn't a guidance or controls problem, but from what I can see it wasn't a technical problem at all. It was a question of settling on reuseability as a goal. In spaceflight, once a goal is set and the funding is adequate, stuff gets done. It's just rocket science.
Quote from: meekGee on 01/07/2017 06:30 pm...a sluggish throttle control...You forgot to mention the most important problem - delay. Nonlinear, unstable systems are relatively easily controllable when there is no delay. Introduce the delay in your control loop and things go south.
...a sluggish throttle control...
Quote from: laszlo on 01/08/2017 03:09 pmI've gotta agree with Jim that it wasn't a guidance or controls problem, but from what I can see it wasn't a technical problem at all. It was a question of settling on reuseability as a goal. In spaceflight, once a goal is set and the funding is adequate, stuff gets done. It's just rocket science.Bingo
I've gotta agree with Jim that it wasn't a guidance or controls problem, but from what I can see it wasn't a technical problem at all. It was a question of settling on reuseability as a goal. In spaceflight, once a goal is set and the funding is adequate, stuff gets done. It's just rocket science.
True, but we had to develop the onboard compute horsepower to support autonomous hoverslam landings. That certainly didn't exist during Apollo nor during Shuttle development.
Quote from: dglow on 01/10/2017 05:22 pmTrue, but we had to develop the onboard compute horsepower to support autonomous hoverslam landings. That certainly didn't exist during Apollo nor during Shuttle development.That wasn't a constraint. Shuttle avionics could have done it.