Author Topic: SpaceX paper on precision landing - and landing technology Thread  (Read 22670 times)

Offline envy887

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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.

Shuttle avionics did support autonomous nav, guidance, and control for landing.

Offline Jim

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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.
« Last Edit: 01/10/2017 06:05 PM by Jim »

Offline Semmel

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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?

Offline Jim

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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?

Intuitively obvious. 

Having to go from orbit at a specific landing site with no propulsion by using energy management.  Going from Mach 25 to zero through the subsonic, transonic, supersonic, hypersonic and high-hypersonic flight regimes with a winged vehicle having variable stability utilizing blended reaction and aerodynamic control.   And this was before GPS (there was TACAN and MSBLS)
« Last Edit: 01/10/2017 06:30 PM by Jim »

Offline john smith 19

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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.
Except on that basis Grasshopper should have shaken out any residual issues and the first landing should have been successful.

It wasn't. 

All the major LV mfg's in the 60's talked about 1st stage reuse. None of them had to do it. SX's experience suggests they would have seriously struggled to do so. The fact that SX only succeed after they fitted grid fins to the top end suggest that any strategy that relied on engine retro thrust was doomed to fail.

The operative word in rocket science is science and science cannot be rushed.  :(



Having to go from orbit at a specific landing site with no propulsion by using energy management.  Going from Mach 25 to zero through the subsonic, transonic, supersonic, hypersonic and high-hypersonic flight regimes with a winged vehicle having variable stability utilizing blended reaction and aerodynamic control.   And this was before GPS (there was TACAN and MSBLS)
You missed having the big lump of the SSME's at the back, demanding a constantly operating computer system driving constantly moving aerosurfaces. Dyna Soar was expected to use a similar hybrid control system and the X15 was able to test such a system over a significant section of the speed / altitude range.

Shuttle was designed to come back to Earth. Since no stage prior to the F9 stage 1 had ever been designed to do so. In that sense the total  problem of stage return is the tougher one.
"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 Jim

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Since no stage prior to the F9 stage 1 had ever been designed to do so. In that sense the total  problem of stage return is the tougher one.

not avionics wise

Offline Lars-J

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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?

Simple. Because it is SpaceX.  ;)

Offline mvpel

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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.

The first modern-era missile defense hit-to-kill intercept (that is, autonomous rendezvous with a non-cooperating target) was the IFT-3 flight test of the Ground Based Interceptor with the Raytheon Exoatmospheric Kill Vehicle which took place in 1999, and having seen an EKV up close I can tell you that there's not much room for compute horsepower, particularly with late-90's embedded systems.

And that aside, the very first hit-to-kill intercept was the Nike Zeus in 1961. Most of the analog electrical engineering experts are retired by now, but it's enjoyable to listen to their stories.
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline dglow

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The first modern-era missile defense hit-to-kill intercept (that is, autonomous rendezvous with a non-cooperating target) was the IFT-3 flight test of the Ground Based Interceptor with the Raytheon Exoatmospheric Kill Vehicle which took place in 1999, and having seen an EKV up close I can tell you that there's not much room for compute horsepower, particularly with late-90's embedded systems.

The first shuttle computer was early 1970s hardware, with .4 MIPS and ~416K of core memory.

http://commons.erau.edu/cgi/viewcontent.cgi?article=2024&context=space-congress-proceedings


Thank you for the anecdotes, especially the Nike! Analog computing is very romantic.  :)

Offline meekGee

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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.

The first modern-era missile defense hit-to-kill intercept (that is, autonomous rendezvous with a non-cooperating target) was the IFT-3 flight test of the Ground Based Interceptor with the Raytheon Exoatmospheric Kill Vehicle which took place in 1999, and having seen an EKV up close I can tell you that there's not much room for compute horsepower, particularly with late-90's embedded systems.

And that aside, the very first hit-to-kill intercept was the Nike Zeus in 1961. Most of the analog electrical engineering experts are retired by now, but it's enjoyable to listen to their stories.

An intercept requires zeroing 3 independent DOFs.  An F9 landing requires zeroing 6 coupled DOFs + their first derivatives, or 12.  Not remotely the same.

Whether STS computers were powerful enough doesn't matter.  This was about the statement that "landing an F9 is easier than docking a spacecraft".

Compute power is just a part of it. Solving the control problem was a bigger part.  Doing the system engineering, yet another. Finding a way to finance it - yet another.

To paint it as "it simply wasn't done since nobody wanted it" is transparent denial and self-gratification...

It wasn't done for many reasons: Nobody had (simultaneously) the vision of doing it, the capability to do it, and the audacity.



ABCD - Always Be Counting Down

Offline Jim

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Give it a rest, that diagram is getting over used.

And, landing an F9 IS easier than autonomous docking a spacecraft.  There is no question.

