Author Topic: SpaceX DragonFly Discussion Thread  (Read 375259 times)

Offline Comga

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Re: SpaceX DragonFly Discussion Thread
« Reply #200 on: 05/26/2014 03:58 am »
[snip]
I understand the efficiency issue in terms of using as much "free" drag delta-v as possible but...
[snip]
The real efficiency issue is not getting as much "free drag delta-v as possible", but in avoiding gravity losses.

Reduction to the absurd:  If the capsule hits zero velocity at some elevated altitude, then it has to do an additional burn to stop after it starts falling again.  That's a gravity loss.

Every additional second the capsule spends off the ground is one second times one g of acceleration needed from the engines.   So the most fuel efficient burn is the shortest, which translates to high acceleration, low altitude.  It's just not the most comfortable, reassuring, or most flexible if something goes wrong.  The goal is to get all of those things without greatly increasing the amount of fuel needed.

Quote
but what I don't know is what would the effect (on ISP) be of short (80% to 100% thrust) bursts?

As for short bursts, it was seen that the Dragon departs from the ISS using series (something like seven and twelve) short bursts from pairs of Dracos.  It would be interesting to know why that's preferable.  Perhaps for very small delta V maneuvers any potential loss of efficiency is not important, if one occurs.  But pulses are preferred under some circumstances.

I just wouldn't want to pulse-width-modulate a 10g system to get less by pulsing on and off. :P
« Last Edit: 05/26/2014 03:58 am by Comga »
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Offline meadows.st

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Re: SpaceX DragonFly Discussion Thread
« Reply #201 on: 05/26/2014 05:07 am »
[snip]
I understand the efficiency issue in terms of using as much "free" drag delta-v as possible but...
[snip]
The real efficiency issue is not getting as much "free drag delta-v as possible", but in avoiding gravity losses.

Reduction to the absurd:  If the capsule hits zero velocity at some elevated altitude, then it has to do an additional burn to stop after it starts falling again.  That's a gravity loss.

Every additional second the capsule spends off the ground is one second times one g of acceleration needed from the engines.   So the most fuel efficient burn is the shortest, which translates to high acceleration, low altitude.  It's just not the most comfortable, reassuring, or most flexible if something goes wrong.  The goal is to get all of those things without greatly increasing the amount of fuel needed.
<snip>

Fair enough.

I understand the gravity loss; what I was talking about was the net (downwards) acceleration (gravity - drag) that must be counteracted by the SDs (only the drag is free).  If the craft slows down too early, the net deceleration required will be larger (more inefficient) - the most efficient method, as you say is not the safest (I think that is true for most/all low/zero margin activities) the trick is going to be finding an acceptable risk/efficiency ratio.  (IANARS and I don't know how reliable the systems) The pulse-width modulation approach was just a question related to efficiency:risk ratio vs., say 25% or 50% (can the SDs throttle this low?) started early (for safety) but I guess that is the question that everyone is trying to answer.
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Offline guckyfan

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Re: SpaceX DragonFly Discussion Thread
« Reply #202 on: 05/26/2014 05:32 am »
This is my uneducated guess:

Do a test fire at an altitude where it is still safe to deploy parachutes if the engines fail. Probably two short bursts of the two largely independend sets of four engines. Decide if it is ok to go for powered landing if one of the two sets does not perform or both sets have to work for redundancy at that point. If necessary go for (thrust assisted) parachute landing.

If the engines are ok at that point it is very very likely they will fire successfully again a few seconds later for the landing burn. If the available fuel allows it go for a 3g landing. 3g for a few seconds should not be too harsh even for sick or injured persons.

Offline TrevorMonty

Re: SpaceX DragonFly Discussion Thread
« Reply #203 on: 05/26/2014 09:10 am »
This is my uneducated guess:

Do a test fire at an altitude where it is still safe to deploy parachutes if the engines fail. Probably two short bursts of the two largely independend sets of four engines. Decide if it is ok to go for powered landing if one of the two sets does not perform or both sets have to work for redundancy at that point. If necessary go for (thrust assisted) parachute landing.

If the engines are ok at that point it is very very likely they will fire successfully again a few seconds later for the landing burn.

My thoughts exactly.

Offline mikes

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Re: SpaceX DragonFly Discussion Thread
« Reply #204 on: 05/26/2014 09:39 am »
Your table has terminal velocity of just over 70 m/s.

To put that into context, the STS orbiters (before they flared for landing) had a descent rate of 10,000fpm which is 50m/s

http://www.century-of-flight.net/Aviation%20history/space/Space%20Shuttle%20history.htm

Offline douglas100

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Re: SpaceX DragonFly Discussion Thread
« Reply #205 on: 05/26/2014 10:37 am »
What if the trunk remains attached for these landings? Weight maybe as much as doubled, terminal velocity higher, burn times longer due to both extra mass and higher initial velocities.  This might get us up toward those 5 second burns in the FAA doc.

