Author Topic: SpaceX paper on precision landing - and landing technology Thread  (Read 64456 times)

Offline LouScheffer

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An engineering view of why landing is harder than rendezvous.  By rendezvous I mean up to stationkeeping, in the same orbit, as a non-maneuvering but otherwise non-cooperative target whose orbit is known.  Actual docking from that point can range from fairly easy (if it's designed in) to hideously difficult (a spinning target not designed for docking).

In both cases we need to match location and velocity, with near-zero angular rates.  In one case the target is in inertial space, in the other an Earth-based frame.  This in itself makes very little difference as rockets already have to deal with both frames, and they are related by the uniform angular velocity of the Earth's rotation.

The variables in rendezvous are separable.  This means you can fix one, and it stays fixed while you adjust the others.  For example, you can start by zeroing angular rates.  If after that, if you thrust through the center of gravity (which you control) the rates stay zero.  And if you do end up with any angular rates, you can re-zero them without screwing anything else up.  Next, you can enter a common orbital plane.  After that, as long as your burns are in plane (which you control) this variable stays fixed.  Then a single burn magnitude matches the apogee, and after that it stays fixed (provided you do futher maneuvers at apogee, which you control).  You can set position in the orbit by timing this burn (also under your control).  The two variables of the burn do not interact in any way, and don't disturb the plane, which you already set.  Finally you burn at apogee to synchonize the orbits, and this does not disturb the timing of the orbit, or the apogee, or the plane, or the angular rates.  It only affects the perigee.

Next, in rendezvous the control inputs are largely orthogonal.  Designers explicitly build in combinations of thrusters that do a single operation - three types of translation, and three types of rotation.  Even the worst combination changes only two closely related variables - a +X burn changes only X and X dot, not anything else. For main engine burns, orthogonality is achievable because of the slow pace of rendezvous - there is time to turn to the needed attitude, so your main engine thrust only affects the variables you want to change.

For landing, the variables are not separable and the controls are not orthogonal.   Suppose you have a perfect trajectory with the only problem that you need a few hundred meters of X translation (perhaps due to winds).  You can only translate the rocket by including the main engine, since the grid fins (and thrusters) do not work through the center of mass.  So a translation in X requires modifying your previously correct Z , Z dot, and at least one angular rate.  Furthermore, you can only reduce Z dot - there is no way to get the rocket to fall faster to get back to the previous solution.  So you need to plan another maneuver later, also affecting all these state variables, that hopefully restores a different but feasible solution.  Overall, rendezvous is like playing Battleship - you keep getting closer to a solution and never backtrack.  Landing is more akin to solving Rubik's cube - there are sequences that do what you want, but they necessarily make lots of other stuff worse, and then hopefully converge back to a good solution.  You need to think ahead.

Next, rendezvous can go as slow as desired, and designers take full advantage of this.  There is a reason why final approach to ISS takes many minutes, with many stops and double checks along the way.  There are very few single opportunity events - you can always stop and try again later, and this usually costs minimal, if any, fuel.  Landing must be conducted in real time, with no second chances.

Next, rendezvous is almost completely insensitive to modelling and performance errors.  If your performance is worse than you expect, or your estimated mass is wrong, it still works fine.  This is because you perform each maneuver by thrusting in the desired direction until the IMU tells you that you have the correct delta V.  This technique has saved many missions where the rocket performance was less than expected.  But for landing, your model needs to be precisely correct. If you are at 100 meters, and sinking with rate X m/s, there is one and only one acceleration that will bring you to zero speed at zero altitude.  If your model (or your hardware) is wrong, and you get any less (or more) acceleration, it's a bad day.

Next, rendezvous has a complete lack of external variation.  There is a reason why rendezvous computations wait until the vehicle is out of the atmosphere.  At this point you can plan the entire rendezvous sequence, and only correct for execution dispersions thereafter.  Landing, on the other hand, needs to compensate for variable and uncertain winds, and must do so as they happen.

So what computers could do these operations?  Almost any computer could solve the rendezvous problem.  After all, it was solved in Gemini 12 by astronauts using charts.  And Apollo did it around the moon, and could do so with no help from the ground, while accounting for most plausible malfunctions.  How about the landing problem?  Apollo, almost certainly not.  It was kept busy just trying to hit a point in 3D space.  How about the Shuttle?  According to The Space Shuttle Primary Computer System, the Shuttle computer was about 75% busy, which already took considerable optimization:
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The eight programs used during a typical mission average about 75 percent CPU utilization for most flight regimes, which leaves us well within the capability of this machine. Very early on in the development phase, we did have some trouble with excessive CPU utilization. We went through a very detailed scrub of the software requirements and the code to achieve the CPU utilization we have today.
Given that rocket landings are several times more fast paced than Shuttle landings, the Shuttle computers seem marginal for this job.  Interestingly, the Delta RIFCA computers seem about an order of magnitude faster than the Shuttle computers, and probably have the compute horsepower for the job.  However, other aspects of the system they are embedded in seem to hobble them quite severely, since they can't even accomodate yaw steering, which Titan, Apollo, and the Shuttle could do.

