Author Topic: SpaceX Falcon 9 v1.1 - SpX-6/CRS-6 DRAGON - Discussion Thread (Read 483362 times)

avollhar · « **Reply #460 on:** 04/15/2015 05:13 am »

Quote from: deltaV on 04/15/2015 04:31 am

Looking at the video (https://vine.co/v/euEpIVegiIx) I'm struck by the dramatic attitude changes. I wouldn't expect a problem with the throttle of the main engine to directly change the rocket's attitude and I don't see why the control system would respond to incorrect thrust by purposefully changing attitude. I'm therefore guessing that the malfunctioning valve is related to attitude control, i.e. either thrusters or the system that gimbals the main engine. But the thrusters are nitrogen gas (no biprop) and the main engine gimbaling is hydraulic (also not biprop) so what's the "biprop throttle valve"?

AFAIK, the engine TVC is fuel-draulic, it runs from (turbopump) pressurized fuel:

http://jasc-controls.com/jasc-industry-listing/space/space-actuators/thrust-vector-control-actuator-part-101424-5/

Falcon8 · « **Reply #461 on:** 04/15/2015 05:14 am »

Elon tweeted "...Even w 1 lit, it can't hover, so always land at high g".

I assume the "it can't hover" means there is not enough fuel left to hover. Or could it mean that a single engine can't throttle low enough to hover with the lightened first stage?

Geron · « **Reply #462 on:** 04/15/2015 05:18 am »

Testing on actual flights is the best way to go. There was considerable cross wind, aside from sticky valve weather variation is an issue that will need to be addressed.

This landing attempt was close enough to good that if they modify the surface with some kind of numerous rapidly deployed grappling mechanisms to secure legs and pull rocket into deck securely they could have less precise landings become successes.

I envision a landing pad at sea or on land with numerous such mechanisms to secure the base of the rocket as its landing. Otherwise the rocket is too tall, and the legs with to small a diameter to assure landing in a majority of landing weather conditions, even with the valve issue solved.

I hope SpaceX starts working at securing the rocket more rapidly immediately to ensure both an initial successes and a higher rate of subsequent successes.

Kabloona · « **Reply #463 on:** 04/15/2015 05:22 am »

Quote from: Falcon8 on 04/15/2015 05:14 am

Elon tweeted "...Even w 1 lit, it can't hover, so always land at high g".

I assume the "it can't hover" means there is not enough fuel left to hover. Or could it mean that a single engine can't throttle low enough to hover with the lightened first stage?

Can't throttle low enough.

meekGee · « **Reply #464 on:** 04/15/2015 05:24 am »

Quote from: Falcon8 on 04/15/2015 05:14 am

Elon tweeted "...Even w 1 lit, it can't hover, so always land at high g".

I assume the "it can't hover" means there is not enough fuel left to hover. Or could it mean that a single engine can't throttle low enough to hover with the lightened first stage?

The second one. A single Merlin, throttled down, produces more thrust then the weight of the empty rocket, by a pretty good margin.

This is not a bad thing. Only human pilots want to hover before landing. It is better to get through the bottom layer of wind as fast as possible, and automated systems are fine landing with T/W>1.

Kabloona · « **Reply #465 on:** 04/15/2015 05:32 am »

Quote from: Geron on 04/15/2015 05:18 am

I envision a landing pad at sea or on land with numerous such mechanisms to secure the base of the rocket as its landing. Otherwise the rocket is too tall, and the legs with to small a diameter to assure landing in a majority of landing weather conditions, even with the valve issue solved.

You may be underestimating its stability. It's quite bottom-heavy with the engines and octaweb structure. IIRC, Elon was asked about its stability and gave an answer to that effect.

And you can bet SpaceX has done landing simulations with varying wind loads and pitch/yaw angles to calculate how much wind is allowable (20 knots) and how much pitch/yaw the stage can have at touchdown and remain vertical. So the allowable pitch/yaw envelope is probably known and may be larger than you might guess.

sdsds · « **Reply #466 on:** 04/15/2015 05:38 am »

I was having trouble with the notion that there even is a "biprop throttle valve" anywhere on the F9 first stage. I had been thinking there were separate LOX and fuel valves feeding the pre-burner, right? But then google tells me:
http://jasc-controls.com/jasc-industry-listing/space/space-fluid-management/bi-propellant-valve/

junk.munk82 · « **Reply #467 on:** 04/15/2015 06:29 am »

cleaned that vine up a bit:

Falcon8 · « **Reply #468 on:** 04/15/2015 06:36 am »

Quote from: meekGee on 04/15/2015 05:24 am

The second one. A single Merlin, throttled down, produces more thrust then the weight of the empty rocket, by a pretty good margin.

It would be nice if the Falcon9 had switchable power profiles for just the center engine. Maybe switch over to a different fuel plumbing circuit prior to landing that would produce less power and a better possibility of hovering.

kevinof · « **Reply #469 on:** 04/15/2015 06:39 am »

Good work. Oh man, that was so so close. Don't you just hate "stiction" :-)

Anyone know if the same valve is used during the ascent phase? I know it was mentioned during the presser that the launch looked a little slow to get off the pad and I thought the same when I was watching it live.

