Author Topic: SpaceX Dragon 2 Updates and Discussion - Thread 2  (Read 559155 times)

Offline uhuznaa

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #60 on: 10/17/2016 10:56 AM »
Does anybody know what SpaceX wants to do with the LES-propellants before splash-down? As far as I know at least the crewed Dragons will land on parachutes for quite a while, so you either dump/use the propellant or you land with full tanks.

Apollo dumped the RCS-propellants during descent, and smashing into the drink with a ton or so of hypergols right next to your elbow may not be a good idea. Or was NASA too cautious here?

You could either use them as a kind of third stage on ascent, use them for a deorbit burn, or be clever and do a simulated mid-air landing with them during descent. Outright dumping probably wouldn't be that clever, Apollo 15 lost one parachute when dumped RCS propellant ate the chute lines.

I'm a bit concerned that a parachute landing with full tanks may be no less dangerous than a propulsive landing. SpaceX starting with propulsive landings with cargo Dragons to learn some lessons and convince NASA would be great.

Offline Lars-J

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #61 on: 10/17/2016 08:56 PM »
Does anybody know what SpaceX wants to do with the LES-propellants before splash-down? As far as I know at least the crewed Dragons will land on parachutes for quite a while, so you either dump/use the propellant or you land with full tanks.

There are no LES specific tanks. Super-draco and draco engines are fed from the same set of tanks. So they would never land with the propellant tanks "full". Either it is all used up during an abort, or it is mostly used up during the mission and only a small part remains.

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I'm a bit concerned that a parachute landing with full tanks may be no less dangerous than a propulsive landing. SpaceX starting with propulsive landings with cargo Dragons to learn some lessons and convince NASA would be great.

You don't have to be concerned, your premise is wrong.
« Last Edit: 10/17/2016 08:56 PM by Lars-J »

Offline uhuznaa

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #62 on: 10/17/2016 10:03 PM »
Does anybody know what SpaceX wants to do with the LES-propellants before splash-down? As far as I know at least the crewed Dragons will land on parachutes for quite a while, so you either dump/use the propellant or you land with full tanks.

There are no LES specific tanks. Super-draco and draco engines are fed from the same set of tanks. So they would never land with the propellant tanks "full". Either it is all used up during an abort, or it is mostly used up during the mission and only a small part remains.

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I'm a bit concerned that a parachute landing with full tanks may be no less dangerous than a propulsive landing. SpaceX starting with propulsive landings with cargo Dragons to learn some lessons and convince NASA would be great.

You don't have to be concerned, your premise is wrong.

Ok... So where is the reserve for propulsive landings coming from when the propellants are used up during a normal mission? The LES or landing propellants should be at least about one order of magnitude more than the RCS propellants, and additionally to these. The delta-v requirements for LES or a landing are much, much higher than what a normal RCS needs (as in Dragon I) and at descent you'll have used up the RCS propellants anyway.

Or to put it more distinctively: When a propulsive landing is planned the engines are tested and only when they work as planned Dragon II proceeds to a propulsive landing. If not it lands with parachutes. At this moment though it will have about 0.5km/s worth of hypergolic propellants left in its tanks. That's quite a lot. Maybe one metric ton or more. I've read about up to two tons. What do they plan to do with all these hypergols if they need to deploy the chutes instead? Dump, burn, or splash down and hope for the best, possibly with some tank, valves or plumbing that failed, because if the engines didn't work as planned something must be broken?

I could understand NASA not being happy about that, that's why I'm asking. You'd be basically landing on a bomb (and with a ton more mass anyway) which negates the safety advantage of an inert capsule drifting down on chutes. When do they burn this off? During ascend adding 0.5km/s of delta-v? Deorbit? When?

Personally I'd prefer a propulsive landing over landing on a ton of hypergolics, chutes be damned. An inert capsule may be somewhat safer than a propulsive landing, but a capsule smashing down onto the water with a ton of hypergolics in its bottom isn't. SpaceX had water intrusion in the pressure vessel and had to install a bilge pump in the space where the propellant tanks and helium COPVs and valves and plumbings are. If you have ever dealt with a boat or ship you will know that water smashing at panels is nothing to be dealt lightly with. And this is a spacecraft, not a boat with a steel bow.