I don't think that's going to happen if the DragonFly tests are designed to support Dragon 2. They may launch with a trunk attached to test behaviour of the combined vehicle for an abort scenario but it would separate before landing. I think the speculation about the trunk remaining attached throughout the flight is a misinterpretation. A vehicle with an integrated trunk (it would really be a payload bay) would be essentially a brand new vehicle and I think such a thing would be further down the line. I now await The Big Reveal to see if I'm wrong!  :)
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Offline Jcc

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Re: SpaceX DragonFly Discussion Thread
« Reply #206 on: 05/26/2014 10:52 am »
A permanently attached trunk would defeat the purpose of having a trunk, in that it would need to be closed off in the back with a heat shield, then you would not be able to use it for external cargo. Besides, the aerodynamics for EDL would be all wrong, and furthermore, there would be a need for an adapter to attach it to the second stage, as the trunk fulfills that purpose now, being an open cylinder.

Offline clongton

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Re: SpaceX DragonFly Discussion Thread
« Reply #207 on: 05/26/2014 11:02 am »
I agree that a permanently attached trunk is unlikely but for a different reason. Dragon is passively stable during re-entry, meaning that its CG is located low enough in the structure that it will always passively present its heatshield first at the first sign of atmosphere resistance. That dynamic completely flips if a now-empty trunk is still attached at re-entry. With the CG now substantially higher in the structure, Dragon will passively flip around and re-enter nose first, providing a bad day for everyone aboard.
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Offline AncientU

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Re: SpaceX DragonFly Discussion Thread
« Reply #208 on: 05/26/2014 12:24 pm »
I agree that a permanently attached trunk is unlikely but for a different reason. Dragon is passively stable during re-entry, meaning that its CG is located low enough in the structure that it will always passively present its heatshield first at the first sign of atmosphere resistance. That dynamic completely flips if a now-empty trunk is still attached at re-entry. With the CG now substantially higher in the structure, Dragon will passively flip around and re-enter nose first, providing a bad day for everyone aboard.
Good point. Pretty much kills the idea.
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Offline fatjohn1408

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Re: SpaceX DragonFly Discussion Thread
« Reply #209 on: 05/26/2014 01:22 pm »
Freefall, 5 sec burn at the last moment, soft touchdown.

Whoever will ultimately ride aboard that kind of flight profile (obviously not during these tests) is a brave man :)

Can anyone model the G-Force curve on that?  It sounds like a rough ride.

If anyone can guess the terminal velocity of the capsule, it should be easy to calculate. If the G-load is evenly spread over 5 seconds, it might not be too bad.

For a very first order estimate I get a terminal velocity of ~150 m/s given a total mass of 17,000 lbs (dry mass plus 3000 lbs of fuel) and a drag coefficient of 0.8.

A 5 second burn to brake from 150 m/s would result in an average of ~3Gs of deceleration.

But assuming you try to decelerate while traveling toward the ground (I know crazy assumption huh) you need another G to counteract gravity. So astronaut Brown Pants will experience 4G.

Offline go4mars

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Re: SpaceX DragonFly Discussion Thread
« Reply #210 on: 05/26/2014 02:19 pm »
I agree that a permanently attached trunk is unlikely but for a different reason. Dragon is passively stable during re-entry, meaning that its CG is located low enough in the structure that it will always passively present its heatshield first at the first sign of atmosphere resistance. That dynamic completely flips if a now-empty trunk is still attached at re-entry. With the CG now substantially higher in the structure, Dragon will passively flip around and re-enter nose first, providing a bad day for everyone aboard.
Good point. Pretty much kills the idea.
Thanks.
I'm not sure it's been fully dismissed.

1) CG issue is only what it is designed to be.  Why assume the trunk would be empty? Perhaps the more dense cargo would preferentially be stowed there (potentially removes center of grav issue).
2) Redundancy.  Potential to re-enter dragon & trunk either attached or unattached if designed to (ideally some emergency way to move from one to the other if need arises -(anyone remember that scene from "Fortress 2"?)
3) Redundancy.  If both pieces have superdracos and tanks, and they enter together, a random engine out becomes less relevant, and the trunk can become the crumple zone if underperformance is too significant.
4) Staging.  Trunk could be used as an extra stage in certain circumstances (either reusably or not). 

Obviously I'm not talking about a slight modification of current trunk.  This would be a new phenotype.
« Last Edit: 05/26/2014 02:21 pm by go4mars »
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Offline douglas100

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Re: SpaceX DragonFly Discussion Thread
« Reply #211 on: 05/26/2014 02:45 pm »

....Obviously I'm not talking about a slight modification of current trunk.  This would be a new phenotype.

Indeed. This is complete speculation with little evidence connecting it to what we know at the moment about the DragonFly tests.
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Online Coastal Ron

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Re: SpaceX DragonFly Discussion Thread
« Reply #212 on: 05/26/2014 02:55 pm »
I agree that a permanently attached trunk is unlikely but for a different reason. Dragon is passively stable during re-entry, meaning that its CG is located low enough in the structure that it will always passively present its heatshield first at the first sign of atmosphere resistance. That dynamic completely flips if a now-empty trunk is still attached at re-entry. With the CG now substantially higher in the structure, Dragon will passively flip around and re-enter nose first, providing a bad day for everyone aboard.
Good point. Pretty much kills the idea.
Thanks.
I'm not sure it's been fully dismissed.

