Author Topic: Pushing the limits of the hoverslam landing  (Read 41934 times)

Online abaddon

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Re: Pushing the limits of the hoverslam landing
« Reply #20 on: 01/08/2016 03:49 PM »
Actually, it's extremely easy to argue against this.  Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades.  It's well proven technology.
I see, so you are asserting that the F9 first stage is identical in size/shape/density/mass distribution to a smart bomb?  I think poking a hole in your "easy" argument is even easier...

Online abaddon

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Re: Pushing the limits of the hoverslam landing
« Reply #21 on: 01/08/2016 03:51 PM »
Is it known that the fins are for guidance and not just stability?
Yes, pretty much.  Earlier flights without fins were only able to achieve a very gross landing area.  Cold gas thrusters were able to maintain attitude successfully (once they added more gas) but that was not sufficient with targeting.  It was the fins that enabled the stage to get in the vicinity of the barge.

Offline rpapo

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Re: Pushing the limits of the hoverslam landing
« Reply #22 on: 01/08/2016 04:12 PM »
The altitudes where winds are the strongest (above 25k ft) are exactly where terminal speed is still significant (but subsonic), hence grid fins will have maximum effect.
Which brings up the question: At which point in the descent (how high up is it still) does the rocket go subsonic?  The fact that the sonic booms were so loud in the December landing indicates (to me, at least) that the rocket remained supersonic until rather late.  Keep in mind that "terminal velocity" is a function of air density (and therefore altitude), so that throughout the descent the rocket is going at some speed above terminal velocity because terminal velocity itself is decreasing.
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Online mme

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Re: Pushing the limits of the hoverslam landing
« Reply #23 on: 01/08/2016 04:13 PM »
It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn.  In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point.  I think this would be difficult to argue against. 
Actually, it's extremely easy to argue against this.  Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades.  It's well proven technology.
Is it known that the fins are for guidance and not just stability?
I can't say it's known, but they water landed stages without grid fins and those landings where within 10 km rather than 10m accuracy.  I'm pretty sure that is mostly do to the grid fins, but I don't have a specific source saying that is the only change.  Also, running out of hydraulic fluid on the first attempt "missed the mark," requiring a huge divert.

As too the rest of the argument, I'm a bit lost.  Clearly the engine is required to avoid lawn darting, and slowing the stage will reduce the effectiveness of the grid fins.  So the engine becomes more important to control in the last few seconds.  That's for an incredibly small portion of the landing and it better be really close.

I'm in the "grid fins are pretty darn accurate AND SpaceX chooses to use engines to divert the IIP while not lawn darting" camp.
« Last Edit: 01/08/2016 04:21 PM by mme »
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Online LouScheffer

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Re: Pushing the limits of the hoverslam landing
« Reply #24 on: 01/08/2016 04:17 PM »
It is true that the grid fins have more control authority at a higher speed, but we don't know that they are capable of a pinpoint landing without any engine burn.  In other words, I am asserting the engine burn is not just to bring vertical velocity to zero, it is also to target the final landing point.  I think this would be difficult to argue against. 
Actually, it's extremely easy to argue against this.  Smart bombs routinely achieve meter-level accuracy using only fins, and have for decades.  It's well proven technology.
Smart bombs have far more control authority and glide capability than a F9R stage.
Of course they have more control authority.  You can't reschedule a war for when the winds are calmer, the launching aircraft may have other things to optimize for (like surviving, avoiding missiles), rather than lining up straight on target, the enemy is unlikely to broadcast ground-level winds for your convenience, etc.

I would assume the SpaceX landing requirements will guarantee they would not run out of control authority.  Then the accuracy will be determined by the magnitude of the disturbances and the combined loop gain of the correcting algorithm, the fins, and the aerodynamics.  Both have similar sideways disturbance accelerations (the GBU-38 has a mass of 250 KG and a surface area of about 1 m^2, the Falcon 9 about 30t and 120 m^2).  The Falcon is about 10x longer so you would expect the control to be about 10x slower (the actuators are 10x further from the center, but the moment of inertia goes like the square, and is 100x more).  So you'd expect the response to slow gusts should be identical (both systems can compensate) but fast gusts could be up to 10x worse than the JDAM.  Overall it should be somewhere between 1x and 10x worse accuracy than JDAM, depending on the temporal structure of wind gusts.  10 meters of accuracy using only the fins seems pretty likely.

