Author Topic: SpaceX Dragon XL  (Read 271660 times)

Offline Jcc

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Re: SpaceX Dragon XL
« Reply #180 on: 03/28/2020 10:16 pm »
Check out the Scott Manley video. Note: the DragonXL trunk is in the front, hence,  “frunk”.




Online TheRadicalModerate

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Re: SpaceX Dragon XL
« Reply #181 on: 03/28/2020 10:39 pm »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.
« Last Edit: 03/28/2020 10:39 pm by TheRadicalModerate »

Offline Norm38

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Re: SpaceX Dragon XL
« Reply #182 on: 03/28/2020 10:53 pm »
private Dragon flight if it could dock to Dragon XL it could have an awesome week in space.
Miniature space station, like Tiangong.

Why not?  The tech allows it.  And right now, nowhere to go but up.
« Last Edit: 03/30/2020 01:17 pm by Norm38 »

Offline Karloss12

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Re: SpaceX Dragon XL
« Reply #183 on: 03/28/2020 11:02 pm »
Even though the hardware is largely derived from legacy equipment, the Earth-Lunar transfer and docking technology will be an important boost for SpaceX experience.
And SpaceX will be making an absolute packet in profit margins for each mission as they know that they are only competing with the astronomically priced Delta IV Heavy.
This is going to be a much celebrated mission for SpaceX.  They will pocket a lot of cash and also develop themselves a new technology.

Offline Comga

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Re: SpaceX Dragon XL
« Reply #184 on: 03/28/2020 11:22 pm »
Even though the hardware is largely derived from legacy equipment, the Earth-Lunar transfer and docking technology will be an important boost for SpaceX experience.
And SpaceX will be making an absolute packet in profit margins for each mission as they know that they are only competing with the astronomically priced Delta IV Heavy.
This is going to be a much celebrated mission for SpaceX.  They will pocket a lot of cash and also develop themselves a new technology.

My bolding
Can you site evidence of anyone making a killing on a NASA contract?
Even Boeing isn't "cleaning up" on SLS.  They are just getting fixed or award fees for the last decade, and that's good enough.
The company I work for has done a many instruments and missions for NASA.  It's a good business.  Higher margin than the commodity stuff.  But it doesn't bloat the bottom line.
A big issue for SpaceX is that this is seems to be a fixed price contract, IDIQ, for a system involved with astronauts, and at any time NASA can increase the requirements.  SpaceX has experience with this, and probably didn't underbid the competition by $1.6B again. 
But it remains a risk, not a bonanza.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Robotbeat

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Re: SpaceX Dragon XL
« Reply #185 on: 03/28/2020 11:42 pm »
private Dragon flight if it could dock to Dragon XL it could have an awesome week in space.

Miniature space station, like Tiangong.

Why not?  The tech allows it.  And right now, nowhere to go but up.
Edit your quote formatting. I didn’t say that.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Cherokee43v6

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Re: SpaceX Dragon XL
« Reply #186 on: 03/29/2020 12:20 am »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.

This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.
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Offline aero

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Re: SpaceX Dragon XL
« Reply #187 on: 03/29/2020 12:43 am »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.

This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.

The Falcon Heavy second stage provides the thrust to deliver the Dragon XL to its destination. The maneuvering thrusters on the Dragon don't contribute to the time to the destination except to maneuver to the berthing portal at the Gateway. Or to the target location if the mission is prior to the Gateway's existence. In any case, there isn't any suitable application for Ion Thrusters.

And by the way, so far we have discussed the issue of disposal of the Dragon XL after the mission is complete, but I don't recall anyone mentioning the ultimate destination of the Falcon Heavy stage 2. It will be in the general vicinity, too. Isn't the Gateway at L1? Or is it L2? Will stage 2 hang around the Gateway for any unreasonable time? It should still have some maneuvering capability, could any use be made of this spent stage?

If the FH needs more total impulse to send the payload to Gateway, then the option of streching the stage 2 tanks is on the table.
« Last Edit: 03/29/2020 02:38 am by aero »
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Offline gemmy0I

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Re: SpaceX Dragon XL
« Reply #188 on: 03/29/2020 02:00 am »
This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.
Indeed, there should be opportunity for optimization here even without employing ion propulsion, and without substantially increasing travel time.