1) CG issue is only what it is designed to be.  Why assume the trunk would be empty? Perhaps the more dense cargo would preferentially be stowed there (potentially removes center of grav issue).

I'm not an engineer, but keeping the trunk attached while the Dragon is returning to Earth does not look like a good idea.

With the attached trunk, I would imagine the aerodynamics of the whole assembly change.  Whereas the capsule by itself would naturally orient it's heatshield to the direction of travel, if the trunk is attached it will have wind resistance and a different CG, and want to flip around and point the capsule into the wind like an arrow - which is wrong for many reasons.

Unless it separates and has it's own re-entry system, I would imagine the trunk is disposable.
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Offline AncientU

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Re: SpaceX DragonFly Discussion Thread
« Reply #213 on: 05/26/2014 04:00 pm »

....Obviously I'm not talking about a slight modification of current trunk.  This would be a new phenotype.

Indeed. This is complete speculation with little evidence connecting it to what we know at the moment about the DragonFly tests.

Not complete...  The FAA license application(impact statement) talked about landing with trunk -- that's what got me thinking along this path. A new phenotype would be to have 'trunk' on crew Dragon/DragonFly be shortened to maybe half of original trunk length and be fitted out with battery, larger fuel tanks, and any other liquid cargo like water or fuel for ISS.  Adding the heat shield to its bottom along with landing legs will move c/g significantly lower. Pressure volume with astros will be fairly fluffy, so still might work.  How low does the c/g need to go to reestablish passive stability on reentry?
« Last Edit: 05/26/2014 04:02 pm by AncientU »
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Offline Geron

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Re: SpaceX DragonFly Discussion Thread
« Reply #214 on: 05/26/2014 04:05 pm »
It still has solar panels they just r not deployed. They wrap around trunk

Offline Joffan

Re: SpaceX DragonFly Discussion Thread
« Reply #215 on: 05/26/2014 05:13 pm »
The FAA license application(impact statement) talked about landing with trunk -- that's what got me thinking along this path.

It doesn't actually talk about landing with the trunk. Here's what the draft EA actually said about the trunk:
Quote
The DragonFly RLV is the Dragon capsule with an integrated trunk (which may or may not be attached during a DragonFly operation) and up to four steel landing legs.
There is also a picture of the Dragon + Trunk in processing which also uses the term "integrated".

The text in brackets means that the trunk is detachable. So speculation about a composite vehicle that extends the Dragon to include the trunk seems unfounded.

"Attached during an operation" doesn't necessarily mean that it stays attached for the entire duration of the flight, either.
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Offline modemeagle

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Re: SpaceX DragonFly Discussion Thread
« Reply #216 on: 05/26/2014 08:57 pm »
I ran the simulation again and made a new chart.  This attachment includes graphs and data for 0% and 20-100% thrust profiles.  I am not running a cosine loss for 10 degrees since it would only be a 1.5% difference which is less than my current error rate.

If I can locate my dragon reentry simulation then I will try to run a more realistic simulation with horizontal velocity to cancel out.

Offline Jcc

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Re: SpaceX DragonFly Discussion Thread
« Reply #217 on: 05/26/2014 09:10 pm »
I agree that a permanently attached trunk is unlikely but for a different reason. Dragon is passively stable during re-entry, meaning that its CG is located low enough in the structure that it will always passively present its heatshield first at the first sign of atmosphere resistance. That dynamic completely flips if a now-empty trunk is still attached at re-entry. With the CG now substantially higher in the structure, Dragon will passively flip around and re-enter nose first, providing a bad day for everyone aboard.

I recall a similar thing happened to a Soyuz, when the service module failed to detach. It reentered nose first until whatever was holding the SM on scorched and burned off, then the capsule could reorientate, it landed normally and the crew survived.
http://www.spacesafetymagazine.com/2011/10/29/soyuz-5-multiply-reentry-failures-endanger-cosmonaut/
« Last Edit: 05/26/2014 09:33 pm by Jcc »

Offline Joffan

Re: SpaceX DragonFly Discussion Thread
« Reply #218 on: 05/26/2014 09:11 pm »
I ran the simulation again and made a new chart.  This attachment includes graphs and data for 0% and 20-100% thrust profiles.  I am not running a cosine loss for 10 degrees since it would only be a 1.5% difference which is less than my current error rate.

If I can locate my dragon reentry simulation then I will try to run a more realistic simulation with horizontal velocity to cancel out.

Haha... I suppose 8g for just under a second (the 100% profile)  is still better than crashing, but hopefully it's not nominal. Interesting that even the gentle 20% 2.5g solution has ignition below 200m.

And hopefully the touchdown g-force would be eliminated with an active control loop, at least on the lower throttle options.
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Offline guckyfan

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Re: SpaceX DragonFly Discussion Thread
« Reply #219 on: 05/27/2014 06:49 am »
The amount of fuel on board will be determined by abort needs. Subtract from that what is needed for on orbit maneuvers then you get what you have for landing. As you will want to burn most of that fuel before touchdown to reduce hypergolic fuel load the spare fuel will determine, how soft you can land.


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