Author Topic: Speculation on new Dragon features  (Read 123522 times)

Offline pathfinder_01

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Re: Speculation on new Dragon features
« Reply #20 on: 01/20/2011 02:17 pm »
This is kind of funny. Like the kremlinology of old, except for spacex. I would not be surprised to see major changes from this promotional video to the final configuration.

One thing I would really like them to do is to change the fuel combination to something more easy to handle. Hydrogen peroxide/Kerosene, like the x37 uses?


Peroxide is not easy to handle and anyways X-37 doesn't use it.

OK. Thanks for the correction.

Compared to nitrogen tetroxide/hydrazine, wouldn't peroxide/kero be easier to handle? At least it is not toxic and carcinogenic. (I did not say easy to handle. No rocket propellant is easy to handle. I said more easy to handle, compared to what dragon uses now)


Peroxide at high concentration is very dangerous and it is not easy to handle. Even at the low concentrations you need a stablizer to keep it from breaking into water and oxygen before you can use it. Anything you use to sterilze something is toxic at enough concentration.
« Last Edit: 01/20/2011 02:18 pm by pathfinder_01 »

Offline douglas100

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Re: Speculation on new Dragon features
« Reply #21 on: 01/20/2011 02:28 pm »
From docmordrid:

Quote
Also, because of a fatal accident in 1934 Werner von Braun nixed it in his designs.

But it was still used to drive the turbopumps of the V2, and is still used to drive the turbopumps of first and second stages of R7 based launch vehicles.

It was even used as the oxidiser for the main engines of a satellite launcher-Black Arrow.

Having said that, I agree with the main opinion here, that the N2O4/MMH is the best propellant combination for Dragon. I think they will stick with this in future versions.
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Offline kevin-rf

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Re: Speculation on new Dragon features
« Reply #22 on: 01/20/2011 03:31 pm »

I am sure N2H4 will be trouble as well. But probably, due to its toxicity, nobody ever tried to use it on board a submarine.

Are you sure? I thought Soviet Era Liquid SLBM's used it... but I could be wrong and am off topic.
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Offline meekGee

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Re: Speculation on new Dragon features
« Reply #23 on: 01/20/2011 04:17 pm »
So SpaceX is weighing three types of properties:

1. Long term storage stability
2. Simplicity and reliability of engine
3. Ease of handling/Toxicity
4. ISP

In that order, pretty much.

#1 is a requirement, plain and simple, for long duration in orbit stays.
#2, because of the manned landing functionality, is a very strong design driver.  It can be somewhat circumvented by having engine redundancy.
#3 Is a desirement.
#4 is a desirement.

 #3/#4 might be reversed - it's like KH2 vs. RP1

Engine weight figures in as well, related somewhat to #2.


Any thoughts about the swing-out panel?  I've looked around and as HMXHMX posted, it's been done before.  Can anyone else see evidence of this in the promotional video? 

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Offline Ben the Space Brit

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Re: Speculation on new Dragon features
« Reply #24 on: 01/20/2011 04:29 pm »
I noticed that the sensor bay panel is shown as open as Dragon lands.  I assume that the altitude and ground drift sensors will be mounted in that bay and will deploy during the parachute descent phase.
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Offline meekGee

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Re: Speculation on new Dragon features
« Reply #25 on: 01/20/2011 09:22 pm »
Guess no one saw what these photographers eyes see: 3 landing struts, not 4 or 6.  If Dragon has 6 of those structures, perhaps 3 carry the bulk of the the launch load and the other can 3 extend for landing.

When I design things that have circular symmetry, I like 3x divisions.

In the case of landing legs (and landing engines), 4 legs are no better than 3, since if one fails, the base no longer contains the CM.  So 5 or 6 would be best.

Of course, this is a small craft - mass and volume are scarce - so it might be 3 for the legs, but I am not happy with the 4 thrusters shown.

Did anyone count how many vertical walls are in the unpressurized ring of the Dragon?