Offline john smith 19

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How about the Shuttle?  According to The Space Shuttle Primary Computer System, the Shuttle computer was about 75% busy, which already took considerable optimization:
Quote
The eight programs used during a typical mission average about 75 percent CPU utilization for most flight regimes, which leaves us well within the capability of this machine. Very early on in the development phase, we did have some trouble with excessive CPU utilization. We went through a very detailed scrub of the software requirements and the code to achieve the CPU utilization we have today.
Given that rocket landings are several times more fast paced than Shuttle landings, the Shuttle computers seem marginal for this job.  Interestingly, the Delta RIFCA computers seem about an order of magnitude faster than the Shuttle computers, and probably have the compute horsepower for the job.  However, other aspects of the system they are embedded in seem to hobble them quite severely, since they can't even accomodate yaw steering, which Titan, Apollo, and the Shuttle could do.
A couple of data points. The first test on a fly by wire system of an unstable aircraft was done using a spare Apollo Guidance Computer running about 32KIPS. The original IBM 4Pi  processor was rated about 400KIPs so 75% of capacity would have been about 300 KIPS. The upgrade version brought it up to about 1 MIPS. IIRC SX uses ARM processors running in the 100s MHz range so an order of magnitude above the RIFCA
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.
Yes that would be a worst case scenario. It seems to have also driven the huge cross range requirement for the Shuttle as well.

It was about the most insane mission I've ever heard of since it would have either had to be mounted during WW III or would probably have started WW III.  :(

Actually STS was used to capture and repair a couple of satellites IIRC (with the owners consent). One at least was randomly tumbling. IIRC the astonaut grabbed on and used their EMU to gradually bring it under enough control that it could be grappled by the arm.
Exactly. It was THE major problems 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.
So it seems Jim is comparing a maneuver that's never actually been done (but is within the range of all possible orbital maneuvers) with one that has been done but only after multiple failures.

I'd suggest that what made this problem hard would be decide what the satellites orientation was and if it was changing (IE tumbling) how to slow it down. That suggests either radar/lidar and/or image recognition. IIRC both can be very compute intensive at high data rates and high throughput rad hard computers were (and are) expensive, unless you go to a more fault tolerant architecture.

BTW for de-orbiting com sats IIRC the tactic seems to approach and lock onto the Apogee motor nozzle since its clearly defined (big round black hole)
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Offline Jim

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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.

Its location is known and programmed into F9

Offline Jim

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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)
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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.

ISS and shuttle have nothing to with this thread

Offline ChrisC

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(facepalm)
PSA #1: EST does NOT mean "Eastern Time".  Use "Eastern" or "ET" instead, all year round, and avoid this common error.  Google "EST vs EDT".
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Offline Jim

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Next, in rendezvous the control inputs are largely orthogonal.  Designers explicitly build in combinations of thrusters that do a single operation - three types of translation, and three types of rotation. 

Not really.  Most vehicles do not do rotational couples.  There is translation with rotation.

Offline Jim

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Next, rendezvous is almost completely insensitive to modelling and performance errors.  If your performance is worse than you expect, or your estimated mass is wrong, it still works fine.  This is because you perform each maneuver by thrusting in the desired direction until the IMU tells you that you have the correct delta V.  This technique has saved many missions where the rocket performance was less than expected.


You are mixing up rendezvous and orbital insertion.  Orbital insertion doesn't care the vehicle enters orbit.

But for landing, your model needs to be precisely correct. If you are at 100 meters, and sinking with rate X m/s, there is one and only one acceleration that will bring you to zero speed at zero altitude.  If your model (or your hardware) is wrong, and you get any less (or more) acceleration, it's a bad day.


That is not changing the actual program, it is only a tweaking of the gains and constants used.
Just add more propellant, increase thrust or burn earlier and longer.  That is what flight testing is for.
The same programming is used.

Again, this isn't about determining all the necessary constants and gains.  This is about the avionics and programming.

« Last Edit: 01/17/2017 09:10 am by Jim »

Offline Lar

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Guys, can we try to get back on topic. rendezvous with ISS is not landing on an ASDS or on land.
« Last Edit: 01/13/2017 05:35 pm by Lar »
"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
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline tdperk

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No.

