Author Topic: SpaceX DragonFly Discussion Thread  (Read 375242 times)

Offline wolfpack

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Re: SpaceX DragonFly Discussion Thread
« Reply #240 on: 05/30/2014 01:47 pm »
OK, a couple of things that I just can't believe. Real spacecraft designers please opine if you will!

1. No switchguards around those center console buttons?

2. A handle on the outside of the hatch?

Offline sghill

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Re: SpaceX DragonFly Discussion Thread
« Reply #241 on: 05/30/2014 01:52 pm »
Regarding the landing burn height,  if we use the video for clues (yes, I know it's not scientific), the burn begins at about 20 miles, not two miles.

Here's a Google Earth comparison from 18 miles with the video when KSC comes into view. The engines are already firing before KSC comes into the frame, so the burn begins around 20 miles.

Well above the BPL.  I'm pleased!

That could all be artistic license on the part of the animators, but it's the only solid thing we have to go on.
Bring the thunder!

Offline Crispy

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Re: SpaceX DragonFly Discussion Thread
« Reply #242 on: 05/30/2014 02:19 pm »
Here's a Google Earth comparison from 18 miles with the video when KSC comes into view. The engines are already firing before KSC comes into the frame, so the burn begins around 20 miles.

Without knowing the field of view for either camera, that comparison is effectively meaningless, unfortunately.
« Last Edit: 05/30/2014 02:20 pm by Crispy »

Offline Elmar Moelzer

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Re: SpaceX DragonFly Discussion Thread
« Reply #243 on: 05/30/2014 02:26 pm »
OK, a couple of things that I just can't believe. Real spacecraft designers please opine if you will!
1. No switchguards around those center console buttons?
I would assume that these buttons are only used in certain situations. There might be some other mechanism that is required to make the active.
Also, I am not concerned about the touch screen. You can program the interface in such a way, that buttons on the screen can not be accidentally pushed (e.g. require a sliding motion followed by a push, or some combination like that).

Offline Lars_J

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Re: SpaceX DragonFly Discussion Thread
« Reply #244 on: 05/30/2014 04:32 pm »
Here's a Google Earth comparison from 18 miles with the video when KSC comes into view. The engines are already firing before KSC comes into the frame, so the burn begins around 20 miles.

Without knowing the field of view for either camera, that comparison is effectively meaningless, unfortunately.

How do you figure?  The land mass would look stretched or shrunken if the field of view differed greatly between the two and it doesn't so the field of view has to be roughly similar.  And since the land looks pretty close to the same in both images, the altitude must also be the same. 

In other words, KSC isn't going to look like that from 2 miles up (versus 20 miles) even with a fisheye lens.

It's a CG rendering. And not a very high fidelity one either. And as someone who does photography, trust me, you cannot make that determination.

Offline Sohl

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Re: SpaceX DragonFly Discussion Thread
« Reply #245 on: 05/30/2014 04:38 pm »
Or it could be "artistic license" in the video, like dglow and mme said.

I'm firmly in the "lots of artistic license" camp.  Dragon2 will not have enough fuel to burn continuously from the depicted height. 

Offline RanulfC

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Re: SpaceX DragonFly Discussion Thread
« Reply #246 on: 05/30/2014 05:47 pm »
Anyone else have the feeling when the "Dragon-2/Trunk" was shown in the video that it would make a really neat sub-orbital hopper?

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Lars_J

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Re: SpaceX DragonFly Discussion Thread
« Reply #247 on: 05/30/2014 05:49 pm »
Why would the trunk be a part of it? It is expendable. I would think a trunk-less Dragon 2 would be a much neater suborbital hopper, even though its range would be just a few miles at best. ;)

Offline notsorandom

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Re: SpaceX DragonFly Discussion Thread
« Reply #248 on: 05/30/2014 05:58 pm »
They might also want to stretch out the landing burn so they can fly out any aiming errors they have. Before the engines come on they only way to change the course is to orient the capsule. Which is something that might not give much control compared to having the super dracos on low thrust for a mile or so before they are needed to slow the capsule down.