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Offline docmordrid

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Re: Speculation on new Dragon features
« Reply #26 on: 01/20/2011 10:23 pm »
8 thrusters - 4 pairs.  Count the exhausts coming off each cluster.  Bulkheads? Count for yourself....
« Last Edit: 01/20/2011 10:24 pm by docmordrid »
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Offline meekGee

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Re: Speculation on new Dragon features
« Reply #27 on: 01/20/2011 11:52 pm »
yeah, but the same argument (for anything BUT 4) holds for pairs as well...  a pair is just a more reliable, 50% throttleable-by-default thruster.

Anyway, bays - I see 4 up front, maybe 2 hiding in the cross section plane, so maybe 10 total.  or 9.  (if their designer likes 3's...)

This image shows at least 6 facing us, so maybe 12?


*I didn't say it, but 12-symmetry makes me really happy, almost to the point of crying.   Much better than 9-symmetry.

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Offline docmordrid

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Re: Speculation on new Dragon features
« Reply #28 on: 01/21/2011 12:54 am »
3's, 9, esp. 12 = OCD? ;)

Besides redundancy & easy 50% throttlability, perhaps it also means easier to design for a lower spec & more mounting flexibility? I was also thinking whoever mentioned a hypergolic Kestral might have a point re: a Dragon service module. I'd be real interested to see a good guesstimate as to what kind of ISP & thrust that might generate.
« Last Edit: 01/21/2011 12:56 am by docmordrid »
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Offline Robotbeat

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Re: Speculation on new Dragon features
« Reply #29 on: 01/21/2011 12:57 am »
If Dragon lands on legs, it will be 3 or 4 legs. Using 3 legs usually means the least mass for the necessary loads. 6 legs don't make sense.
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Offline meekGee

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Re: Speculation on new Dragon features
« Reply #30 on: 01/21/2011 12:59 am »
OCD is a requirement for an engineer.
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Offline meekGee

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Re: Speculation on new Dragon features
« Reply #31 on: 01/21/2011 01:12 am »
If Dragon lands on legs, it will be 3 or 4 legs. Using 3 legs usually means the least mass for the necessary loads. 6 legs don't make sense.

I don't see any value in 4 legs.  If one fails, you're likely to topple over.  Make it 3 to save weight, or 5 or more for redundancy.

Thrusters, same thing.  4 is useless.  you can have 8 in 4 pairs, but that's just inefficient - no better than 3 pairs, but worse than 6 spread all around, since then any single or any non-adjacent pair can fail)

And remember - more engines, without allowing for more failures, is a decrease in overall system reliability/availability.

The exact cross-over point depends on the probability of failure of an engine (or of a pair, if it's something like a feed system that fails).  It's not difficult to calculate, but you need to know the real failure probabilities or the results are meaningless.


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Offline hop

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Re: Speculation on new Dragon features
« Reply #32 on: 01/21/2011 04:30 am »
Peroxide at high concentration is very dangerous and it is not easy to handle. Even at the low concentrations you need a stablizer to keep it from breaking into water and oxygen before you can use it.
Soyuz uses H202 monoprop for descent module ACS. The only reason I can see for this choice is crew safety vs hypergols. Decomposition is an issue (it limits Soyuz to ~6mo ISS stays), but a biprop system could use lower concentration and/or stabilizers.

All decent oxidizers are nasty on one way or another.

Offline starsilk

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Re: Speculation on new Dragon features
« Reply #33 on: 01/21/2011 05:34 pm »
If Dragon lands on legs, it will be 3 or 4 legs. Using 3 legs usually means the least mass for the necessary loads. 6 legs don't make sense.

I don't see any value in 4 legs.  If one fails, you're likely to topple over.  Make it 3 to save weight, or 5 or more for redundancy.