Quote from: junk.munk82 on 04/15/2015 06:29 am

cleaned that vine up a bit:

QuantumG · « **Reply #470 on:** 04/15/2015 06:41 am »

Quote from: kevinof on 04/15/2015 06:39 am

Don't you just hate "stiction" :-)

No.. but in this case I think there was a fraction too much.

john smith 19 · « **Reply #471 on:** 04/15/2015 07:08 am »

Quote from: sdsds on 04/15/2015 05:38 am

I was having trouble with the notion that there even is a "biprop throttle valve" anywhere on the F9 first stage. I had been thinking there were separate LOX and fuel valves feeding the pre-burner, right? But then google tells me:

What pre-burner?
Merlin is a GG cycle engine

john smith 19 · « **Reply #472 on:** 04/15/2015 07:17 am »

Quote from: Space Ghost 1962 on 04/15/2015 12:25 am

More complicated than that. You'd need to compare telemetry/tracking with sims.

Depends on where/when the deviation occurred. Remember that the stage bends - can act like a whip if you apply too much force at the wrong time. So even considerable increase isn't an instant answer.

Once you find where the issue is (by matching a sim to actual), you rerun the sims with increased thrust til it begins to work, then you exhaustively simulate variations to prove you've adequately bounded the situation. Tedious.

And that's the interesting question about this.

SX, like all major players, design their LV's and spacecraft in CAD systems. In principle a design should be available for input to dynamics simulation models almost from the day it's frozen. Modern computer time is cheap so they would have been able to lots of sims of this process and their models should have been high fidelity.

So what was missed this time?

I've just looked at the cleaned up footage and wow does that puppy waggle.

Either I'm recalling only stills from previous landing movies or this was really flapping about, like a failed launch from the 1950's

I beleive SX will get there but it's been interesting to see how something that is conceptually simple, and which should be pretty well simulated has taken 4 landings and still not resulted in a complete stage ready for reuse or analysis.

Baysian statistics would have to penalize your chances of #5 being successful.

Jarnis · « **Reply #473 on:** 04/15/2015 07:27 am »

Each time has failed for a brand new reason (2nd try for a completely external one - bad weather, no barge).

...and it is getting closer. Hopefully they'd run out of ways to fail soon.

macpacheco · « **Reply #474 on:** 04/15/2015 07:28 am »

Its not uncommon to see 30km/h wind shear from 1000ft to sea level. Given how fast the stage descents, the issue might be how quickly the stage can sense the wind. We routinely see 50km/h wind shear in my home town (southerly 10 kts @ 1000ft, northeast 20kts @ 50ft).
A means to upload wind estimates for sea level, 500ft, 1000ft, 1500ft to the stage right before launch might be very helpful, if the rocket could incorporate wind estimates into its lateral profile.
The rocket seemed to be compensating very aggressively towards dead center, perhaps its taking a wee bit too long to fully sense the wind.
Windy conditions are common both at sea and at the coast. This would be an enhancement that might pay off in land and sea scenarios (although sea typically have stronger winds).
Learn from aviation, every decent airport transmit fresh wind data to landing craft (ATIS, tower frequency, wind socks visually read).
But it could be that this is just a natural fine tuning as a consequence of prioritizing landing hard as close as possible to bulleye vs landing softly at the edge of the barge, and now that they accomplished it, they can balance the two goals.
Coming down a little slower would help if possible, but with T/W > 1 they might already be at the edge of how early they can slowdown.
Wind shear/gusts probably a much bigger problem than total wind strength @ sea level.

guckyfan · « **Reply #475 on:** 04/15/2015 07:29 am »

Assuming they expected some problems. Elon Musk indicated it by saying before the launch that the success probability is still low.

Then probably this is the reason why they cancelled the New Mexico tests with Falcon 9R-dev. Every test failure costs a stage. Better do it with stages from orbital launches. And they are getting closer.

As a sidenote. The video shows how much the engines can gimbal. Very impressive to see the exhaust flame wiggle. Even though it indicates a control issue.

john smith 19 · « **Reply #476 on:** 04/15/2015 07:30 am »

Quote from: meekGee on 04/15/2015 01:37 am

It is unarguable that active guidance (the rocket doesn't need to communicate with the barge for that) is more precise, since by using absolute coordinates you add the errors in barge and rocket station navigation.

I don't know if SX have an active guidance system or not but I can provide a data point.

Radar altimeters can provide accurate height above ground and velocity. In the 1950's 4 antennas (front, back, left, right) at slight angles to the airframe to give deduced "ded" reckoning were simpler and more reliable than inertial systems (back when INS meant spinning metal lumps in very precise bearings).

Modern units use mm microwaves and are available for drones. They have ranges in the 500-1000m and weigh a few Kg.

Such a system requires no comm link to the barge and should work properly against any surface you'd choose to use as a landing site IE no boulders strewn across it.

sdsds · « **Reply #477 on:** 04/15/2015 07:33 am »

Quote from: john smith 19 on 04/15/2015 07:08 am

What pre-burner?
Merlin is a GG cycle engine

OK fine. Can I call it, "The place where the combustion that powers the turbine happens?"

rubtest · « **Reply #478 on:** 04/15/2015 08:31 am »

just a remark: the barge is probaby filmed/viewed from her short side and almost from sea level . that is why it looks so small. it is only the barge side cross section view. (look for the blue container on the barge for orientation ) . Great amazing success spacex !

Radical_Ignorant · « **Reply #479 on:** 04/15/2015 10:44 am »

Quote from: Jim on 04/15/2015 12:51 am

Why can people understand that what is done on the barge will be the exact thing used on land?

Because all thing is still in progress and until it's established everything can change? And you are just repeating this: "barge is no go" like if more times you say it more likely people will listen to you? And temorary (as oposed to long term goals) can mean something like over 10 years?