The inference/speculation around here seems to be that Dragon XL will be deployed in "high Earth orbit", something short of a full TLI (although TLI is technically a "high Earth orbit" if we want to be pedantic), with its own on-board Draco thrusters and propellant completing the transfer as a de facto third stage. This would be similar to how most GEO comsats do it, i.e. the launch vehicle sends them to GTO and they use an integrated "apogee kick stage" to get themselves the rest of the way. Just how much of the job is left for Dragon XL to do is a question I'm very curious to know the answer to (although we won't know until we get more solid numbers on its dry and wet mass).

Each launch vehicle has a different tradeoff optimum point for when it ceases being more valuable to add fuel mass to the payload's integrated stage in trade for the launch vehicle delivering that heavier total payload to a lower separation orbit.

For Falcon 9 and Heavy, that optimum point is - as best I can figure it - all the way down in LEO. This is a consequence of the fact that F9/H has a low-Isp, high-thrust upper stage. That's great for minimizing gravity losses during launch to LEO but adds a huge amount of dry mass that is more painful the higher you're going. Merlin's high thrust is more or less useless for in-space maneuvering since gravity losses aren't a factor. (The Oberth effect can factor in to a degree but unless you're switching to something extremely low-thrust like ion propulsion, it shouldn't be the dominant factor.)

The reason the optimum staging point should be all the way in LEO is that Falcon's kerolox propulsion has Isp roughly the same as what a hypergolic stage integrated onto the payload would have. So by shifting as much of the delta-v equation as possible to the payload, you're burning (approximately) just as efficiently but with far lower dry mass. There's some dry mass penalty to increasing tankage on the payload, but it's certainly far less than hauling a Merlin Vac along. At some point it might become necessary to increase the payload's own engine thrust to keep from losing too much Oberth effect, but even taking that to a hypothetical maximum, I don't think even a full ~65 tonne payload maxing out FH's capability to LEO would need anything close to a Merlin-class engine.

We've seen a gradual shift in the GEO comsat industry to account for this tradeoff since Falcon 9/H became major market players. Previously, comsats were optimized for GTO-1800 or GTO-1500 synchronous or supersynchronous transfer orbits, which was reasonably optimal for the "old school" launchers, most of which used super-efficient hydrolox upper stages (Ariane V, Atlas V, Delta IV; Proton doesn't use hydrolox but it has a relatively lightweight upper stage designed for in-space maneuvering, so its tradeoff point is probably similar). But now that comsats are being designed with Falcon in mind, we're seeing them built extra-heavy, packing lots of fuel on the satellite in exchange for a subsynchronous deployment. The satellite has to do more of the job getting from there to GEO but it results in more useful payload mass being delivered end-to-end.

Since Dragon XL is being designed expressly for Falcon Heavy, it makes sense to load as much of its own fuel on it as possible without forcing a major redesign (e.g. a bump-up to engines more powerful than Draco). We won't know to what extent SpaceX has done this until we get some hard numbers on Dragon XL (specifically its dry mass and fuel mass), but I would guess they'll pack as much fuel as they can by aggregating off-the-shelf Dragon fuel tank modules.

In principle there's no limit to how many of such tank modules you can include but at some point the plumbing would get absurd. My guess is that the plumbing/design considerations will be the limiting factor rather than the thrust-to-weight ratio of the Draco engines and Oberth effect considerations. They seem to be (sensibly) going for a "minimum viable product" here with as many off-the-shelf parts as possible, which would preclude optimizations like making all-new custom tanks, new engines, etc. Hence why it'll get deployed in "high Earth orbit" instead of in LEO, which would otherwise be the optimal point. Falcon Heavy has so much performance margin that they can easily meet NASA's requirements without having to do much optimization.

As you noted, including ion propulsion on Dragon XL could offer even more extreme optimization opportunities - I've been thinking much the same thing. The trick would be to find a way to do it without substantially increasing investment into new design and non-off-the-shelf components. If they could make it work with off-the-shelf Starlink parts that would be ideal. I could see such a vehicle being very useful as a cheap Mars orbiter or the like (e.g. something to get some close-up pictures/scans of prospective landing sites in advance of Starship, ideally to be sent in the 2020 transfer window). Starlink thrusters might be too tiny for something of this size, though. You'd have to aggregate a bunch of them and it'd likely require upsizing (or multiplying) the solar panels.
« Last Edit: 03/29/2020 02:04 am by gemmy0I »

Offline yg1968

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Re: SpaceX Dragon XL
« Reply #189 on: 03/29/2020 02:11 am »
Based on the quotes in the articles below, I am guessing that there won't be a second logistics service provider any time soon as NASA wants to keep its funds for the lander.