So I assume that a Dragon II that is planned to land on parachutes will do this with empty tanks. I really don't  see the RCS Dracos burning a ton of propellants without whizzing in circles around the ISS.
 
(I don't like to be told I'm wrong without being told what is right, preferably with some numbers in it, sorry.)

Offline Negan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #63 on: 10/17/2016 10:14 PM »
IIRC Dragon 2 was always supposed to be able to land with parachutes only if the Super-Dracos failed even on land. How is this any different than a water landing?

Edit: Here's an article that gives more info on this subject. http://www.nasaspaceflight.com/2014/08/dragon-v2-rely-parachutes-landing/

“The whole landing system is designed so that it’s survivable if there’s no propulsive assist at all.  So if you come down chutes only with the landing legs, we anticipate no crew injury. It’ll be kind of like landing in the Soyuz.”
« Last Edit: 10/18/2016 01:03 AM by Negan »

Offline Khadgars

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #64 on: 10/17/2016 10:39 PM »
He has a valid point though.  If they do not do a propulsive landing, what happens to the propellant?

Offline Lars-J

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #65 on: 10/17/2016 10:57 PM »
Does anybody know what SpaceX wants to do with the LES-propellants before splash-down? As far as I know at least the crewed Dragons will land on parachutes for quite a while, so you either dump/use the propellant or you land with full tanks.

There are no LES specific tanks. Super-draco and draco engines are fed from the same set of tanks. So they would never land with the propellant tanks "full". Either it is all used up during an abort, or it is mostly used up during the mission and only a small part remains.

Quote
I'm a bit concerned that a parachute landing with full tanks may be no less dangerous than a propulsive landing. SpaceX starting with propulsive landings with cargo Dragons to learn some lessons and convince NASA would be great.

You don't have to be concerned, your premise is wrong.

Ok... So where is the reserve for propulsive landings coming from when the propellants are used up during a normal mission? The LES or landing propellants should be at least about one order of magnitude more than the RCS propellants, and additionally to these. The delta-v requirements for LES or a landing are much, much higher than what a normal RCS needs (as in Dragon I) and at descent you'll have used up the RCS propellants anyway.

Well... You can either believe it or not believe it. And no, the landing propellant is not an order of magnitude more than RCS. And just be clear, the "RCS" in this case is used for ALL orbital maneuvers. Orbit raising burns, de-orbit burns, in addition to the normal small maneuvers. Again, I'm not sure if you really understand how the Dragon 2 propellant tanks are set up. This is NOT (I repeat NOT) like Orion or CST-100, where each set of different thrusters has their own tanks.

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Or to put it more distinctively: When a propulsive landing is planned the engines are tested and only when they work as planned Dragon II proceeds to a propulsive landing. If not it lands with parachutes. At this moment though it will have about 0.5km/s worth of hypergolic propellants left in its tanks. That's quite a lot. Maybe one metric ton or more.

0.5 km/s? The landing needs is probably less than half of that, the Dragon 2 is going to be subsonic and decelerating.

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So I assume that a Dragon II that is planned to land on parachutes will do this with empty tanks. I really don't  see the RCS Dracos burning a ton of propellants without whizzing in circles around the ISS.
 
(I don't like to be told I'm wrong without being told what is right, preferably with some numbers in it, sorry.)

If you don't like the response, make less blanket assumptions. Especially when *your* numbers are pulled from thin air.
« Last Edit: 10/17/2016 10:59 PM by Lars-J »

Offline uhuznaa

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #66 on: 10/17/2016 11:14 PM »
IIRC Dragon 2 was always supposed to be able to land with parachutes only if the Super-Dracos failed even on land. How is this any different than a water landing?

Dragon II was always supposed to aim at a water landing at the coast, test the engines and then either divert to land on the ground with the engines or splash down in water with parachutes if there was something wrong with the engines. NASA than decided that they didn't want to have anything to do with the propulsive landings and wanted to land with parachutes in the water like capsules are supposed to land since ancient times.

