Author Topic: SpaceX paper on precision landing - and landing technology Thread  (Read 18766 times)

Online john smith 19

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STS did not go from orbit to landing site with no propulsion.  The OMS engines delivered the impulse to direct STS to it entry interface corridor.  This is analogous to F9's boostback burn. 


Wrong.  The OMS just changed the orbit to one that intersected the atmosphere, so it is not analogous to the boost back burn.

Sounds like Woody telling Buzz Lightyear he's not flying, but just falling with style.

But what do you really think about the tools that could enable precision landing?  You correctly pointed out the enabling technologies that STS used, will any of those have application to Mars EDL or other likely destinations?
There is a study of landing error Vs improved models for the European reentry demonstrator (the lifting body I think it was called Colibri?)
Fixed g to proper evaluation of the inverse square law went from 120 x 50 Km --> 8x5 Km landing ellipse
Moving to an interpolated L/D ratio dropped that to 4 x 3.5Km
Going from an exponential to a US std model of the atmosphere dropped that to 0.7x1.7Km

IIRC Image recognition could bring that down to 0.4x0.4Km.

So a really good model of the precise gravity field around a planet (from a fixed number to the the 300 term and now 2000+ term from the satellite part funded by the USAF) helps a lot.

Likewise a good model of the atmosphere. On Earth the outer fringes of which can vary 10x by hour of the day and season. Or you have some kind of on board sensor (they seem to like the name "sounder") that maps the atmosphere on a pass and uses those values for the day.

Astronav systems using 40 guidance stars (which are daylight visible with the right filters) can pin down a location on Earth to 6m, provided you have an accurate time reference.

My instinct is that INS with a good enough baseline can do a lot. There drift over the time for decent is small enough not to be a problem. Worst case puts a radar altimeter on the vehicle. It may only have a 500m range but the ground is a pretty big target.

GPS has become so ubiquitous that people don't seem to be able to consider navigation problems without it.   It makes life simple but I don't believe it's essential.
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Offline Jim

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I'm a software engineer, just looking at the problem from where I am, docking is an easier SW problem than landing the F9 booster. Call it intuition, since you seem to accept that as valid.

That is because you don't understand orbital mechanics.  It is not just docking, it is rendezvous.  The landing spot for F9 never changes, the vehicle's attitude at landing is the same.  The flight path back to the it is never out of plane.     
 
Software?  Easy - most of it is just a continuation of the launch flight program.  The flip around, boost back and braking burns are something most other launch vehicles avionics could execute.  Adding booster ACS and grid fins aren't hard additions from a software POV.  Landing has been done by many vehicles on many planets.

The challenge for Spacex was to keep enough propellant reserves to cover dispersions to ensure landing but at the same time provide enough dV to the second stage.  Spacex was able to determine this incrementally though flight tests during launches and grasshopper flights.  Spacex was also able to determine control gains through these tests.   These tests helped determine time and efficiency of all the burns after separation. 

Online Semmel

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Jim, I take your statement as a working assumption: "software is easy".

If software is easy, how is orbital mechanics hard? Newtons laws are fantastically simple. You dont need fancy long term trajectory prediction (over centuries, which are complicated) for a rendezvous. So if software is easy and your computer knows the orbit of your target, the software can predict its position far enough into the future for a rendezvous and subsequent docking. The rendezvous procedure is also fantastically simple. Here is the pseudocode:

1. Approach from a lower orbit
2. Align orbital planes
3. Raise your orbit slowly timed such that you approach your target from behind
4. Initiate docking

For the docking, the target docking port needs to provide a signal of some kind that you can target. Keeping stationary requires frequent adjustments using thrusters, but thats all software which is, as you say easy. So here is the pseudo code for docking:

1. Align your spacecraft and the docking port in a defined state
2. Approach the docking port within a defined trajectory envelope
3. if envelope is violated, abort docking and go back to step 1.
4. otherwise approach until docked

This is also not very hard. trajectory prediction requires orbital mechanics. But thats software which is easy. So.. how is docking hard?

From your proposition that software is easy and no other statement about the difficulty of the task, we arrive at the conclusion that docking is easy. Which contradicts your statement that docking is hard. So one has to go or you need to provide more information what exactly is hard here.

Also, I want to reiterate my last request: Please state your metric that you use to measure difficulty. Its really the hinging point of this conversation. Unless we agree on the metric, we will never agree on the difficulty statement.

Offline llanitedave

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Jim, I take your statement as a working assumption: "software is easy".

If software is easy, how is orbital mechanics hard? Newtons laws are fantastically simple. You dont need fancy long term trajectory prediction (over centuries, which are complicated) for a rendezvous. So if software is easy and your computer knows the orbit of your target, the software can predict its position far enough into the future for a rendezvous and subsequent docking. The rendezvous procedure is also fantastically simple. Here is the pseudocode:

1. Approach from a lower orbit
2. Align orbital planes
3. Raise your orbit slowly timed such that you approach your target from behind
4. Initiate docking

For the docking, the target docking port needs to provide a signal of some kind that you can target. Keeping stationary requires frequent adjustments using thrusters, but thats all software which is, as you say easy. So here is the pseudo code for docking:

1. Align your spacecraft and the docking port in a defined state
2. Approach the docking port within a defined trajectory envelope
3. if envelope is violated, abort docking and go back to step 1.
4. otherwise approach until docked

This is also not very hard. trajectory prediction requires orbital mechanics. But thats software which is easy. So.. how is docking hard?