Offline Stan-1967

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Re: Pushing the limits of the hoverslam landing
« Reply #25 on: 01/08/2016 04:24 PM »
Warning: Wall of text incoming!    :o

tl;dr:  Looks like SpaceX could save fuel/weight by landing more aggressively.  Could they? Will they?

......

So, first off can anyone point out any obvious math errors? (i know "total impulse" for breaking + Gravity losses is probably the wrong terminology)  Beyond the feasibility of my scenarios, I would like to know if I didn't even calc the fuel use to within an order of magnitude.

But more interestingly, how useful is a fuel savings of about 2000kg? or even just 600kg?  Is there any reason to expect that SpaceX will attempt to land more aggressively and really push the limits? 

I don't want to say that I was disappointed by the landing, but it wasn't exactly as "brown pants" of a maneuver as I was expecting. But maybe my perception betrays just how difficult of landing it already was?

Anyway, after doing all these calcs I figured that with how much use I get out this forum, that I should try to give back and hope someone else finds this interesting as well.  It is now way too late for me to still be awake please forgive typos.  :)

It was a long read, I appreciated it & it was worthwhile.   Thanks for contributing to the discussion. 

I think what you have done is mapped out a "window" where it is mathematically possible to land the stage.   There are infinite mathematical solutions to landing the stage, but functionally, the physical limitations of the hardware and fidelity of control responses narrow down what is possible.  ( i.e grid fins, wind, valve response time etc.)  When I have thought through the calculations you were going through, in my mind I draw out a flight regime on a graph:   

The left vertical axis has height above the landing zone where the landing burn begins, and it ends at zero elevation.

The bottom horizontal axis has thrust level,  say 70-100% is the range of interest.

The top horizontal axis has probability of failure figures throughout the flight regime  that assigns increasing risk as higher thrust levels progress rightward on the horizontal axis.  For arguments sake, maybe failure probability ranges from 5% to 90% as throttle goes from 70-100%.

As you start the burn lower everything has to happen much faster and all the sensor and mechanical responses will overlay their uncertainty & variability into greater risk of failure.  Some factors may favor higher speed, like wind response, but overall I think the risk increases with low elevation/high throttle landing.

In the end, I think you get a landing window for a given amount of propellant available, and then you try to optimize your landing probability based on your characterization of your electrical/mechanical/software systems.  You could theoretically do a "brown pants" landing every time, but the real goal is a rocket standing on the pad undamaged. 

 I recall E. Musk citing a 10% increased probability of successful landing when he explained delaying the launch by a day.   He cited "Monte Carlo" simulations as justification of this.  That statement tells me they are modeling each landing based on all variables they have characterized that are inputs to the landing algorithms and process.


Again, thank for posting your ideas!
« Last Edit: 01/08/2016 04:28 PM by Stan-1967 »

Offline cambrianera

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Re: Pushing the limits of the hoverslam landing
« Reply #26 on: 01/08/2016 04:35 PM »
Nobody realized that saving 2000 kg of propellant in the landing burn decreases the mass during reentry burn (and boostback burn, if done).
Therefore saving could be easily double that, because propellant for reentry burn (and boostback burn) can be less.
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Offline Kabloona

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Re: Pushing the limits of the hoverslam landing
« Reply #27 on: 01/08/2016 07:01 PM »
Nobody realized that saving 2000 kg of propellant in the landing burn decreases the mass during reentry burn (and boostback burn, if done).
Therefore saving could be easily double that, because propellant for reentry burn (and boostback burn) can be less.