Thrusters, same thing.  4 is useless.  you can have 8 in 4 pairs, but that's just inefficient - no better than 3 pairs, but worse than 6 spread all around, since then any single or any non-adjacent pair can fail)

And remember - more engines, without allowing for more failures, is a decrease in overall system reliability/availability.

we've seen this before from SpaceX - Falcon *9*. ie: more than is ideal.

maybe the answer to the question: "how much thrust / what ISP does a hypergolic kestrel have" would explain whey they feel the need to have eight of them.

Offline Namechange User

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Re: Speculation on new Dragon features
« Reply #34 on: 01/21/2011 05:52 pm »
maybe the answer to the question: "how much thrust / what ISP does a hypergolic kestrel have" would explain whey they feel the need to have eight of them.

That's absolutely part of it.  The engines must primarily serve as a LAS.  I'm sure the SpaceX folks are smart enough to design the system and requirements around the worse-case scenerio, which would be max-q with an acceptable rate of acceleration from the troubled rocket. 

Couple those system requirements with size of the engines for a given thrust, the packaging volume available, location of other systems/equipment, mass, CG, controlability, redundancy and the desire to use them (if all goes well during ascent) for a powered landing and you likely end up with something very close to what you see. 

I'd say so far so good personally. 
« Last Edit: 01/21/2011 05:58 pm by OV-106 »
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Offline simcosmos

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Re: Speculation on new Dragon features
« Reply #35 on: 01/21/2011 06:30 pm »
( . . . )
A lot of this discussion is speculative, but it will be fun to keep track of posts when design details are eventually revealed by SpaceX.

Numerical speculation is encouraged.  E.g. "I estimate the total LAS impulse to be XXXX N-Sec", or "There will be 6 motors, XXX lbf each".  It is ok to base speculation on calculations and other reverse-engineering techniques, not just direct interpretation of pictures and graphics.

Being "wrong" is 100% alright - that's why it is called "speculation".

Hi meekGee:

Following the spirit of the thread's original post :)

The first thing that I did when saw the 'Dragon landing on an helipad' video was to update my Falcon9-Dragon 'calculation' book, in particular the spreadsheet dedicated to  the conceptual Dragon with integrated LAS-Landing-OMS-RCS.

As a side note, from some time that I do have a brainstorm going on about a ~10t to 12t capsule with integrated Landing-OMS-RCS (although using something else other than hypergolics) but... the abort function would probably require more aggressive assumptions (which, for now, I left to a separated, more standard solid tractor LAS).


But focusing then on SpaceX: one limitation that can be used to constraint the brainstorm  could be the expected delivery capability of something like the Falcon9 block2... This gives about 10t up to 10.45t or so mass to play with for an 'advanced' Dragon...


The integrated SM already seems a bit crowded: I'm not sure if the fully integrated system (for Abort + Landing functions and for OMS + RCS) might or not reduce pressurized space (and/or lead to a slightly enlargement of the capsule), in particular if keeping the parachutes... This would require extra number / 3D crunch.


Anyway, IF using the video as any hint of intended capabilities, I would guess that something like a 9.8t up to 10t Dragon (including nose aerocover on this number) could perhaps – in pure conceptual terms - make an abort burn of about 6s (using 8 engines) while having ~12s (after nose aerocover jettison) of available power time (if using 4 engines at full thrust: in reality the burn time would be higher if using throttle control)...


IF keeping the number of engines at a maximum of 8 engines for the abort / landing system (sharing tanks with OMS / RCS) the Thrust / ISP assumed was 80kN / 268s per engine being that, for landing, less engines (4?) could be used and/or they could need to be capable of some throttle range... Haven't really researched / detailed yet about the feasibility of such engine specs vs available 3D space, required development, etc...

Moving on: the ideal abort dV would be from 404m/s up to ~413m/s or so with 490m/s up to ~503m/s being available for landing (while keeping a margin of about 5% or so)... This means that had to assume an higher propellant load than the standard 1290 kg assumed on Dragon... (had to assume a little more than 2x increment)... If no abort (nominal ascent), the 'abort dV' would be available for OMS/RCS. 