Quote from: SN
“We can now tell them 100% positively [Gateway] will be there because we’ve changed that program to a much more what I would call solid, accomplishable schedule,” he said. He added there were unspecified changes to the Gateway design to reduce its cost “so I don’t get into a struggle” between the cost of the Gateway and human lunar landers, suggesting there were cost overruns with the Gateway.

https://spacenews.com/nasa-takes-gateway-off-the-critical-path-for-2024-lunar-return/

Quote from: SFN
Mark Wiese, deep space logistics manager at NASA’s Kennedy Space Center in Florida, said the agency considered selecting more than one cargo transportation provider for the Gateway, but eventually settled on picking a single contractor. NASA could open up the Gateway Logistics Services contract to more companies in the future, but there is no specific timetable to do so, he said.

https://spaceflightnow.com/2020/03/27/nasa-picks-spacex-to-deliver-cargo-to-gateway-station-in-lunar-orbit/
« Last Edit: 03/29/2020 02:12 am by yg1968 »

Offline gongora

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Re: SpaceX Dragon XL
« Reply #190 on: 03/29/2020 03:17 am »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.

This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.

The Falcon Heavy second stage provides the thrust to deliver the Dragon XL to its destination. The maneuvering thrusters on the Dragon don't contribute to the time to the destination except to maneuver to the berthing portal at the Gateway. Or to the target location if the mission is prior to the Gateway's existence. In any case, there isn't any suitable application for Ion Thrusters.

And by the way, so far we have discussed the issue of disposal of the Dragon XL after the mission is complete, but I don't recall anyone mentioning the ultimate destination of the Falcon Heavy stage 2. It will be in the general vicinity, too. Isn't the Gateway at L1? Or is it L2? Will stage 2 hang around the Gateway for any unreasonable time? It should still have some maneuvering capability, could any use be made of this spent stage?

If the FH needs more total impulse to send the payload to Gateway, then the option of streching the stage 2 tanks is on the table.

We haven't seen any evidence that the FH second stage will do the TLI burn.  To the contrary, the only thing we've seen said the spacecraft would be released in High Earth Orbit, which would mean the Draco thrusters would be used to get to Gateway from Earth orbit.  This type of mission could be done with electric propulsion, but the vehicle would need more electrical power.

Offline Nathan2go

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Re: SpaceX Dragon XL
« Reply #191 on: 03/29/2020 03:18 am »
...
I'm wondering if the Dragon XL is actually modular.  I.e. that smaller section which mates to the FH upper stage is really all power and propulsion (maybe the artist moved the solar panels forward incorrectly).  Then the forward pressurized section is an optional part that gets replaced with external cargo on some missions?"
(I guess I was thinking of Cygnus).  Looking at the Scott Manley video, I'm now on board with the interpretation that the pressurized section is aft.  Scott included a screenshot from Caspar Stanley which increased the apparently length of the aft section; now the aft section appears to have much more volume than Dragon I, i.e. it seems like the right size.  The longer forward section is not just "frunk" for unpressurized cargo, it would also have the propellant tanks.

Scott also said he thinks the dry mass will be significantly less than Dragon I's 4 tons, due to lack of heat shield etc.  In that case there's less incentive to want to build a version without the pressurized section.  But they might want to make the frunk shorter if there are Gateway modules that are too wide to fit inside.

Scott talked about the minimum necessary Delta-V, but did not speculate they might go higher for reasons that Gemmy0I mentioned with regard to FH or I mentioned with regard to Starship.

For the question of how much thrust is needed: if they did want to stage from the FH stage 2 in GTO, and wanted the TLI burn to finish in 20 minutes to limit gravity losses (maximize Oberth effect), that would be an acceleration of 630 m/s in 1200 seconds= 0.53 m/s/s = 0.053 gees.  If the gross mass is 15 tons, then it needs 1,750 lbs of thrust, which would be 19 Draco thrusters, or one Super Draco operated at 10% thrust.  If the burn is spread over 3 perigee firings of 32 minutes each, then four Dracos is enough.
« Last Edit: 03/29/2020 05:11 am by Nathan2go »

Offline aero

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Re: SpaceX Dragon XL
« Reply #192 on: 03/29/2020 03:26 am »
Quote
... the only thing we've seen said the spacecraft would be released in High Earth Orbit, ...