They did buy the LES fueled by the same engines and the same fuel though. Now they're stuck with splashing down on a ton of unused hypergolics or (and that is what I'm wondering about) dumping/burning them off before even deploying the chutes to make sure that you don't hit the water on a ton of hypergolics next to your bum. I would really hate to do that and I guess NASA doesn't feel good about it too. Anybody who would feel good about this except in an emergency would need to be crazy.

Mind you, this WAS only meant to happen in an emergency (if the engines don't work), in which case it would be fine (using a backup landing system always is like attempted suicide to escape certain death), but somehow now a water landing is supposed to be the rule instead of the exception. So they either need to spend this fuel way before the landing or they will land on a bloody hypergolic bomb. Of course you can do some things to mitigate risks (like venting the high-pressure helium COPV tanks because these going off at a shock would be very bad news for the tanks with the hypergolic propellants right next to them), but this again means another point of failure, because some valves or pyros to vent the pressure vessels you need to pressurize your LES engines normally would be considered crazy too. These tanks are not meant to lose pressure in any other way than pressing fuel to the engines.

And that's why I'm asking at which point of the mission they want to get rid of these propellants or if they really want to splash down on them.

I could understand SpaceX to be fuming at NASA wanting to make the backup plan the rule and by this making things harder, but NASA is the customer and pays the bill.

SpaceX often is aggressively ambitious, but being that and then having safety blocks thrown in front of your feet doesn't make things easier.


Offline uhuznaa

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #67 on: 10/17/2016 11:39 PM »
Well... You can either believe it or not believe it. And no, the landing propellant is not an order of magnitude more than RCS. And just be clear, the "RCS" in this case is used for ALL orbital maneuvers. Orbit raising burns, de-orbit burns, in addition to the normal small maneuvers. Again, I'm not sure if you really understand how the Dragon 2 propellant tanks are set up. This is NOT (I repeat NOT) like Orion or CST-100, where each set of different thrusters has their own tanks.


Ok, so please tell me how the tanks are set up (what does it matter how the tanks are set up?) and how much delta-v Dragon II needs for landing. After spending all the fuel for the RCS needs it will STILL have to have enough fuel for a propulsive landing. I don't care if this is half a ton or one ton or two. Half a ton of hypergolics when hitting the water is enough to make an big ugly orange fireball in the sea if there's a leak in plumbing or a valve giving way. Apollo didn't dump the little RCS fuel it had for no reason. Hitting the water is easily the hardest shock a capsule experiences during a flight, just as the front wheel coming down at landing was the driver for the stiffness of the shuttle fuselage. These are spacecraft, not boats. One Dragon had its pressure vessel taking on water on impact.

I agree that I don't have precise numbers. Do you? But when this thing is supposed to be able to do a propulsive landing it will have to have the propellants for that before deciding to either splash down or try a propulsive landing. And if it doesn't do a propulsive landing it will land with the fuel still on board.

Or it will have to dump or burn the fuel way before that and this was the only thing I was asking about in the first place. I don't want to argue. If you know it, tell me. If you want to discuss things, discuss. But don't tell me I'm wrong without telling me what's right.

Ah, forget it. I will go lurking again and come back in a year or so when Dragon may be actually landing, I have better things to do than to argue on a forum.


Offline IainMcClatchie

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #68 on: 10/18/2016 12:21 AM »
That's an interesting point about Apollo 15.  I hadn't heard that before.

The hypergol load isn't hard to figure out.  The Dragon 2 is supposed to be 8900 kg max on landing (6400 kg dry + 2500 kg return payload).  The SuperDracos are supposed to have 240s Isp.  So you just have to guess what the delta-V needed for landing is.  If the burn is 10 seconds long, that's 100 m/s of gravity loss, plus another 200 m/s or so to cancel Dragon 2's terminal velocity.

300 m/s delta-V is 1200 kg of propellant.

uhuznaa, I know you did this calc already.  I'm just pointing out that your guess of 1 tonne propellant isn't really just a guess.