From your proposition that software is easy and no other statement about the difficulty of the task, we arrive at the conclusion that docking is easy. Which contradicts your statement that docking is hard. So one has to go or you need to provide more information what exactly is hard here.

Also, I want to reiterate my last request: Please state your metric that you use to measure difficulty. Its really the hinging point of this conversation. Unless we agree on the metric, we will never agree on the difficulty statement.


Way too much mistaking of intuition for objective fact on this thread.  Engineers should know better.
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Online Robotbeat

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The idea that software is easy... Is wrong. :)
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Online Semmel

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The idea that software is easy... Is wrong. :)

Thats what I wanted to show. But maybe there are other things that are hard that Jim is getting at. I like to understand that.

Also, I want to reiterate my last request: Please state your metric that you use to measure difficulty. Its really the hinging point of this conversation. Unless we agree on the metric, we will never agree on the difficulty statement.
« Last Edit: 01/12/2017 03:29 PM by Semmel »

Offline Jim

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computer knows the orbit of your target

3.  Raise your orbit slowly timed such that you approach your target from behind

For the docking, the target docking port needs to provide a signal of some kind that you can target.


I said an uncooperative target.  No data is exchanged between vehicles nor its the target attitude known.

3. Why?  What says the docking adaptor is in that location

Online Semmel

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I must have missed the uncooperative part in the past. Why do you assume an uncooperative target? Thats not done unless you try to attack something. Why is that even part of the conversation?

Online meekGee

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Orbital mechanics, or orbital bodies, are the most predictable things there are.

Time spans for rendezvous and docking are hours..  minutes at the end... 

Meanwhile landing occurs in wind, in seconds, with control time constants that are also measured in seconds.

And the visiting vehicles have so many more control inputs...

Those sort of differences are not "software", they are inherent parameters of the control problem.

ISS may be non-cooperative, but it's certainly not trying to evade. Given its own very poor control authority, suppose it was cooperating, what exactly would it have done other than  just hold 3 axis stable, and wait for the VV to take its time, line up, and then glide forward?

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Offline rpapo

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I must have missed the uncooperative part in the past. Why do you assume an uncooperative target? Thats not done unless you try to attack something. Why is that even part of the conversation?
I could imagine rescue missions as "uncooperative", but I don't believe that was ever part of this conversation.
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Offline Jim

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I must have missed the uncooperative part in the past. Why do you assume an uncooperative target? Thats not done unless you try to attack something. Why is that even part of the conversation?

It is part of the conversion because it is hard. "Uncooperative" means it doesn't have any interaction with the chaser spacecraft.

That is what autonomous robotic servicing spacecraft will have to do.   They will have approach a spacecraft that is not designed for routine rendezvous and docking.  It will have to use onboard sensors to find the target spacecraft and then will have to find an area such as the launch adapter as a mating point. 

Offline kch

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I must have missed the uncooperative part in the past.

Yes, you did:


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.

Online Semmel

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So Jim, you take a particular hard case that is very special which requires a dedicated mission of docking with an uncooperative target and compare it with a routine operation of landing the first stage? Why would you do that? A comparison with a routine docking operation i.e. the ISS would make much more sense.

Out of curiosity.. when was there ever a docking with an uncooperative target?

Offline Jim

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So Jim, you take a particular hard case that is very special which requires a dedicated mission of docking with an uncooperative target and compare it with a routine operation of landing the first stage? Why would you do that? A comparison with a routine docking operation i.e. the ISS would make much more sense.



The point was software and modern avionics were not the enabler for the ability to land the stage.

https://forum.nasaspaceflight.com/index.php?topic=41935.msg1627456#msg1627456
« Last Edit: 01/12/2017 05:02 PM by Jim »

Online kevinof

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Doesnt the ASDS or the landing pad qualify as an "uncooperative" target? It's stationary ok but it's not exchanging any data with the vehicle just like the ISS.

For me the F9 landing is more than just the sw to get it there. Theres a lot of systems work gone into the vehicle to make this happen from attitude control, restartable engines, shielding and so on. All been done before in different ways but never the way to F9 does it.

As an ex realtime process control sw head I think the F9 sw work is top notch. It's a difficult problem to solve and yes they have tuned it time and time again with their landings and perfected it , but still a difficult problem to solve.

Online meekGee

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Doesnt the ASDS or the landing pad qualify as an "uncooperative" target? It's stationary ok but it's not exchanging any data with the vehicle just like the ISS.

For me the F9 landing is more than just the sw to get it there. Theres a lot of systems work gone into the vehicle to make this happen from attitude control, restartable engines, shielding and so on. All been done before in different ways but never the way to F9 does it.

