SpaceX Falcon 9/Dragon CRS-2 SpX-2 MISSION GENERAL DISCUSSION

[Robotbeat]

I have no understanding of orbital mechanics so can someone please explain to me why Dragon uses near instant launch windows but can free drift for hours while fixing issues then can simply recompute the course to station?

Play some Kerbal Space Program and you will "get it" after a couple of launches

Or even better the Orbiter simulator: http://orbit.medphys.ucl.ac.uk/

According to a Reddit AMA (ask me anything) with SpaceX software guys, SpaceX uses Kerbal:

Redittor: "First question: do any of you guys play Kerbal Space Program?"
SpaceX: "Are you kidding? That’s how we design our rockets!"
http://www.reddit.com/r/IAmA/comments/1853ap/we_are_spacex_software_engineers_we_launch/

[Antares]

Frozen/really thick N2O4.  H2O contamination in the He.  Those are my two hypotheses.  I don't like the latter because there's no heat sink that could freeze water.

One question: 4 cardinal directions, 4 thruster banks.  1 thruster bank working initially.  Sun rises in the east.  Was the working thruster bank nearest to the east while sitting on the pad?  Or maybe it was on the windward side while flying angles of attack during pitchover?

[blazotron]

If 3 of 4 He valves got blocked or stuck - this does not look like manufacturing flaw

Agreed, which is what I first thought. So to be clear, 3 out of four independent systems failed?  It wasn't a single point, one valve, that took out three thrusters?

How about if the system looks like

Gas  -> ----- -> ----- -> ----- -> ------
             T1       T2        T3       T4

where each "->" is a check valve?  Then if the second one sticks, T1 works and T2-4 have low pressure.

That's not how it would be plumbed. It would be like a fork with 4 tines, and a check valve at the end of each tine. Reason being, you don't want a single stuck check valve to disable all 4 branches.

Then how did they pressure hammer it?

Agree there is no point to plumb sequentially. The point of check valves on these systems would be to provide isolation between downstream systems. Plumbing sequentially only provides isolation from downstream tanks to upstream ones, rather than from each to every other.

If the obstructions were leaking through slowly (which you might expect based on Elon's "trending positive" tweet), simply shutting an upstream isolation valve, allowing the volume between the valve and the obstruction to bleed down, then opening it again would provide the ability to slam the obstruction without needing additional paths to vent overboard.

[Kabloona]

Frozen/really thick N2O4.  H2O contamination in the He.  Those are my two hypotheses.  I don't like the latter because there's no heat sink that could freeze water.

One question: 4 cardinal directions, 4 thruster banks.  1 thruster bank working initially.  Sun rises in the east.  Was the working thruster bank nearest to the east while sitting on the pad?  Or maybe it was on the windward side while flying angles of attack during pitchover?

Elon reported late in the presser that the NTO tanks were at 70F...granted that was a few hours after the initial failure to pressurize...but still, even if the NTO froze at some point earlier, how would that prevent the He from pressurizing the tanks? I assume these are diaphragm tanks, so no way for NTO to migrate into the He plumbing, and the implication from Elon was clearly that they believed something was preventing the He from reaching three of the four tanks.

Also, since they apparently do have temp transducers on the tanks, and Elon reported only good temp data (his reference to 70F) and never said anything about off-nominal temp data, frozen NTO sounds unlikely....

[Lars_J]

On a side note - While the frequency of launches is not up to what we hope (yet), I have to congratulate the F9 launch team on being able to launch directly on the first launch window for these last two flights. No delay, right on time.

It certainly shows that the operations team is maturing - IMO.

[john smith 19]

I listened to the recording of the teleconference after the flight and Musk mentioned that crewed Dragon would run on batteries only, no solar arrays.

I guess this is because batteries are adequate (you'll be on board ISS or de orbiting fairly quickly) and the arrays have a lot of bought in space rated parts, which are expensive.

Was this fact already known?

[MP99]

Frozen/really thick N2O4.  H2O contamination in the He.  Those are my two hypotheses.  I don't like the latter because there's no heat sink that could freeze water.

I'm sure one of the computers was quoted as getting close to a "blue line" (cold) according to the feed from LCC before launch.

ISTM the only thing between the LOX tank in the upper stage and the heatshield under Dragon is the air gap in the trunk.

Presuming that's what was chilling the computer, how cold does the air in the trunk get? Would low temperatures at the launch pad exacerbate that?

I'm wondering whether this area already gets a dry nitrogen purge to avoid ice forming in the trunk, and whether they could (or already do) keep a steady flow of warm N2 to stop this from getting too cold.

cheers, Martin

[R7]

I assume these are diaphragm tanks, so no way for NTO to migrate into the He plumbing

..unless the diaphragm has developed a leak.

[paycom]

but perhaps there's a reason to plumb it that way - for example, if they need to pressurize each tank one after another, instead of all at the same time? if that was a requirement, it's easier to see why that configuration could have been picked..