Not really, because the dominant factor is the 26,000 kg dry mass on the first stage, so carrying 2,000 kg less propellant through boostback and re-entry reduces vehicle mass by less than 10%, and thus gives you less than 10% additional propellant savings for a given deltaV.
« Last Edit: 01/08/2016 07:02 PM by Kabloona »

Offline PreferToLurk

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Re: Pushing the limits of the hoverslam landing
« Reply #28 on: 01/08/2016 08:16 PM »
Thanks for all the great replies.  40m/s to payload certainly doesn't seem worth the risk.   I wonder though if they could use the extra propellant for more boost back burn? 3 more seconds and around 150m/s extra boost back might enable rtls for more missions, or enable a less lofted trajectory. Certainly lots of variables at play here, and a safe/reliable landing is priority one. Leads me to believe we may still see testing at Spaceport America. Even if they only squeeze out another 1000kg of saved fuel, you can triple that for FH.

Offline rpapo

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Re: Pushing the limits of the hoverslam landing
« Reply #29 on: 01/08/2016 08:23 PM »
...or enable a less lofted trajectory.
Careful what you wish for: More loft is more air time, which means less velocity back to landing site required, which means less boostback required.  On the other hand, it isn't worth it to loft the stage yet more, as that would require propellant too.
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Offline cambrianera

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Re: Pushing the limits of the hoverslam landing
« Reply #30 on: 01/08/2016 09:04 PM »
Nobody realized that saving 2000 kg of propellant in the landing burn decreases the mass during reentry burn (and boostback burn, if done).
Therefore saving could be easily double that, because propellant for reentry burn (and boostback burn) can be less.

Not really, because the dominant factor is the 26,000 kg dry mass on the first stage, so carrying 2,000 kg less propellant through boostback and re-entry reduces vehicle mass by less than 10%, and thus gives you less than 10% additional propellant savings for a given deltaV.
Propellant consumed in boostback and reentry is more than 25,000 kg, so saving less than 10% doubles the saving in braking burn.
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Offline PreferToLurk

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Re: Pushing the limits of the hoverslam landing
« Reply #31 on: 01/08/2016 09:22 PM »
...or enable a less lofted trajectory.
More loft is more air time, which means less velocity back to landing site required, which means less boostback required.

Right, but the more lofted the trajectory, the less the first stage contributes to the required horizontal orbital velocity. And we just saved 1000-2000kg of fuel for the extra boostback.

In the OG2 mission, there was over 250 seconds of free flight from the first stage between boostback and reentry burns. lets say less lofted trajectory cuts off 50 seconds of ballistic free flight, and adds 100m/s (horizontal) to the second stage. 150m/s of extra boostback burn completely cancels the extra horizontal velocity while adding 50m/s in the opposite direction. So the final question is will 10km of extra boostback distance be enough to compensate for the extra distance downrange and the loss of 50 seconds of free return?

We would need to know just how much dV the original boostback burn provided (likely proprietary information) to really know.   They don't announce boostback completion on the webcast, but from the cheers it seems like it is about 15-20 seconds long. At a minimum the boostback burn is 50% cancelling out downrange velocity (probably more like 60%-75%, but just for arguments sake...) so 8-10 seconds of building up rtls velocity at most. if you add 2-3 seconds to that burn you get at least 20% more rtls velocity with 20% reduction in "hang time". 
At the very worst it would appear to still get you back to your landing point while providing more dv to the payload than simply delaying MECO1 by one second. 


Anyway, like I said, TONS of variables at play here.  But I've got to think that SpaceX can find a good use for an extra 1000kg of fuel. Also, there are a lot of good reasons for SpaceX being conservative in this landing.  Sticky valve from the last attempt not being the least of them.  As they gain confidence I would expect to see incrementally more aggressive landings. Getting the center core of a FH even back to a barge is going to be a lot more difficult, and they may not be able to afford such a conservative landing.

Offline whitelancer64

Re: Pushing the limits of the hoverslam landing
« Reply #32 on: 01/08/2016 09:59 PM »
Time for a smaller analogy.