Not to be splitting hairs, but to split hairs - Technically, Gateway is in high Earth Orbit. Just a very high, high Earth Orbit.
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Offline su27k

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Re: SpaceX Dragon XL
« Reply #193 on: 03/29/2020 03:33 am »
Quote
... the only thing we've seen said the spacecraft would be released in High Earth Orbit, ...

Not to be splitting hairs, but to split hairs - Technically, Gateway is in high Earth Orbit. Just a very high, high Earth Orbit.

I think it's more accurate to say TLI put you into a very High Earth Orbit, because its C3 is slightly negative?

So technically if FH send Dragon XL to TLI, it would still be releasing it in a (very very) High Earth Orbit...

Online TheRadicalModerate

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Re: SpaceX Dragon XL
« Reply #194 on: 03/29/2020 05:02 am »
This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.

Back when NASA still had the HLS option analysis up (I saved a copy of it here, p. 27), they addressed the topic of why they had ruled out SEP for 15 t payloads:
Quote
• A SEP Tug comparable to Gateway PPE would have a minimum Time-of-Flight (ToF)
greater than 420 days to deliver a 15 t HLS element from GTO to NRHO
(300+ days of spiral time GTO-TLI; 120 day ballistic transfer TLI-NRHO).

• This transfer time is more than 20% of the remaining schedule for HLS and it was judged
to be an impractical approach for achieving the 2024 mission timeline.

As always, to use SEP effectively, you either need tiny payloads or you have to be really, really sure of your schedule.

Online TheRadicalModerate

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Re: SpaceX Dragon XL
« Reply #195 on: 03/29/2020 05:13 am »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.

This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.
And by the way, so far we have discussed the issue of disposal of the Dragon XL after the mission is complete, but I don't recall anyone mentioning the ultimate destination of the Falcon Heavy stage 2. It will be in the general vicinity, too. Isn't the Gateway at L1? Or is it L2? Will stage 2 hang around the Gateway for any unreasonable time? It should still have some maneuvering capability, could any use be made of this spent stage?

It ought to be pretty easy to put the FH S2 into a free return trajectory for disposal.  Barring that, TLI is really just a 200 x 385,000-ish elliptical orbit.  A couple of m/s (Literally 2 m/s) of delta-v at apogee will drop the perigee into the atmosphere for disposal.

Online TheRadicalModerate

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Re: SpaceX Dragon XL
« Reply #196 on: 03/29/2020 05:35 am »
For Falcon 9 and Heavy, that optimum point is - as best I can figure it - all the way down in LEO. This is a consequence of the fact that F9/H has a low-Isp, high-thrust upper stage. That's great for minimizing gravity losses during launch to LEO but adds a huge amount of dry mass that is more painful the higher you're going. Merlin's high thrust is more or less useless for in-space maneuvering since gravity losses aren't a factor. (The Oberth effect can factor in to a degree but unless you're switching to something extremely low-thrust like ion propulsion, it shouldn't be the dominant factor.)

The reason the optimum staging point should be all the way in LEO is that Falcon's kerolox propulsion has Isp roughly the same as what a hypergolic stage integrated onto the payload would have. So by shifting as much of the delta-v equation as possible to the payload, you're burning (approximately) just as efficiently but with far lower dry mass. There's some dry mass penalty to increasing tankage on the payload, but it's certainly far less than hauling a Merlin Vac along. At some point it might become necessary to increase the payload's own engine thrust to keep from losing too much Oberth effect, but even taking that to a hypothetical maximum, I don't think even a full ~65 tonne payload maxing out FH's capability to LEO would need anything close to a Merlin-class engine.

The MVac has a vacuum Isp of about 348 s, which is comfortably higher than what you could get out of storables.  One of the big problems with using cryogenics on a spacecraft that's designed to be launched inside the fairing is that there's currently no way to load or unload propellants to the payload while it's sitting on the pad.  Beyond that, you need to vent boiloff outside the fairing to prevent a rather spectacular explosion.  (That's what that little thing sticking out of the Atlas V 5xx fairings is for, since the Centaur is inside the fairing.)

That's not to say that SpaceX couldn't do quite nicely with a hydrolox tug in the FH fairing, especially the proposed Category C fairing.  But it's a fair amount of work to do so.  My guess is that they're doing DXL because it's easy and gives them a testbed for some things they'll want to try on Starship.  Redesigning the entire payload processing system and putting cryogenics into the fairing seems a bit beyond the pale, though.

