Author Topic: SpaceX DragonFly Discussion Thread  (Read 375274 times)

Offline AJA

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Re: SpaceX DragonFly Discussion Thread
« Reply #160 on: 05/23/2014 05:27 pm »
OK, I haven't been through all 11 pages on this thread, so someone forgive me if this has already been mentioned.

Where in the report does it say the engine firing will be continuous? They can fire for 10 x 0.5 seconds for a total of 5 seconds. Since the acceleration will not be sustained for a long period of time, they can get away with higher-G's too... possibly making the development of the SuperDracos easier..?

Offline Joffan

Re: SpaceX DragonFly Discussion Thread
« Reply #161 on: 05/23/2014 05:33 pm »
Well, they are referring to the FAA-report, which most of us have probably read. I think that the only mention of the trunk is:

Quote from: FAA
The DragonFly RLV is the Dragon capsule with an integrated trunk (which may or may not be attached during a DragonFly operation)...

My guess is that the Waco Tribune simply extrapolated "may be attached during a DragonFly operation" to meaning the whole operation including landing instead of launching with it and then detaching. The report does not differentiate between these scenarios...

This seems most likely to me. However the launched-then-detached trunk option makes me wonder if it would be straightforward (or even possible) to fit a small recovery chute in the trunk - for these Dragonfly tests only, of course, not for operational Dragon landings.
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Offline Llian Rhydderch

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Re: SpaceX DragonFly Discussion Thread
« Reply #162 on: 05/23/2014 05:54 pm »
OK, I haven't been through all 11 pages on this thread, so someone forgive me if this has already been mentioned.

Where in the report does it say the engine firing will be continuous? They can fire for 10 x 0.5 seconds for a total of 5 seconds. Since the acceleration will not be sustained for a long period of time, they can get away with higher-G's too... possibly making the development of the SuperDracos easier..?

Well the FAA report mentions engine throttling on several occasions.  My question to the rocket engine folks here is:  with respect to throttling on rocket engines, does throttling always refer to reduced thrust on a continuously-firing engine?  or can throttling also be accomplished with a less-than-100% duty cycle on engine firing?  (I'm asking about normal usage in the chemical rocket engine literature, as well as speculation about what might be expected here with SuperDracos.)

It seems to me that, for hypergolic RCS engines, the reaction force needed is often obtained from multiple-short firings of any specific thruster.  For something 100x as large in thrust, as the SuperDraco is over the Draco, might a sort of Pulse Width Modulation (PWM) be used to obtain a much more granular throttling, just like PWM is often used in varying the effective voltage in torsion motor drive electrical circuits like those used in some electric vehicles?
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Offline Elmar Moelzer

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Re: SpaceX DragonFly Discussion Thread
« Reply #163 on: 05/23/2014 05:54 pm »
Well, they are referring to the FAA-report, which most of us have probably read. I think that the only mention of the trunk is:

Quote from: FAA
The DragonFly RLV is the Dragon capsule with an integrated trunk (which may or may not be attached during a DragonFly operation)...

My guess is that the Waco Tribune simply extrapolated "may be attached during a DragonFly operation" to meaning the whole operation including landing instead of launching with it and then detaching. The report does not differentiate between these scenarios...

This seems most likely to me. However the launched-then-detached trunk option makes me wonder if it would be straightforward (or even possible) to fit a small recovery chute in the trunk - for these Dragonfly tests only, of course, not for operational Dragon landings.
Of course it could also mean that the trunk stays on the ground (in case of a simulated pad abort situation of sorts).

Offline clongton

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Re: SpaceX DragonFly Discussion Thread
« Reply #164 on: 05/23/2014 06:11 pm »
Has there been any analysis what would happen if Dragon(Fly) would lose a number of SDs?

It's hard to loose even 1 hypergolic engine, let alone "a number" of them. No moving parts in the power stream, just a couple of valves to open or close the 2 propellant streams, which are pressure fed. Open the valves, propellant flows and the engine self-ignites. Close the valves and the engine shuts off. Simple. That's a big reason why Apollo CSM/LM went with hypergols. Practically foolproof.
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Offline AJA

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Re: SpaceX DragonFly Discussion Thread
« Reply #165 on: 05/23/2014 06:23 pm »
It's hard to loose even 1 hypergolic engine, let alone "a number" of them.

It's already happened. On orbit. To Dragon. (albeit to Draco, and not SuperDraco) Requirements for redundant systems aren't defined by probability of failure alone, but by the product of severity of failure x probability of failure.

In this case, if there's no parachute, or they're below minimum safe altitude for a parachuted landing, the severity is loss of/serious injury to crew. There's no way they're simply going to go ahead with "Yeah, hypergolics don't fail".

Also, engine control can be lost due to software glitches too.
« Last Edit: 05/23/2014 06:24 pm by AJA »

Offline sghill

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Re: SpaceX DragonFly Discussion Thread
« Reply #166 on: 05/23/2014 06:26 pm »
Has there been any analysis what would happen if Dragon(Fly) would lose a number of SDs?

