Author Topic: SpaceX paper on precision landing - and landing technology Thread  (Read 21857 times)

Online FutureSpaceTourist

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Great paper by Lars Blackmore (principal rocket landing engineer at SpaceX) on the challenges of precision landing:

Quote
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.

https://twitter.com/larsblackmore/status/815354936741412864
« Last Edit: 01/01/2017 06:41 AM by FutureSpaceTourist »

Online FutureSpaceTourist

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The attached figure shows how dispersions (largest possible trajectory variations) vary through the different stages of flight.

Online FutureSpaceTourist

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Some good references in Lars' article. Way out of my field but I found the discussion in the last section, about knowing accurately where you are on Mars with no GPS, particularly interesting. The following terrain navigation paper he cites is atrached:

Quote
Real-time terrain relative navigation test results from a relevant environment for Mars landing

Johnson, Andrew E.; Cheng, Yang; Montgomery, James; Trawny, Nikolas; Tweddle, Brent; Zheng, Jason

URI: http://hdl.handle.net/2014/45631
Date: 2015-01-05
Keywords: EDL; Lander Vision System; GNC; Pin-point Landing; Mars 2020
Publisher: Pasadena, CA : Jet Propulsion Laboratory, National Aeronautics and Space Administration, 2015
Citation: AIAA Scitech 2015, Kissimee, Florida, January 5-9, 2015

Abstract:
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.

Offline john smith 19

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Great paper by Lars Blackmore (principal rocket landing engineer at SpaceX) on the challenges of precision landing:
A nice round up of the problem (it's hard), and how results have been improving over the last 4 decades. Not much on how SX gets from 10Km on Mars to 10m on Earth though.
[EDIT Beyond basically we run CVXGen to produce a chunk of custom software that solves the problem on the lander. It's not clear if they create a new version for each launch DOLILU style or if they ran the system for a while to get a really well optimized program this class of problem ]
« Last Edit: 01/01/2017 01:00 PM by john smith 19 »
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Online AnalogMan

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I extracted the article by Lars Blackmore from The Bridge periodical linked/attached in the opening post.
« Last Edit: 01/01/2017 12:37 PM by AnalogMan »

Online Robotbeat

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Sweet.
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Offline rpapo

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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.
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Offline Lar

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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.

After I read it I felt like I hadn't learned much. But then MAYBE I'm a sigma (or 3!! ) above the average reader in my knowledge in this area (thanks, NSF!!!!)  :)
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
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Offline meekGee

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(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.
« Last Edit: 01/07/2017 08:42 PM by meekGee »
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Online Robotbeat

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Docking isn't trivial, either. You can afford to take longer. But you have to hit a smaller target and the pieces are less forgiving for relative velocity.

Landing on a barge is much harder though. You have one shot and no change of abort and redo.
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Offline rpapo

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Landing on a barge is much harder though. You have one shot and no change of abort and redo.
You don't get to abort or redo on land either.  Not with thrust/weight > 1.

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.
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Offline Jim

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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.
« Last Edit: 01/08/2017 02:09 PM by Jim »

Offline laszlo

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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.

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.

Offline MarekCyzio

(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.


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.


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Offline DOCinCT


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.

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.
One NASA planning document outlined a reusable lander that would make multiple trips to LMO to bring additional cargo to the surface.  Required MethaLox engines and ISRU production of fuel.

Online Oersted

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...
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.

He did mention delay when he referred to a "sluggish throttle control".
« Last Edit: 01/10/2017 11:04 AM by Oersted »

Offline Jim

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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.

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.


Bingo

Offline dglow

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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.


Bingo

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.

Offline Jim

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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.

That wasn't a constraint. Shuttle avionics could have done it.

Offline dglow

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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.

That wasn't a constraint. Shuttle avionics could have done it.

No.

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