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

How do you know that?

He doesn't.   His experiences lead his to certain biases.

Consider the matter, if you are on top of a 4 story building, and you have a hang glider and a pogo stick with a brake, and your task is to land safely--which do you want to use?

The Shuttle is the glider.

The Falcon is the pogo stick.

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.

Its location is known and programmed into F9

But AFAIK it does not maneuver to "catch" the Falcon.  It is more or less successful and holding a position and attitude.
« Last Edit: 01/13/2017 07:09 pm by Chris Bergin »

Offline Semmel

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Wrong, the ability to fly autonomously from orbit to runway landing is actually more difficult than autonomous  RTLS and hoverslam landing.
How do you know that?
He doesn't.   His experiences lead his to certain biases.

Consider the matter, if you are on top of a 4 story building, and you have a hang glider and a pogo stick with a brake, and your task is to land safely--which do you want to use?

The Shuttle is the glider.

The Falcon is the pogo stick.


1. Please dont speak for Jim. I dont know who you are but speaking for Jim is just asking for trouble. You can be lucky if your post is not deleted. I would hive it a dislike if that would exist.

2. I am not looking for an analogy. I am looking for hard, cold, engineering reasoning. Its always harder than you think. LouScheffer gave some good insight. But thats probably all we can get.

3. Lar gave us a shot in front of our metaphorical bow. If we continue with the "I know better than you what is more difficult", posts get deleted. Arguments get hurt and ultimately, threads get closed. So I would very much appreciate to get back on topic and discuss the challenges of landing falcon first stages. Discussing whats more difficult is not helpful.

4. The initial question "why wasnt booster landing done before?" got answered way back: The solution was not interesting enough for the rocket builders of the past. It had nothing to do with the difficulty of the job, far less with being impossible for the computer hardware. It might have been impossible, but it was not tried so we will not know.
« Last Edit: 01/13/2017 08:30 pm by Semmel »

Offline tdperk

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1. Please dont speak for Jim. I dont know who you are but speaking for Jim is just asking for trouble. You can be lucky if your post is not deleted. I would hive it a dislike if that would exist.

2. I am not looking for an analogy. I am looking for hard, cold, engineering reasoning. Its always harder than you think. LouScheffer gave some good insight. But thats probably all we can get.

3. Lar gave us a shot in front of our metaphorical bow. If we continue with the "I know better than you what is more difficult", posts get deleted. Arguments get hurt and ultimately, threads get closed. So I would very much appreciate to get back on topic and discuss the challenges of landing falcon first stages. Discussing whats more difficult is not helpful.

4. The initial question "why wasnt booster landing done before?" got answered way back: The solution was not interesting enough for the rocket builders of the past. It had nothing to do with the difficulty of the job, far less with being impossible for the computer hardware. It might have been impossible, but it was not tried so we will not know.

I have yet to claim to speak for Jim.  Were I to speak for Jim, I would unavoidably be more loquacious  :P

Without numbers we don't have, its all analogy.

I'm responding to Jim, whose post Lar chose not to remove, and which post was peremptory almost to the point of rudeness as well as being obviously incorrect from an engineering standpoint.  Without numbers we don't have, we have analogy.  To put more flesh and relative energy accuracy on the analogy.
a) STS is like jumping off a 9 or 10 story building with a hang glider and an open field in front of us.  You have to land within 1" inch of on orange cone which is in the middle of your glide pattern, but you can take a few steps to zero forward momentum out.
b) Falcon is like jumping off a 4 story building with a ballistically deployed parachute which has a high enough sink rate it's use alone will cause you to RUD.  You also have a pogo stick with a brake, you have to use the brake to come to a halt vertically and balanced without bouncing or hitting too hard.
B is harder.  Neither is for the faint hearted.

" The initial question "why wasnt booster landing done before?" got answered way back "  <--  Well then the threads already over, isn't it?  It was not, as matter of fact, an engineering goal to reduce the cost of a pound to LEO--where there might be a pound of people (or several hundred pounds).  So the best means of doing so was not pursued.
« Last Edit: 01/14/2017 12:30 am by tdperk »

Offline Jim

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He doesn't.   His experiences lead his to certain biases.

Yes, I do and also it is not biases.
« Last Edit: 01/14/2017 02:44 am by Jim »

Offline mvpel

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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.

One of the old timers at work was giving a presentation last year, and in discussing the present-day work on the problem of collaborative swarm navigation of autonomous vehicles[e.g.] while GPS-denied in the presence of enemy jamming, he noted that before the 90's, ALL navigation was "GPS-denied."  ;D
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Offline john smith 19

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Mulling things over I think the comparison between Shuttle and the F9 1st stage is unfair.