Offline RanulfC

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Re: SpaceX DragonFly Discussion Thread
« Reply #249 on: 05/30/2014 06:18 pm »
Why would the trunk be a part of it? It is expendable. I would think a trunk-less Dragon 2 would be a much neater suborbital hopper, even though its range would be just a few miles at best. ;)

Think= "non-expendable" Trunk with extra propellant etc to provide more performance. Yes I know all the arguments against that for an Orbital vehicle but we'd be talking a "de-rated" D-V2 (possibly a "retired" orbital model??) for short-hop suborbial runs. Trunk-less would be very neat of course but is only a few miles altitude "worth" the trouble?
(Really. To ever hit "gas-and-go" EM/SpaceX is going to have to get away from the current propellant mix. Hmmm, wonder if EM has thought about applying for Federal "Flex-Fuel" incentives on the D-V2? :) )

(To use an older "scale" a friend and I worked out it would be a Class-1 Sub-Orbital, straight-up/straight-down mission mode)

That's why I noted when it "first" showed. Fins, capsule, etc, heck I immediatly thought "That's a Spaceship!" :)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline RanulfC

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Re: SpaceX DragonFly Discussion Thread
« Reply #250 on: 05/30/2014 06:19 pm »
They might also want to stretch out the landing burn so they can fly out any aiming errors they have. Before the engines come on they only way to change the course is to orient the capsule. Which is something that might not give much control compared to having the super dracos on low thrust for a mile or so before they are needed to slow the capsule down.

Well there are some other options as well. CG shift for example.
"Everyone lean as far as you can to the left!" :)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline StephenB

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Re: SpaceX DragonFly Discussion Thread
« Reply #251 on: 05/30/2014 07:12 pm »
Why would the trunk be a part of it? It is expendable. I would think a trunk-less Dragon 2 would be a much neater suborbital hopper, even though its range would be just a few miles at best. ;)

Think= "non-expendable" Trunk with extra propellant etc to provide more performance. Yes I know all the arguments against that for an Orbital vehicle

Is this an obstacle?: If the fuel is below the nozzles, how does it get from a tank in the trunk against both gravity and the acceleration from the super dracos?

Offline RanulfC

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Re: SpaceX DragonFly Discussion Thread
« Reply #252 on: 05/30/2014 07:37 pm »
Is this an obstacle?: If the fuel is below the nozzles, how does it get from a tank in the trunk against both gravity and the acceleration from the super dracos?

The propellants are delivered by high pressure. The point was made that a high-pressure, "seperable" connection system would be problimatical due to the "weak-link" point of the seperation mechanism. This holds "true" with the suggested aplication as well unless there are significant changes to the D-V2/Trunk. But then again that's pretty much what I'm suggesting anyway :)

First of all the (suborbital) D-V2 and trunk don't normally seperate at all during operation, the vehicle being a "Single-Stage-To-Sub-Orbital" vehicle.

Further thought though suggests it might just be "simpler" all around to mount a "standard" D-V2 to a specifically designed and built "trunk-stage" and retain serperate operations capability.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Llian Rhydderch

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Re: SpaceX DragonFly Discussion Thread
« Reply #253 on: 05/30/2014 08:22 pm »
Could we maybe get this thread back to the more narrow scope of the DragonFly test vehicle, flying 30-odd test flights in Texas?   :)

Maybe leave all the Dragon V2 design, CG, animation, altitude of burns on orbital descents, etc. to the threads where those topics are already being discussed.   ::)
Re arguments from authority on NSF:  "no one is exempt from error, and errors of authority are usually the worst kind.  Taking your word for things without question is no different than a bracket design not being tested because the designer was an old hand."
"You would actually save yourself time and effort if you were to use evidence and logic to make your points instead of wrapping yourself in the royal mantle of authority.  The approach only works on sheep, not inquisitive, intelligent people."

Offline modemeagle

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Re: SpaceX DragonFly Discussion Thread
« Reply #254 on: 05/30/2014 10:23 pm »
So modemeagle posted this discussion with his simulation of a 10% to 20% thrust profile

I am going to do my chart again as I did more work on the equations, which is going to effect the results. 