Offline docmordrid

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Re: SpaceX Dragon XL
« Reply #197 on: 03/29/2020 05:55 am »
>
Looking at the Scott Manley video, I'm now on board with the interpretation that the pressurized section is aft.  Scott included a screenshot from Caspar Stanley which increased the apparently length of the aft section; now the aft section appears to have much more volume than Dragon I,
>

Here's the original Twitter thread; rotating video, still and FH integration guesses.

https://twitter.com/Caspar_Stanley/status/1243894081534808066
DM

Online TheRadicalModerate

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Re: SpaceX Dragon XL
« Reply #198 on: 03/29/2020 06:01 am »
We haven't seen any evidence that the FH second stage will do the TLI burn.  To the contrary, the only thing we've seen said the spacecraft would be released in High Earth Orbit, which would mean the Draco thrusters would be used to get to Gateway from Earth orbit.  This type of mission could be done with electric propulsion, but the vehicle would need more electrical power.

I'd take that "high earth orbit" mention with multiple grains of salt.  It appears to be on the caption of the artwork, and those are often written at the last minute by some editor who isn't exactly an expert.  It could mean something silly like the DXL detaches from the S2 at a high altitude in its lunar transfer orbit.  There is simply no way that a small number of Dracos are going to do the bulk of a TLI burn.

My guess is that the S2 and DXL will be inserted in a somewhat eccentric orbit, simply because it doesn't need to be circular.  The S2 will then coast to perigee and provide all of the delta-v to TLI.  The DXL will then separate, and the Dracos will provide the delta-v necessary to insert into NRHO via a lunar flyby, as well as whatever is needed for rendezvous and docking.  It comes out to about 450 m/s.

If we assume a 6 t DXL with 3 t of MMH/NTO for the Dracos, I get the following max payloads to TLI, using the remaining prop from the S2:

FHE:  9.2 t
FH2R: 6.5 t
FH3R: 1.0 t

Offline soyuzu

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Re: SpaceX Dragon XL
« Reply #199 on: 03/29/2020 06:15 am »
Dragon XL will use a faring on FH and looks to be sized to use the current tankage diameter. If there were ever to be an XXL, what is the putative maximum fairing size for FH that might be used to launch it?

I think that the current fairing, which has a cylindrical static envelope of 6.7m high x 4.6m wide, has to be stretched an extra 5 m to be Category C compliant, which would make the cylindrical portion of the static envelope inside the fairing about 11.7m x 4.6m wide.  I suppose you could make an odd-shaped frunk to use some of the frustum space if you really wanted to push things, but my guess is that you'll hit mass to TLI limits if you stretch much more than what DXL appears to be doing.

This brings up a question I've wondered about a bit.

This is cargo.  It's not necessarily 'speed sensitive' on delivery.  Therefore, would it not be a reasonable consideration to increase cargo mass by going with a different proven thruster technology. Say an Ion Thruster something like DS-1 used?  Trading fuel mass for cargo mass?  So it takes weeks instead of days to arrive.  Again, it's not necessarily time critical on the delivery.

The Falcon Heavy second stage provides the thrust to deliver the Dragon XL to its destination. The maneuvering thrusters on the Dragon don't contribute to the time to the destination except to maneuver to the berthing portal at the Gateway. Or to the target location if the mission is prior to the Gateway's existence. In any case, there isn't any suitable application for Ion Thrusters.

And by the way, so far we have discussed the issue of disposal of the Dragon XL after the mission is complete, but I don't recall anyone mentioning the ultimate destination of the Falcon Heavy stage 2. It will be in the general vicinity, too. Isn't the Gateway at L1? Or is it L2? Will stage 2 hang around the Gateway for any unreasonable time? It should still have some maneuvering capability, could any use be made of this spent stage?

If the FH needs more total impulse to send the payload to Gateway, then the option of streching the stage 2 tanks is on the table.

We haven't seen any evidence that the FH second stage will do the TLI burn.  To the contrary, the only thing we've seen said the spacecraft would be released in High Earth Orbit, which would mean the Draco thrusters would be used to get to Gateway from Earth orbit.  This type of mission could be done with electric propulsion, but the vehicle would need more electrical power.
Doesn’t normal TLI put spacecrafts in a kind of High Earth orbit?

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