As an ex realtime process control sw head I think the F9 sw work is top notch. It's a difficult problem to solve and yes they have tuned it time and time again with their landings and perfected it , but still a difficult problem to solve.

ISS keeps itself stable in three axis, and there's positional feedback for closing the loop at end-of-docking.

Additionally, before the closed loop is enabled, ISS tells everyone where it is

That's not very uncooperative.

In a control sense of the word, ISS doesn't actively try to "dock back", so will it counter (AFAIK) any impulse given to it by the VV, but given the difference in sizes, that's immaterial.  It had more relevancy when talking about a Gemini-Gemini docking, or something like that.   As far as the VV is concerned, ISS is an inertial and stable target - best thing you can ask for.
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Online john smith 19

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It is part of the conversion because it is hard. "Uncooperative" means it doesn't have any interaction with the chaser spacecraft.
Except that does not apply to ISS IIRC. All contract winners had to provide systems to ISS to allow it to stop the berthing (not docking so far that will be for CRS2)
Quote
That is what autonomous robotic servicing spacecraft will have to do.   They will have approach a spacecraft that is not designed for routine rendezvous and docking.  It will have to use onboard sensors to find the target spacecraft and then will have to find an area such as the launch adapter as a mating point.
True, but that's not a description of what happens (and what did happen with Shuttle) WRT ISS.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online john smith 19

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And the visiting vehicles have so many more control inputs...

Those sort of differences are not "software", they are inherent parameters of the control problem.

ISS may be non-cooperative, but it's certainly not trying to evade. Given its own very poor control authority, suppose it was cooperating, what exactly would it have done other than  just hold 3 axis stable, and wait for the VV to take its time, line up, and then glide forward?
True.

You also forgot the big ones.

F9 stage is mostly empty and has very high aspect ratio. It is (relatively) "floppy" causing a moving Cg. You've got 2 tank ends part way up as a point mass and another at the top to act as a 2nd point mass.

As it has turned out the control problem is in fact impossible without the use of grid fins since the engines simply lack the control authority over the structure.  :(

Realizing that, figuring what to do about it and making the solution work is rocket science.

OT This seems to be a recurring theme of aerospace problems. The physically simplest systems (in terms of geometry) seem to have very complex behaviors and require a lot of manual tuning. I'm thinking of ramjets, SCramjets and lifting bodies. High AR tank structures would also seem to be in this category.

I suspect there is something in the underlying mathematics of all these problems that makes them sensitive to apparently minor changes in geometry. Unfortunately I have no idea what that is.  :(
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline woods170

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computer knows the orbit of your target

3.  Raise your orbit slowly timed such that you approach your target from behind

For the docking, the target docking port needs to provide a signal of some kind that you can target.


I said an uncooperative target.  No data is exchanged between vehicles nor its the target attitude known.

3. Why?  What says the docking adaptor is in that location

Exactly. It was THE major problem that had to be solved by the folks of ConXpress (and other orbital recovery projects in the early 2000's): How to rendez-vous and dock with an uncooperative target? There is some good documentation out there about that particular problem. And it confirms what Jim has been stating for the past few days.
« Last Edit: 01/13/2017 07:50 AM by woods170 »

Online Semmel

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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.

Wrong, the ability to fly autonomously from orbit to runway landing is actually more difficult than autonomous  RTLS and hoverslam landing.

With some time to think about what you said, I might have a clue what you are getting at.

I dont remember where I heard that from and I dont find the source, but shuttle was designed to catch a satellite and carry it back to earth for analysis. The satellite didnt necessarily have to be operational for that capture operation or cooperative for that matter (i.e. the requirement was to be able to capture a Russian spy satellite, it was cold war after all). I dont know if that capability was ever used, but I think to remember that this was a requirement by the air force to the space shuttle. Therefore, it needed the capability to rendezvous and "dock" with a non-cooperative target.

Also, there was a requirement for the space shuttle to land back at the cape after just one orbit. This requirement actually meant that it needed extreme cross range capability and quite tough control algorithms.

I dont know if you meant these things but if so, it would have been easier to actually name them as example when you made your initial statements. At least we would have known what you specifically are talking about and didnt need to have this "believe me because I am an authority on the topic". I get that you sometimes have to restrict your comments to simple statements because of some stuff you are working on is secret. But in case of things that are public knowledge like the above, it is not necessary. You could simply follow your statement with an argument why things are the way they are.

It makes sense that "docking" with an uncooperative target is a tough problem to solve, but I don't understand why you think its tougher than the hoverslam landing of an F9. I dont say the hoverslam is harder, I simply dont know. So please provide your argument in a way that actually provides some insight into the matter.

For the record, I think a hoverslam landing was possible in the shuttle area. In the 70th and 80th, people simply didnt care to solve the problem because they didnt think it was worthwhile or maybe because they never thought of that idea in the first place. SpaceX can now prove that they are able to deliver the cheap re-usability that the shuttle promised but never was able to deliver. In terms of economic re-usability, shuttle was a failure. I hope F9 is the opposite.

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