Here is a possible nominal profile with ignition at ~ 1,000 meters at 10% thrust with an increase to 20% for the final stop.  This should allow plenty of time to check operation of the Super Draco engines and shutdown any that are not up to par.  I think this also shows that very low thrust is needed for the final brake and touchdown.  Starting the 10% thrust later and 20% sooner will use less propellant due to gravity loses.

I suspect the excess delta-v remaining will be used up while dropping the horizontal velocity.

The duration is ~17.6 sec.
As modemeagle found the velocity never hits zero at 10% the remainder of the 26 seconds shown in tonight's video must be more 10% thrusting, pre-testing the engines with time to deploy the chutes, targeting for landing on the pad "like a copter" and generally reducing the BPL factor.


Or it could be "artistic license" in the video, like dglow and mme said.

Comga,
The actual burn was 32.56 seconds as the 10% burn started at 997 meters and 35.01 seconds from drop.  Shut down was at 67.57 seconds from drop.

I would expect the "test" burn to be higher and sooner, probably higher than 5 km so there is sufficient time for parachutes.  This is only a guess on my part and I have no indications it is correct.

Offline eriblo

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Re: SpaceX DragonFly Discussion Thread
« Reply #255 on: 05/31/2014 12:48 am »
More than 5 km sounds very careful considering that the drop test was at less than half that (8000 feet) :)

Offline rpapo

Re: SpaceX DragonFly Discussion Thread
« Reply #256 on: 05/31/2014 10:19 am »
More than 5 km sounds very careful considering that the drop test was at less than half that (8000 feet) :)
But the drop test didn't start at terminal velocity either.
Following the space program since before Apollo 8.

Offline eriblo

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Re: SpaceX DragonFly Discussion Thread
« Reply #257 on: 05/31/2014 10:44 am »
More than 5 km sounds very careful considering that the drop test was at less than half that (8000 feet) :)
But the drop test didn't start at terminal velocity either.
True that :) But it did start with a tumbling capsule that fell for at least 5 seconds before being stabilized and slowed by the drogues, so think the difference is less than say 1 km. If they are tumbling out of control at 3.5 km (or 5 km) they have my permission to deploy the drogues ;)

Offline adrianwyard

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Re: SpaceX DragonFly Discussion Thread
« Reply #258 on: 05/31/2014 09:19 pm »
While powered landing tests with DragonFly will certainly grab the headlines, based on discussions over in the Dragon V2 Reveal thread it seems to me that parachute land-landing tests will be important too in order to gain confidence in the whole system.

While parachutes are a backup available in case the powered landing is not possible, they present their own challenges, these include:

+ A completely unpowered land landing will be very hard. With hypergolics onboard you do not want those tanks to breach, so verifying the survivability of a hard landing would be a worthwhile test. Note: if some residual Draco/SuperDraco functionality is available this could be used to cushion the touchdown (a la Soyuz).

+ It appears as though Dragon V2 will hang at an angle under the chutes (as did V1) which is best for water impact after a launch abort. But this will mean that during a parachute land landing just one, or possibly two landing struts will need to take the brunt of the impact force. (And again, if some SD power is available it could bring the Dragon to level flight before impact.)

+ Under parachutes the horizontal velocity at landing could be quite high due to crosswinds. While a powered landing can take any crosswinds out, an unpowered parachute landing Dragon will need to be able to skid to a halt. If you're unlucky, the Dragon will already by 'tipped over' in the direction of crosswind travel under the chutes, and so be in danger of tumbling once it hits the ground.

For the prior discussion see here: http://forum.nasaspaceflight.com/index.php?topic=34828.msg1207214#msg1207214

« Last Edit: 05/31/2014 09:52 pm by adrianwyard »

Offline adrianwyard

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Re: SpaceX DragonFly Discussion Thread
« Reply #259 on: 05/31/2014 09:50 pm »
I should say that I doubt if any of these concerns will prove to be showstoppers, especially if your goal is simply to survive a crash landing. But ISTM hard landings and/or toppling over are more of a concern for Dragon V2 than other spacecraft because it could have have a substantial amount of hypergolic propellant onboard.

I suppose it may be possible to burn off unneeded propellant when under the chutes, but I doubt if dumping it overboard would be considered...

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