Turning it on its head the analogy would be Shuttle with SRB, SSME and OMS but almost no RCS thrusters and NASA flying several missions as they decided the minimal numbers to add to carry out their reference missions (for example because mass is phenomenally critical).

I know that is an analogy stretched almost to the point of absurdity but SX have turned a structure whose key design task (until now) has never been to return to base and many of whose best design choices actively discourage you from doing so into doing exactly that with minimal changes (IE no changes to main structures load directions, so no wings. No secondary engines etc).

This really is turning a sows ear into a silk purse.  It's hugely impressive.
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Offline john smith 19

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One of the old timers at work was giving a presentation last year, and in discussing the present-day work on the problem of collaborative swarm navigation of autonomous vehicles[e.g.] while GPS-denied in the presence of enemy jamming, he noted that before the 90's, ALL navigation was "GPS-denied."  ;D
Exactly.

And the modern solution (outside military systems) is to switch to another satellite constellation.  :(

On Earth GPS and WAIS can deliver high accuracy on terminal approach but on Mars this will not be an academic exercise. The closer ITS can land to each other the less transport that has to take place before useful work can begin.  :(
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Offline Nomadd

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 One thing I've didn't find in this paper or anywhere else was terminal, as in the last few feet, guidance. I keep remembering the way the barge was moving all over the place including up and down during the first landing that ended vertical, but have never found any mention of final adjustments via altitude or relative position sensors on the rocket. Or maybe I should say velocity sensors, since the barge position is not only uncertain in seas but it's rate of movement in all directions is constantly changing.
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Offline mvpel

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Or maybe I should say velocity sensors, since the barge position is not only uncertain in seas but it's rate of movement in all directions is constantly changing.
I reckon that's a benefit of the hoverslam - everything is over before the barge has a chance to move much.


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Offline LouScheffer

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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.   
From the navigation point of view, these statements seem wrong.  The pad location never changes in Earth based coordinates.  But that's not what inertial navigation systems use - they use (surprise) inertial coordinates.  (They could in theory use Earth based coordinates, but then you need to add Coriolis forces and other complications.  So I believe the inertial navigation systems are indeed inertial.)  In inertial space system, the pad location is constantly changing.  Basically it's moving in a circular orbit with a period of 24 hours, with a center offset from the center of the Earth.  Of course when you get close you will switch to Earth based coordinates, just as in rendezvous, where when you get close you start measuring your distance and rates relative to the target, not the Earth.

The same applies to the vehicles attitude at launch and landing.  They are both vertical, but only in a rotating Earth based frame.  From the INS point of view, the rocket's body vertical will vary as 360 degrees/24 hours* (landing-launch times).  This is about 2.1 degrees for a typical 500 second launch-landing sequence, and so can't be neglected.   (This particular effect was the cause of a Soviet launch failure.  They aligned the INS, then called a hold.  30 minutes later, the INS calculated the rocket was 8 degrees from vertical, and triggered the abort system as being out of allowable range.)
Quote
The flight path back to the it [landing spot] is never out of plane.
This seems wrong as well.  The initial plane is defined by the launch azimuth and the center of the Earth.  This plane also (nearly) includes the landing site, but only at the time of launch.  By the time the rocket get back, about 500 seconds later, the rotation of the Earth has carried the landing site about 200 km to the East.  This is a quite large correction - the Atlas launches to ISS could only cope with a 5 minute (300 second) offset from the launch plane.

Offline dglow

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I don't know, Lou, I'm starting to come around to Jim's point of view.
The evidence? SpaceX just made that landing look really easy.

Offline meekGee

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Or maybe I should say velocity sensors, since the barge position is not only uncertain in seas but it's rate of movement in all directions is constantly changing.
I reckon that's a benefit of the hoverslam - everything is over before the barge has a chance to move much.


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That IMO is critical, and why I never like the posts advocating adding "hover" abilities.

Slam it, baby!

The faster you go through the last layer of variable surface winds, the less you have to worry about them, or at least about their variability.  The stage has some notion of winds aloft, but wind sheer in the last 100 m has an effect and needs to be minimized. We've seen how the stage "leans" in the last few seconds, and basically "uncrabs" just in time for touch down.

(Show me a docking craft doing THAT....)

-----------------------

So hey - Suppose SpaceX shows 10 consecutive landings that are within say 0.5 m, and thatthey have cores to spare.

Will we see a legless S1 trying to nail a cradle landing?   Or is the concept to difficult to pull off with a tiny little rocket like F9 and will have to wait for larger ones?

I get bored easily.
« Last Edit: 01/14/2017 07:49 pm by meekGee »
ABCD - Always Be Counting Down

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