Author Topic: SpaceX Falcon 9 v1.1 - Dragon - CRS-5/SpX-5 -Jan. 10, 2015 - DISCUSSION (Read 618082 times)

SoulWager · « **Reply #800 on:** 01/10/2015 05:14 pm »

Quote from: Herb Schaltegger on 01/10/2015 04:50 pm

Quote from: cscott on 01/10/2015 04:45 pm

Yeah. I doubt it uses RP-1 as the working fluid, since the grid fins are on the 'wrong side' of the core for that.

Probably dedicated high-pressure fluid; maybe shares the LOX pressurant.

Isn't the LOX tank self-pressurizing?

Not to my knowledge, I think that's a planned feature of the BFR or MCT(using some of the gaseous turbopump exhaust to pressurize the tanks).

cscott · « **Reply #801 on:** 01/10/2015 05:14 pm »

Quote from: rcoppola on 01/10/2015 05:06 pm

So if they "Already" have 50% more fluid planned, that means they are and/or continue to be ridiculously fast at iterative design or more likely in this case, already knew through modeling they'd need more margin. Or both.

Or, as I posted earlier, they were planning more fluid anyway in order to allow the grid fins to dynamically stabilize an earlier leg deployment in the next test flight.

Kabloona · « **Reply #802 on:** 01/10/2015 05:15 pm »

Not the first rocket to run out of hydraulic fluid...the Conestoga failure illustrates why F9 uses RP-1 instead of hydraulic fluid for TVC actuators:

http://en.m.wikipedia.org/wiki/Conestoga_(rocket)

Quote

. The launch took place from a clamshell gantry, which included power and environmental control, at the south end of Wallops Flight Facility on 23 October 1995; the rocket launched normally, but broke up 46 seconds into the flight. EER determined that an unknown source of low frequency noise had caused the guidance system to order course corrections when none were needed, causing the steering mechanism to eventually run out of hydraulic fluid.

That's the problen with open-loop hydraulics; if you underestimate the duty cycle you can run out of fluid.

sghill · « **Reply #803 on:** 01/10/2015 05:16 pm »

Because the grid fin control was lost just before landing, it could have landed slightly off of true vertical like we saw in all the ocean landing videos. It wouldn't take much to snap a leg or buckle the stage if the rocket touched down off axis at all.

cartman · « **Reply #804 on:** 01/10/2015 05:18 pm »

That is 5.5 Mach according to http://exploration.grc.nasa.gov/education/rocket/mach.html
CRS-4 staged at Mach 6. Maybe the difference comes from the time they did the readout of these values

cro-magnon gramps · « **Reply #805 on:** 01/10/2015 05:19 pm »

Quote from: Elmar Moelzer on 01/10/2015 04:48 pm

Elon Musk @elonmusk
Grid fins worked extremely well from hypersonic velocity to subsonic, but ran out of hydraulic fluid right before landing.

Elon Musk @elonmusk
Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.

Elon Musk @elonmusk
Am super proud of my crew for making huge strides towards reusability on this mission. You guys rock!

Chris I know your busy, but when you get a chance, can you confirm with the PAO how much real damage to the ASDS there is and how soon it will be repaired, if any? not sure if this is L2, but suspect it isn't something that SpaceX wants widely announced, so if you want to move it there. ok..

edit to make case for L2

Herb Schaltegger · « **Reply #806 on:** 01/10/2015 05:19 pm »

Quote from: sghill on 01/10/2015 05:16 pm

Because the grid fin control was lost just before landing, it could have landed slightly off of true vertical like we saw in all the ocean landing videos. It wouldn't take much to snap a leg or buckle the stage if the rocket toucheddar down off axis at all.

Especially so if it was dealing with 30 kph winds, as indicated earlier this morning.

cscott · « **Reply #807 on:** 01/10/2015 05:19 pm »

Quote from: lark on 01/10/2015 04:59 pm

At 7:42 in that video, you can see a deployed grid fin on the monitor at upper left. At 11:10, I believe you can see what looks like the landing burn video reflected off the control room's back window, also at upper-left.

They called out loss of first stage telemetry quite late -- at T+8:26. This is roughly 30 seconds before first stage touchdown. So they probably had full telemetry from the first stage throughout three of the four engine burns. (And grid fins are deployed quite early, either between burns 1 and 2 or between 2 and 3.)

(The rest of the telemetry data was captured by the ASDS -- it began receiving telemetry at T+1:45 or so. It will take some time for that to be downlinked to SpaceX mission control; depending on their satellite connection Hans indicated "a day or two".)

So I'm saying that it checks out.

matthewkantar · « **Reply #808 on:** 01/10/2015 05:19 pm »

Quote from: lark on 01/10/2015 04:59 pm

At 7:42 in that video, you can see a deployed grid fin on the monitor at upper left. At 11:10, I believe you can see what looks like the landing burn video reflected off the control room's back window, also at upper-left.

Nice catch.

Mattthew

Tonioroffo · « **Reply #809 on:** 01/10/2015 05:26 pm »

Quote from: Elmar Moelzer on 01/10/2015 04:48 pm

Elon Musk @elonmusk
Grid fins worked extremely well from hypersonic velocity to subsonic, but ran out of hydraulic fluid right before landing.

Elon Musk @elonmusk
Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.

Elon Musk @elonmusk
Am super proud of my crew for making huge strides towards reusability on this mission. You guys rock!

So... trivial! It's always the little things. Also.. "landing attempt next month" - with DSCOVR NET jan 29th, does that mean no attempt on this flight, or do I take Mr. Musk too literal?