It's hard to loose even 1 hypergolic engine, let alone "a number" of them. No moving parts in the power stream, just a couple of valves to open or close the 2 propellant streams, which are pressure fed. Open the valves, propellant flows and the engine self-ignites. Close the valves and the engine shuts off. Simple. That's a big reason why Apollo CSM/LM went with hypergols. Practically foolproof.

Except for that whole Gemini 8 thingy....

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Offline savuporo

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Re: SpaceX DragonFly Discussion Thread
« Reply #167 on: 05/23/2014 06:45 pm »
Note: 500m is still below the "Brown Pants Line" (BPL) in this pilot's opinion.  I'd want to know the motors are running up around 1000m, even if they are turning on a few at a time to provide some emotional comfort and stability at low altitude.
Having pulled the reserve twice so far, both above the  established hard deck, i would say that BPL is a mischaracterization, as your sphincter ani externus actually tends to rapidly contract.
AAD common firing altitudes are at 750 feet AGL by the way, which is of course determined by nominal terminal velocity and typical reserve canopy deployment sequence. I think BRS has performed saves as low as 200 feet AGL.

EDIT: well , actually much lower, but obviously very few of this saves were performed in vertical terminal descent
http://www.sky-walker.aero/pdfs/sonstiges/Lives-Saved.pdf
http://www.brs-vertrieb.de/en/news/
« Last Edit: 05/23/2014 06:52 pm by savuporo »
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Offline clongton

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Re: SpaceX DragonFly Discussion Thread
« Reply #168 on: 05/23/2014 06:50 pm »
BPL - Brown Pants Line. On descent, the altitude at which you will sh** deficate in your pants if you are still in free fall.

Fixed that for ya. Now it's legal and Chris can add it to the list. :)
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Offline Elmar Moelzer

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Re: SpaceX DragonFly Discussion Thread
« Reply #169 on: 05/23/2014 06:59 pm »
In this case, if there's no parachute, or they're below minimum safe altitude for a parachuted landing, the severity is loss of/serious injury to crew. There's no way they're simply going to go ahead with "Yeah, hypergolics don't fail".
Also, engine control can be lost due to software glitches too.
A few thoughts on that.
1. I think that a catastrophic software failure on ANY US manned spacecraft since Apollo would have probably resulted in a loss of crew. The timely deployment of parachutes and/or airbags will require software too.
The Shuttle and the Dream Chaser probably wont fly well without software to control them.
So this is not any different for any of the competing space craft.

2. If anything the powered landing adds another level of redundancy to the whole system. I am sure that SpaceX will test various emergency scenarios for the whole system. That will most likely include any combination of hardware, software, valve, engine and other failures.

3. If the normal method for landing a Dragon2 will indeed be with the trunk attached (and this is only one possible interpretation of the FAA permit), I presume that the trunk could act as an additional crush/crumble zone, that could improve the survivability even with severely reduced engine power and very late parachute deployment. Modern cars have quite good survivability in head on collisions, even at high speeds. Of course a capsule like Dragon has added risk because of fuel leaks and other things...

Offline savuporo

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Re: SpaceX DragonFly Discussion Thread
« Reply #170 on: 05/23/2014 07:03 pm »
The timely deployment of parachutes and/or airbags will require software too.
In a piloted flight, not necessarily. Light aircraft rely on a pilot pulled handle, and in skydiving manual reserve deployments are far more common than AAD firing. I'd be suprised if they didnt have a handle in there for deployment.
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Offline dlapine

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Re: SpaceX DragonFly Discussion Thread
« Reply #171 on: 05/23/2014 07:12 pm »

3. If the normal method for landing a Dragon2 will indeed be with the trunk attached (and this is only one possible interpretation of the FAA permit), I presume that the trunk could act as an additional crush/crumble zone, that could improve the survivability even with severely reduced engine power and very late parachute deployment. Modern cars have quite good survivability in head on collisions, even at high speeds. Of course a capsule like Dragon has added risk because of fuel leaks and other things...

Um, wouldn't that assume that no hypergolic propellent is stored in trunk? I don't believe that is and that the stores in the capsule are probably shock isolated to a reasonable extent, but wasn't there some talk of storing additional supplies in the trunk earlier in this thread.