One should also be aware of the fact that there's only very little space in Dragon's service section, compared to traditional service modules. This might influence the layout of the plumbing.

[R7]

but perhaps there's a reason to plumb it that way - for example, if they need to pressurize each tank one after another, instead of all at the same time? if that was a requirement, it's easier to see why that configuration could have been picked..

One should also be aware of the fact that there's only very little space in Dragon's service section, compared to traditional service modules. This might influence the layout of the plumbing.

It's no excuse to do this:

He--->--T1-->--T2-->--T3-->--T4

instead of this:


He__________________
     |    |    |    |
     V    V    V    V
     |    |    |    |
     T1   T2   T3   T4

To be redundant against stuck open/stuck close each check valve should be quadrupled like this:

   _|_
  |   |
  V   V
  |   |
  V   V
  |___|
    |

(ascii-art is back!)

[paycom]

but perhaps there's a reason to plumb it that way - for example, if they need to pressurize each tank one after another, instead of all at the same time? if that was a requirement, it's easier to see why that configuration could have been picked..

One should also be aware of the fact that there's only very little space in Dragon's service section, compared to traditional service modules. This might influence the layout of the plumbing.

It's no excuse to do this:

He--->--T1-->--T2-->--T3-->--T4

instead of this:


He__________________
     |    |    |    |
     V    V    V    V
     |    |    |    |
     T1   T2   T3   T4

I can still imagine that the first option is the most compact one.

[R7]

I can still imagine that the first option is the most compact one.

No it's not. Both options require T-joints from the main He-line to the tanks. Only difference is having check valve in the main line or in the line from T-joint to the tank.

[paycom]

I can still imagine that the first option is the most compact one.

No it's not. Both options require T-joints from the main He-line to the tanks. Only difference is having check valve in the main line or in the line from T-joint to the tank.

That's the point.

[R7]

Both options require T-joints from the main He-line to the tanks. Only difference is having check valve in the main line or in the line from T-joint to the tank.

That's the point.

Yes? Does not mean you require any other length of line from the T-joint to the tank except the check valve. If you really want to compact things machine the check valve and T-joint into same body. edit: and if you want to compact things even further with redundancy, machine T-joint and the check valve quad redundancy into same body.

[yg1968]

I listened to the recording of the teleconference after the flight and Musk mentioned that crewed Dragon would run on batteries only, no solar arrays.

I guess this is because batteries are adequate (you'll be on board ISS or de orbiting fairly quickly) and the arrays have a lot of bought in space rated parts, which are expensive.

Was this fact already known?

No, it wasn't known before. See this post on the cost of the solar arrays:
http://forum.nasaspaceflight.com/index.php?topic=30385.msg1020178#msg1020178

[guckyfan]

No, it wasn't known before. See this post on the cost of the solar arrays:
http://forum.nasaspaceflight.com/index.php?topic=30385.msg1020178#msg1020178

I have not seen a reference to that statement. It's hard to believe. We do know that quite standard non-space-rated cells are used so the cell cost is almost negligible. Of course mounting them in that foldable format costs money, but that much? Skipping them would eliminate a possible point of failure but cost?

Edited typo.

[cambrianera]

From:
http://forum.nasaspaceflight.com/index.php?topic=30385.msg1020178#msg1020178

Author: krytek
IIRC it was mentioned the panels cost upwards of several millions $.
Good idea to reuse that.

Sorry but without a reference I can't buy this: SpaceX made his own solar panels in house, obviously freely picking in the huge knowledge base of Solar City. I personally doubt the panels, quite surely made from commercial silicon cells, are that expensive.

So said, I'm sure that a "only battery" Dragon could be considered (not necessarily done!): cheaper, lighter, simpler.

In the pic, widely circulating image of in house testing of the panels.

[smoliarm]

2. CO2 -- since most of He production goes from natural gas, some processing schemes give He with substantial CO2 content. However, it is difficult to imagine that CO2 would form a solid ice blocking valve.

Well the He is released from very high pressure reservoir, it expands, does work, cools, no? But does it cool enough to freeze CO2, dunno. Moisture more likely would freeze.

Agree on both comments.
Also, about moisture -- it is not only freezes more easily, it is also pretty easy to get as contaminant. It can get in He at production, transportation, and at propellant/pressurizer loading just as well.

[yg1968]

Here is a video of the launch from SpaceX' channel:

[Kabloona]

I assume these are diaphragm tanks, so no way for NTO to migrate into the He plumbing

..unless the diaphragm has developed a leak.

In that scenario, three tanks would have had to leak NTO past their diaphragms and into their respective He presuurization lines....highly unlikely. Diaphragm tanks just don't leak unless they've been badly abused, and for three diaphragm tanks to leak on the same vehicle is unheard of, AFAIK. It would have taken a major processing mistake for SpaceX to damage three tank diaphragms, and I just don't see that happening.

Frozen condensate or contaminant in the He lines, or stuck check valve(s) much more plausible, IMO.