JBF · « **Reply #810 on:** 01/10/2015 05:27 pm »

Quote from: nadreck on 01/10/2015 05:03 pm

Quote from: JBF on 01/10/2015 04:55 pm
Quote from: rcoppola on 01/10/2015 04:53 pm
So is he saying that losing grid fin authority was "the" cause of this off-nominal landing? Or a contributing factor?
I mean, if they got that close and this quickly determined the only thing keeping them from success is more Hydraulic fluid, which they already have planned, then that is quite remarkable.

This is probably just the first issue identified.

I agree with your point JBF, not in a pessimistic way, but the fact is that whether or not the landing would have been a success with more hydraulics, this is not the only bit of data that they will uncover. Part of their work in determining cause of failure will result in some amount of 'overdetermination' of the failure and probably cleaning up other issues that could be marginal contributors to failure.

Had the stage survived the landing perfectly there would still have been a fault analysis of the landing to find any elements that exceeded planned tolerances. Because there is less data (some parts will have been too damaged for accurate analysis) many possible causes will be examined as well as certain causes.

I didn't mean it in a negative way; I've been part of a lot of fault analysis where the first issue identified turned out to be a minor one that was masking the real problem. My main point was is that it is still early in the discovery process.

schaban · « **Reply #811 on:** 01/10/2015 05:28 pm »

Or loss of control means it landed on some of the equipment boxes on the sides and fall of the barge

Could explain damaged equipments

savuporo · « **Reply #812 on:** 01/10/2015 05:38 pm »

If he is talking about hydraulic actuation loss pre landing, that would be TVC actuation, not grid fins as grid fins would not be providing a lot of control authority pre landing. It's the TVC+throttle that does the final approach and landing, not the fins.

Kabloona · « **Reply #813 on:** 01/10/2015 05:44 pm »

Quote from: Tonioroffo on 01/10/2015 05:26 pm

Quote from: Elmar Moelzer on 01/10/2015 04:48 pm
Elon Musk @elonmusk
Grid fins worked extremely well from hypersonic velocity to subsonic, but ran out of hydraulic fluid right before landing.

Elon Musk @elonmusk
Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.

Elon Musk @elonmusk
Am super proud of my crew for making huge strides towards reusability on this mission. You guys rock!

So... trivial! It's always the little things. Also.. "landing attempt next month" - with DSCOVR NET jan 29th, does that mean no attempt on this flight, or do I take Mr. Musk too literal?

He probably knows that NET Jan 29th really means February.

.

F9 reportedly has plenty of performance margin for DSCOVR, so no reason not to carry legs and try landing.

Herb Schaltegger · « **Reply #814 on:** 01/10/2015 05:50 pm »

Quote from: savuporo on 01/10/2015 05:38 pm

If he is talking about hydraulic actuation loss pre landing, that would be TVC actuation, not grid fins as grid fins would not be providing a lot of control authority pre landing. It's the TVC+throttle that does the final approach and landing, not the fins.

Read his tweets - he specifically refers to grid fins.

Further, TVC is fuel-draulic. So long as the engine has fuel and is firing, there is TVC authority.

https://twitter.com/elonmusk/status/553963793056030721

https://twitter.com/elonmusk/status/553964281025548289

EDIT: Added links to tweets.

savuporo · « **Reply #815 on:** 01/10/2015 05:53 pm »

oh ok. then that must have happened at pretty high speed I guess - i.e. before engine was at power to have the full control authority.

Kabloona · « **Reply #816 on:** 01/10/2015 05:57 pm »

Quote from: savuporo on 01/10/2015 05:53 pm

oh ok. then that must have happened at pretty high speed I guess.

He said the loss of grid fin control authority happened "right before landing." And remember, when the grid fins lock up, they're suddenly providing unbalanced pitch/yaw forces, because it's probable they were at non-zero angles when they locked up. So now the grid fins are destabilizing, instead of stabilizing, and that can be a problem even at low speed, even if the engine is firing.

antiquark · « **Reply #817 on:** 01/10/2015 05:58 pm »

Does the Falcon 9 use fuel as hydraulic fluid? (Like the Saturn V did). I.e., running out of fluid actually means running out of gas?

mme · « **Reply #818 on:** 01/10/2015 06:06 pm »

Quote from: savuporo on 01/10/2015 05:53 pm

oh ok. then that must have happened at pretty high speed I guess - i.e. before engine was at power to have the full control authority.

Why do you assume it was at high speed? My understanding is the the stage would be down to terminal velocity long before the landing burn.

speedevil · « **Reply #819 on:** 01/10/2015 06:07 pm »

Quote from: savuporo on 01/10/2015 05:38 pm

If he is talking about hydraulic actuation loss pre landing, that would be TVC actuation, not grid fins as grid fins would not be providing a lot of control authority pre landing. It's the TVC+throttle that does the final approach and landing, not the fins.

Final approach and landing - yes.
The grid fins have no effect for the last of the trajectory.
However - it's reasonable to say that they're going to have some effect even at 1/2 freefall velocity, which is some 15s before landing.

Random numbers.
From the image of the grid-fins when they were announced, they look around 1.8m*1.8m.
The area they have is actually comparable to the cross-section of the whole stage.
They may be quite useful in counteracting wind loads till the stage hits 3* or so of the speed of the wind.

Whatever their precise control authority at various speeds - if the algorithm relies on them being there, it will be at best terribly tuned for landing in a given spot without them.
Even worse if they are stuck in an unhelpful attitude.

With TVC alone, it's hard to control both position of the base of the stage, and its orientation independantly in the face of variable crosswinds.

With the fins it gets lots easier. They can modulate the cross-section of the vehicle - even at ~0 vertical speed.