Hypergolic Tanks + crumple zone = Hollywood movie car crash. :)

Offline Elmar Moelzer

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Re: SpaceX DragonFly Discussion Thread
« Reply #172 on: 05/23/2014 07:26 pm »
Um, wouldn't that assume that no hypergolic propellent is stored in trunk? I don't believe that is and that the stores in the capsule are probably shock isolated to a reasonable extent, but wasn't there some talk of storing additional supplies in the trunk earlier in this thread.
Hypergolic Tanks + crumple zone = Hollywood movie car crash. :)
I am quite convinced that even if (and that is a big if), the trunk is indeed meant to stay attached during normal operation (and landing) of the Dragon, I am very sure that there is no propellant stored on the trunk.
If anything, I would assume that some of(!) the battery packs that presumably will replace the solar panels ( think Musk said that) for crewed flight, will be in the trunk. That could match a (speculative!) flexible architecture, where trunks with different outfits are attached to the Dragon capsule, depending on the mission.
I think there once was a mention of a bigger trunk with more space for unpressurized payloads. In addition to that, I could see trunks with solar panels, trunks with bigger heat shields, trunks with bigger batteries and so on (yes very speculative).
Either way, I am skeptical of the idea of having something as integral as the hypergolic fuel tanks in the trunk, but then I might be completely wrong here. All this is a lot of speculation anyway. I cant wait for the reveal in a few days, when we will finally know for sure what the thing will look like.
« Last Edit: 05/23/2014 07:27 pm by Elmar Moelzer »

Offline AJA

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Re: SpaceX DragonFly Discussion Thread
« Reply #173 on: 05/23/2014 07:44 pm »
...
The Shuttle and the Dream Chaser probably wont fly well without software to control them.
So this is not any different for any of the competing space craft.

Sorry, but this had to be done

I concur that they'll add redundancy. However, I don't think the trunk can be counted on as one such redudancy. After all, it'll presumably be packed with a lot of downmass. Then again, if all that downmass is squishy human waste etc. - then I guess it still works.

Having pulled the reserve twice so far, both above the  established hard deck, i would say that BPL is a mischaracterization, as your sphincter ani externus actually tends to rapidly contract.

It's different for different people, and different stimuli. (And possibly also dependent on whether your bowels are currently full, or empty). There are different forcing agents, but adrenaline apparently leads to a relaxation of the external sphincter.

Even if having the "shit scared out of you" doesn't literally happen... there's still a good chance that there would be a BPL. You'd very likely still soil your pants once you're down and safe, and the physiological tension of the fight or flight response gives way to.. "HOLY LIVING SH%#!". Don't forget that the occupants would have fluid-loaded, and that they would have a downward fluid shift, increasing pressure on the bowels. (Even IF they hadn't had >1g accelerations in the past few seconds, this would probably make them want to poop).

Or, in a less desirable situation, post-mortem sphincter distension would also leave their pants soiled (if the impact forces didn't also lead to RUD of the body). Not to mention the trunk squishing I've mentioned above.

Online Lar

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Re: SpaceX DragonFly Discussion Thread
« Reply #174 on: 05/23/2014 07:49 pm »
Thanks for the biology lessons but I think we're probably done with it on this thread, right?
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Offline Space OurSoul

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Re: SpaceX DragonFly Discussion Thread
« Reply #175 on: 05/23/2014 08:25 pm »
Re: trunk attach. I wonder if the LAS process, were it to occur, would disconnect both the trunk from the second stage and the trunk from the dragon simultaneously. Since it's an emergency situation, who knows if either of those separation points will actually succeed. So you have to plan for the case when the trunk separates from the stage but not from the dragon, and then land with the two still connected. It would kinda suck if the software didn't take this possibility into account and then they broke the crew in a real abort just because the extra weight threw off the landing algorithm.


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Re: SpaceX DragonFly Discussion Thread
« Reply #176 on: 05/23/2014 09:03 pm »
Another step on the road to Mars.

Offline AncientU

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Re: SpaceX DragonFly Discussion Thread
« Reply #177 on: 05/23/2014 11:13 pm »
An interesting feature of all this is that SpaceX will be flying the capsule all the way from orbit to within a km or two of the landing pad... That will be a ride!

Edit: Not actually DragonFly, but the real version.
« Last Edit: 05/23/2014 11:17 pm by AncientU »
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Offline 411rocket

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Re: SpaceX DragonFly Discussion Thread
« Reply #178 on: 05/23/2014 11:40 pm »
An interesting feature of all this is that SpaceX will be flying the capsule all the way from orbit to within a km or two of the landing pad... That will be a ride!

Edit: Not actually DragonFly, but the real version.

I would think, they would like to nail the landing pad. But did you mean, the launch pad???

Offline AncientU

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Re: SpaceX DragonFly Discussion Thread
« Reply #179 on: 05/24/2014 12:00 am »
An interesting feature of all this is that SpaceX will be flying the capsule all the way from orbit to within a km or two of the landing pad... That will be a ride!

Edit: Not actually DragonFly, but the real version.

I would think, they would like to nail the landing pad. But did you mean, the launch pad???
Landing.  Might be on left coast, at least at first.  My point was reentry all the way to within a km or so of the ground without drogues, mains, nothing... and flying/maneuvering the capsule until the last tens of seconds before landing when the burn starts... what an entry to watch! Or ride!!!
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