Another issue with these assumptions (adding to space requirements, development targets, etc) is that the T/W *for a pad abort* could be from ~6.7 (at start) to about 7.8 (at end of the ~6s abort burn)... I'm not sure if this could be enough for that specific abort case neither for a number of other extra abort cases (such as at maxQ or if Falcon9 got the first stage engines 'stuck' at max. thrust without MECO1 and in case 'something else' was happening, etc).


I'm being a little 'cryptic'  with these assumptions (instead of sharing an explicit set of numbers on a clear table) mostly because:

- this is just one of the sets of possibilities that have here
- haven't yet fully checked the calculations
- and also because this would require extra simulation work, outside of the spreadsheet (in slow camera mode in order to be able to 'code' some kind of more automatic thrust control method to then run the simulation in real time and play with some parameters).

Meanwhile, hope what I wrote provides a fun reading
(at least for the moment)

António

EDIT: example of other options: reducing the ISP to 200s and keeping the prop. load could give about 300m/s for abort and a little more than 360m/s for landing... increasing the ISP assumption could reduce prop. load or augment available dV for the same load, etc... similar things could be written regarding combined sets of mass estimations, thrust specs and so on.
« Last Edit: 01/21/2011 06:42 pm by simcosmos »
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Offline Patchouli

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Re: Speculation on new Dragon features
« Reply #36 on: 01/21/2011 07:16 pm »
I can see them making a trunkless version with enhanced batteries for specific use for LEO station ferry missions using a two to three orbit rendezvous.
Sorta like the solar wingless ferry Soyuz.

I can also see a propulsion module for lunar missions.
Other lunar modifications deletion of three to four seats adding a full WCS,improved galley,and various modifications for the deep space thermo environment.

Offline zaitcev

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Re: Speculation on new Dragon features
« Reply #37 on: 01/23/2011 04:30 am »
I don't see any value in 4 legs.  If one fails, you're likely to topple over.  Make it 3 to save weight, or 5 or more for redundancy.

Thrusters, same thing.  4 is useless. (...)
You are right about 4 legs, but very wrong about 4 engines. If thrust of 3 engines is sufficient, 4-engine rocket can take an engine fault and still fly even if the engines use 1-DoF thrust vectoring. Completely counter-intuitive, but trust me, it works.

Interestingly enough that was the original Masten's configuration but they never announced an engine-out capability, to the best of my recollection.

Offline meekGee

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Re: Speculation on new Dragon features
« Reply #38 on: 01/23/2011 04:46 am »
simcosmos - looking forward to seeing some of your calcs - I've been doing some (primitive) calcs myself here, it would be good to compare assumptions and conclusions...

Zaitcev - I didn't consider 1 DOF vectoring.  I thought they'd only throttle them.  If you do 1-DOF vectoring, it depends I guess on whether the line of force of the odd rocket can be made to go both above and below the intersection of the lines of force of the other two, right?   So if the rockets can swivel, I agree.

But couldn't you build the lander with fixed rockets?  It's a slower response time, but you will travel cross-range like a helicopter does - you throttle one side, the capsule tilts, the main thrust vector shifts, and off you go.   Less nimble, but simpler to build.

1-DOF swivel OTOH is not that complex, especially if you can tilt the whole panel+cluster. And a failure in the tilting mechanism is not at all fatal.

Nice.
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Offline Nathan

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Re: Speculation on new Dragon features
« Reply #39 on: 01/23/2011 05:11 am »
Could the current heat shield, large parachutes and projected soft landing system enable a landing on Mars from Mars Orbit?
Note that I'm not talking about full aerocapture from a MTO, rather just entry from Mars orbit.
My rough calc's say yes but I get lost on the impact of L/D ratio's and trajectories.

If so, or with minimal modifcation (extra fuel etc) then Dragon could become an early Mars logistics carrier when launched upon a Falcon 9H with an appropriate MOI stage.
Given finite cash, if we want to go to Mars then we should go to Mars.

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