Just as "it simply wasn't done since nobody wanted it" is reality.  Trying to paint it otherwise is just more sycophancy.

I have provided plenty of support.  Just some people are too biased to accept the truth.
The landing site is known vs the unknown location of the target spacecraft.  Stopping at the end is not big deal.  We have landed on other bodies before.
As for autonomous docking, you have made the assumption that the docking adapter is point towards the approaching vehicle.  As I stated before, it is an uncooperative target, meaning it does not provide location or attitude to the chaser spacecraft.
« Last Edit: 01/11/2017 02:37 AM by Andy USA »

Offline meekGee

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God I hope you're kidding but I suspect you aren't.
Waiting for this exchange to be nuked already.
Peace - out.
ABCD - Always Be Counting Down

Offline Stan-1967

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Intuitively obvious. 

Having to go from orbit at a specific landing site with no propulsion by using energy management.  Going from Mach 25 to zero through the subsonic, transonic, supersonic, hypersonic and high-hypersonic flight regimes with a winged vehicle having variable stability utilizing blended reaction and aerodynamic control.   And this was before GPS (there was TACAN and MSBLS)

Wrong
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.  Both have their own needs in regards to accuracy. 

Comparing STS to F9 is not helpful.  Specifically from an energy management perspective as you did, not to mention complete different types of vehicles.  There was a good thread comparing F9 to New Shepard.  In the end even those two are very imperfect comparisons, STS vs. F9 is worse.

STS's challenge was in shedding enormous amounts of energy.  To do that it had plentiful aerodynamic means to do so.  The STS's crossrange is testament to that.  F9 doesn't face the high hypersonic regime, but is challenged in the hypersonic to subsonic regimes because it has a near insignificant L/D to manage energy, whereas STS has ample aerodynamic L/D options and trajectory solutions to manage energy.  ( look at the wide turns it made on mid range ground tracks, as well as the turn to final approach.  The radius of the STS turn to final is where energy can get fine tuned for the landing )  Once STS is within close range of the runway, there is little doubt that it will land intact barring pilot error or landing gear collapse.

F9 on the other hand is energy poor from the perspective of nulling the vertical & horizontal velocity components.   It has just enough energy in the propellant for a single shot at landing.  STS had a runway to null the last remaining energy in the vehicle.  There is no 15,000 ft deep net to catch it should its velocity be not exact when it's over the numbers at the end of it's runway.  ( errr..drone ship)

The interesting part of this thread is what landing techniques lend themselves to precision.  For landings on earth, GPS is the obvious choice with no immediate rivals.  What about Mars, the Moon, or Titan?  Do you first orbit a constellation of GPS satellites? Do you land a MLS type system onto the surface?  Or do you need a combination of the two to get good precision?




Online FutureSpaceTourist

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The interesting part of this thread is what landing techniques lend themselves to precision.  For landings on earth, GPS is the obvious choice with no immediate rivals.  What about Mars, the Moon, or Titan?  Do you first orbit a constellation of GPS satellites? Do you land a MLS type system onto the surface?  Or do you need a combination of the two to get good precision?

Yes, please let's get the thread back to this topic (unless there's another thread that's already covered it, in which case please link).

Offline Jim

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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.

Offline Stan-1967

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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?

Offline Jim

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The interesting part of this thread is what landing techniques lend themselves to precision.  For landings on earth, GPS is the obvious choice with no immediate rivals.  What about Mars, the Moon, or Titan?  Do you first orbit a constellation of GPS satellites? Do you land a MLS type system onto the surface?  Or do you need a combination of the two to get good precision?

Extra propellant is needed until a local navigation system is provided.

Offline Joel

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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 ]
My understanding is that the trajectory planning uses a first-principle model of the reentering stage, higher-fidelity model for Earth than for Mars as stated in the paper. By using CVXGen, you can solve the resulting convex problems (should be a Second-Order Cone Program) very fast, probably in milliseconds. The controller is then fast enough to e.g. react to a wind shift. Embedded optimization like this first appeared in the chemical process industry in the 1980ies, with much slower processes. Only spread to aerospace applications in the last decade or so. Is still very much a hot research topic, both in terms of algorithms and software.

Offline JamesH65

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Give it a rest, that diagram is getting over used.

And, landing an F9 IS easier than autonomous docking a spacecraft.  There is no question.


Sorry, I don't believe you - you have not shown this in any of your posts. And I doubt you can provide any SW evidence to back up this claim.

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.
« Last Edit: 01/12/2017 08:34 AM by JamesH65 »

Offline Semmel

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Maybe we should define what "difficulty" means in this context. We can run in circles all day long if we all have different metrics on the term "difficulty".

So please, define your metric and then we can measure the "difficulty" of landing F9, landing Shuttle, docking and others activities.

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