Let's say you have a car that gets 40 mpg. This car has a 10 gallon tank, so for each tank you can go 400 miles. Put another way, to go one mile your car burns about 3.2 ounces of gas.

Let's say you think you might save gas by coasting in downhill sections of roadway rather than keeping your foot on the gas. Let's say your calculations show that by doing this you will save 0.5% of your car's tank of fuel, which is 6.4 ounces of gas. This enables you to drive approximately 2 additional miles.

Given the car normally goes 400 miles on a tank anyway, is this extra 2 miles going to be worth it?
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Offline CJ

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Re: Pushing the limits of the hoverslam landing
« Reply #33 on: 01/08/2016 10:21 PM »
Time for a smaller analogy.

Let's say you have a car that gets 40 mpg. This car has a 10 gallon tank, so for each tank you can go 400 miles. Put another way, to go one mile your car burns about 3.2 ounces of gas.

Let's say you think you might save gas by coasting in downhill sections of roadway rather than keeping your foot on the gas. Let's say your calculations show that by doing this you will save 0.5% of your car's tank of fuel, which is 6.4 ounces of gas. This enables you to drive approximately 2 additional miles.

Given the car normally goes 400 miles on a tank anyway, is this extra 2 miles going to be worth it?

It is absolutely worth it *if* the next gas station is 402 miles away. :)

IMHO, a likely scenario in future is that available margin will dictate the landing profile. If it's really tight on margin due to payload requirements, they'll fly a very aggressive (fuel-minimal but higher risk) profile plus land on the ASDS. They'd do this because if they don't, the lose the stage for sure. If there's a bit more margin, they'll use it. If there's plenty of margin, they'll fly a very conservative profile. In a nutshell, my guess is they'll adapt the profile to the specific mission as needed.   

Offline MrHollifield

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Re: Pushing the limits of the hoverslam landing
« Reply #34 on: 01/08/2016 10:35 PM »
As mentioned upthread, this looks like a perfect case for testing in NM.

Each case can be simulated then the most promising flight tested without risking a customer's payload and a new, unflown F9FT. The ones that work can be tried out on LEO launches then made a standard return method if they improve payload to orbit or margin for landing error. If the recovered stage in NM (F9-19 is my guess) lawn darts, then they'll know not to try that landing sequence on a future flight.

Offline Norm38

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Re: Pushing the limits of the hoverslam landing
« Reply #35 on: 01/09/2016 03:37 AM »
If we're talking about reusable vehicles, then I don't get the need. FedEx doesn't pack extra cargo onto a 737 by coasting it into the airport on fumes. They use a 757.

If the F9 needs another 200kg of payload mass, then SpaceX will build a bigger rocket. Which it's doing.
Landings should be boring, safe, routine, with plenty of margin.

Offline guckyfan

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Re: Pushing the limits of the hoverslam landing
« Reply #36 on: 01/09/2016 08:00 AM »
Landings should be boring, safe, routine, with plenty of margin.

I agree with boring, safe, routine. We need to get there. But I disagree with plenty of margin. Nothing in space is done with plenty of margin, except for the first tries. They aim for just enough margin. Of course many flights will have plenty, it's about the largest payloads possible without going to the next more capable and expensive means. They will home in on that and risk some failures on the way.

Offline Llian Rhydderch

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Re: Pushing the limits of the hoverslam landing
« Reply #37 on: 01/09/2016 02:32 PM »
I wonder if we might get this thread back on topic.  The OP said:

"Looks like SpaceX could save fuel/weight by landing more aggressively.  Could they? Will they? "

This is the sort of rocket equation/propellant mass flow/trajectory stuff that is typically catnip for NSFers.  Let's talk about that.