Offline Negan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #69 on: 10/18/2016 05:11 PM »
That's an interesting point about Apollo 15.  I hadn't heard that before.

The hypergol load isn't hard to figure out.  The Dragon 2 is supposed to be 8900 kg max on landing (6400 kg dry + 2500 kg return payload).  The SuperDracos are supposed to have 240s Isp.  So you just have to guess what the delta-V needed for landing is.  If the burn is 10 seconds long, that's 100 m/s of gravity loss, plus another 200 m/s or so to cancel Dragon 2's terminal velocity.

300 m/s delta-V is 1200 kg of propellant.

uhuznaa, I know you did this calc already.  I'm just pointing out that your guess of 1 tonne propellant isn't really just a guess.

Is there a requirement to launch with a full load of propellant?

Edit: Thinking about it since this is the LES propellant too, I guess yes it would so really Starliner would also have the same issue with regard to either burning it, dumping it, or landing with it.
« Last Edit: 10/18/2016 05:48 PM by Negan »

Offline gongora

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #70 on: 10/18/2016 05:44 PM »
Edit: Thinking about it since this is the LES propellant too, I guess yes it would so really Starliner would also have the same issue with regard to either burning it, dumping it, or landing with it.

Starliner is completely different system, they dump the service module.

Offline Negan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #71 on: 10/18/2016 05:50 PM »
Edit: Thinking about it since this is the LES propellant too, I guess yes it would so really Starliner would also have the same issue with regard to either burning it, dumping it, or landing with it.

Starliner is completely different system, they dump the service module.

Thanks.

Offline whitelancer64

Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #72 on: 10/18/2016 06:04 PM »
0.5 km/s is 1,100 mph

Terminal velocity for the Dragon, without parachutes, is going to be more like 250 mph. With parachutes it's more like 30 mph.
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Offline Negan

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #73 on: 10/18/2016 07:25 PM »
How much propellant did the shuttle land with?

Offline rpapo

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #74 on: 10/19/2016 03:27 PM »
How much propellant did the shuttle land with?
Enough to where every landing was a hazmat situation, to some extent.
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Offline sdsds

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #75 on: 10/19/2016 05:38 PM »
Would there be anything preventing them from doing a burn to depletion just before parachute deployment?
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Offline rpapo

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #76 on: 10/19/2016 06:15 PM »
Would there be anything preventing them from doing a burn to depletion just before parachute deployment?
Nothing, but that would leave nothing for a possible soft(er) landing.  And frankly, there's no reason I (at least) can think of why they couldn't just fire the upward pointing thrusters (not the SuperDracos) after landing until all the fuel is depleted.  The thrust wouldn't be enough to do anything, and by burning normally, all, or almost all the toxicity would be removed.

But you guys are worried about the tank cracking on landing...
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Offline IainMcClatchie

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #77 on: 10/19/2016 06:38 PM »
0.5 km/s is 1,100 mph

Terminal velocity for the Dragon, without parachutes, is going to be more like 250 mph. With parachutes it's more like 30 mph.

If they are abandoning propulsive landing entirely, even as a backup, then you are quite right that there is no need to carry the required propellant all the way down to a landing.

You're right about the velocity too.  Even assuming it's 2 km up and aerobraking at 1.5 Gs, it'll be at 236 mph.  A 2.5 G landing burn (39% full thrust) would use around 175 m/s delta-V.  That's more reasonable than I expected.

Offline envy887

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #78 on: 10/19/2016 06:41 PM »
To my understanding, NASA only wants water landings for crew, which don't require a propulsive assist to soften the touchdown, so all the hypergols could be burned off or dumped prior to popping the chutes.

Propulsive assisted soft landings on dry land could dump/burn nearly all the hypergols before deploying the chutes, such that the landing assist burn consumes all but a small margin.

Offline docmordrid

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Re: SpaceX Dragon 2 Updates and Discussion - Thread 2
« Reply #79 on: 10/19/2016 07:22 PM »
OTOH, if the parachutes fail you may want that propellant for an emergency propulsive landing - though ISTM, as someone who has had to use a reserve chute, the odds of success favor using the hypergolic rockets as the primary.
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