Re arguments from authority on NSF:  "no one is exempt from error, and errors of authority are usually the worst kind.  Taking your word for things without question is no different than a bracket design not being tested because the designer was an old hand."
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Offline PreferToLurk

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Re: Pushing the limits of the hoverslam landing
« Reply #38 on: 01/10/2016 11:26 PM »
I'll Cross quote from the SES-9 thread.   Dante80 put together a very interesting analysis of upcoming launches, masses, and RTLS/Barging implications. As GuckyFan just stated, this is likely to be of most importance when looking at squeezing an expendable flight into a barge flight, or a barge flight into a RTLS flight. The absolute performance gain is not as important as moving the break-points in your favor. 

To re-iterate a little more about the projected capability for FT.

1. F9 v1.1 has "demonstrated" an expendable capability of ~ 4,850kg to GTO-1800. Two sets of data compound to that assertion.

a. The biggest GTO payload was the 4,707kg TurkmenAlem52E. It was placed in a roughly GTO-1765 orbit (180x36600x25.5)
b. Thaicom 6 (a 3,016kg sat) got to a GTO-1500 equivalent orbit (295x90000x22.5). This stretched S2 fuel reserves to almost complete depletion (according to USAF, which evaluated this flight as part of the SpaceX EELV certification procedure).

2. F9 FT as a whole is reported to be around 30% more capable than F1 v1.1

3. DPL (barging) costs about 15% payload.

This means that F9 FT has a theoretical capability of about 6,300kg to GTO-1800
Barging moves it to 5,355kg.
SES9 is 5330 kg.

Its looking very close. Depending on whether the rocket goes to DPL or not, as well as what the end orbit is, we are going to get a lot of info about the current Falcon variant capabilities. 

If this pans out, and RTLS removes another 15% of performance, F9 FT would be able to RTLS after sending a 4410kg payload to GTO-1800. This number is interesting for some of the following missions (quoting from here):

Thaicom 8                      3100kg   GTO    Yes
ABS 2A, Eutelsat 117 West B   ~4000kg?  GTO    Possibly (based on ABS-3A, Eutelsat 115 West B mass)
JCSAT-14                      ~3400kg?  GTO    Probably (based on JCSAT-15 mass)
BulgariaSat-1                 ~3400kg?  GTO    Probably (based on JCSAT-15 mass, same SSL-1300 bus)
JCSAT-16                      ~3400kg?  GTO    Probably (based on JCSAT-15 mass)
KoreaSat-5                     4465kg   GTO    Possibly
Es'hail-2                     ~3000kg   GTO    Probably

If F9 FT performance upgrade over v1.1 I listed above is correct (some say its more, like 33%), then almost all the missions above would be eligible for RTLS, OR a better orbit than GTO-1800 + DPL.

For Barging especially, a few extra seconds of breaking burn paired with an aggressive landing barging might be the difference between throwing away a stage and getting it back.  Or also likely, the difference between SpaceX bidding on a launch worth millions and not bidding at all. Ariane 5  is still winning a lot of launches, at least in part because there are plenty of large comsats that SpaceX can't/wont bid on.  Being able to win even one or two of those launches seems like a very large reason for SpaceX to at least test out aggressive landings.

Although unless someone can figure out the most efficient use of 1-2k kg's of fuel (i still think boostback/breaking burn paired with a lower loft trajectory, but i can't back that up) I think this discussion may have already gone about as far as it can. 

Offline Joffan

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Re: Pushing the limits of the hoverslam landing
« Reply #39 on: 03/06/2016 06:07 PM »
I think the legs could burn with rocket exhaust if exposed too soon.

If the deploy mechanism would be changed to allow this they could do a partial deploy first. Make it look like an arrowhead. It would provide drag and keep the legs away from the flames. It should cause less stability issues too.

They could do this really early, at the start of the re-entry burn. One difficulty is that the force of the burn is acting to deploy the legs, but you want them to stop at this intermediate position. So as well as the partial-deploy latch (which holds the legs open), there would need to be a deploy-limit mechanism which would then need to be disabled for the full leg deploy during the landing burn.

Talking of the burn acting to deploy the legs - I wonder how well the legs fared in the SES-9 deploy at high speed/high thrust?
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