SpaceX Dragon 2 Updates and Discussion - Thread 3

[woods170]

Were they able to reclaim the space taken over by the SuperDraco internally (inside the pressured shell) ? Or they just cover it up on the surface without changes inside?

The SuperDracos are mounted on the OUTside of the pressure shell of Crew Dragon.
Cargo Dragon 2 uses the same pressure vessel as Crew Dragon, even the window cutouts are there (but  with pressure plates bolted over them). Barren internal volume between the two is the same.

The SuperDraco fairings on Crew Dragon house several non-SuperDraco systems as well. The same non-SuperDraco systems are present on the Cargo Dragon 2 so it made sense to feature the SuperDraco fairings on Cargo Dragon 2 as well.
Naturally the SuperDracos themselves are NOT present on Cargo Dragon 2. The SuperDraco exhaust indent on Crew Dragon is therefore covered with a plate on Cargo Dragon 2.

The mass of SuperDracos NOT being present on Cargo Dragon 2 directly translates in additional available upmass. However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

[king1999]

However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

No true. Internal volume Dragon 2 is less.
Dragon1: 10 m³ / 350 ft³
Dragon2: 9.3 m³ / 328 ft³

[soltasto]

However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

No true. Internal volume Dragon 2 is less.
Dragon1: 10 m³ / 350 ft³
Dragon2: 9.3 m³ / 328 ft³

9.3 m³ is for Crew Dragon 2, Might be (and probably is) different for Cargo Dragon 2.

[king1999]

However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

No true. Internal volume Dragon 2 is less.
Dragon1: 10 m³ / 350 ft³
Dragon2: 9.3 m³ / 328 ft³

9.3 m³ is for Crew Dragon 2, Might be (and probably is) different for Cargo Dragon 2.

No. They have the same pressure vessel.

[gongora]

Dragon 2 was designed to hold a larger volume of pressurized cargo than Dragon 1.  If you're just picking numbers off of Wikipedia I really wouldn't trust them.

[jpo234]

However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

No true. Internal volume Dragon 2 is less.
Dragon1: 10 m³ / 350 ft³
Dragon2: 9.3 m³ / 328 ft³

9.3 m³ is for Crew Dragon 2, Might be (and probably is) different for Cargo Dragon 2.

No. They have the same pressure vessel.

But no life support and all the other crew enhancements.

[cohberg]

However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

No true. Internal volume Dragon 2 is less.
Dragon1: 10 m³ / 350 ft³
Dragon2: 9.3 m³ / 328 ft³

Per the NASA OIG, usable cargo space for both vehicles is:
Dragon 1: 6.6 m³
Dragon 2: 8.6 m³

A couple notes for this (borrowing from my reddit post):

I spent a lot of time researching where Dragon 1 "lost" a lot of its pressurized volume (with the SpaceX essentially stating that only 6.6m3 of 11 m3 pressurized volume was usable). After a lot of modeling and reference materials I've come up with the following list.

Also, we all have to keep in mind that Dragon is very different than Cygnus and reusability / return is part of the volumetric constraints. Additionally, the most plausible explanation that I have come across is early F9's significantly lower payload which in turn governed D1's size / weight (and to a degree D2's).

Hatch Size Efficiency
Smaller diameter of NDS hatch on D2 consumes less space / volume when opened
Helps in the same way as Cygnus' design choice to use a CBM with the actual hatch size being smaller

Hatch that opens on the same side as side hatch (vs D1 that had the CBM opening to port (off to the side)). This means that the dead space needed for the hatches to open are shared in D2 and thus more volume efficient.

Top hatch utilizes "hole" needed for actuation for late load cargo. There is lots of space savings from recovered volume otherwise needed for CBM hatch.

Smaller bag sizes
No central stack of M01 bags (M01 bags are too large to fit through NDS hatch). This allows the vehicle to be more densely filled up.

Possible fuel efficiency (speculation: implied that the service section can be a bit slimmer)
Dragon 2 forward bulkhead dracos thrusters have no cosine losses or body impingement
Dragon 2 forward bulkhead dracos thrusters have larger expansion ratio with possibly higher ISP

Better Packaging
More compact ECLSS takes up less of the floor, and there is more room now for powered lockers

[Nomadd]

The mass of SuperDracos NOT being present on Cargo Dragon 2 directly translates in additional available upmass. However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

Wouldn't the fuel weight for non Super Draco capsules be much less?

[Lars-J]

The mass of SuperDracos NOT being present on Cargo Dragon 2 directly translates in additional available upmass. However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

Wouldn't the fuel weight for non Super Draco capsules be much less?

No, they share propellant tanks with Draco thrusters. And if the reduced SD mass is used for more cargo, then the same propellant load is necessary.

[Comga]

The mass of SuperDracos NOT being present on Cargo Dragon 2 directly translates in additional available upmass. However, like Cargo Dragon 1 the Cargo Dragon 2 is volume limited, despite the fact that cargo volume on '2' is substantially larger than on '1'.

Wouldn't the fuel weight for non Super Draco capsules be much less?

No, they share propellant tanks with Draco thrusters. And if the reduced SD mass is used for more cargo, then the same propellant load is necessary.

Please explain you logic further.
Why and how would Cargo Dragon carry the (3000 kg? of) launch escape propellants that could never be burned through the standard Draco thrusters?  Carrying that mass down also requires the fourth parachute for the safety of the astronauts. 
Are you sure the tankage is identical and filled for launch?
That doesn't make sense.
edit:  I think your "same propellant load" logic is faulty.

With the reduced safety margins (from n 9's to (n-m) 9's ) and the lower reentry mass will Cargo Dragon 2 still use 4 chutes for commonality?

[Lars-J]

No, they share propellant tanks with Draco thrusters. And if the reduced SD mass is used for more cargo, then the same propellant load is necessary.

Please explain you logic further.
Why and how would Cargo Dragon carry the (3000 kg? of) launch escape propellants that could never be burned through the standard Draco thrusters?  Carrying that mass down also requires the fourth parachute for the safety of the astronauts. 
Are you sure the tankage is identical and filled for launch?
That doesn't make sense.
edit:  I think your "same propellant load" logic is faulty.

But they do use the same propellant. There is no “launch escape propellant”. The same propellant source drives both types of thrusters. Crew Dragon does not land with 3 tons of propellant - it is mostly used up during the mission. I thought this was common knowledge.

But please explain your logic to me. Am I mistaken? It wouldn’t be the first time.

(But a slightly reduced propellant load could be plausible)

With the reduced safety margins (from n 9's to (n-m) 9's ) and the lower reentry mass will Cargo Dragon 2 still use 4 chutes for commonality?

The point of using a cargo variant of the crew capsule is for maximum commonality. So I would be VERY surprised if it has a different parachute system.

[darkenfast]

The Super Draco system and its tanks were designed to fly a normal mission (if not used for an abort), and then a re-entry burn, followed by a powered landing.  I would think that the fuel allowance for that powered landing is about the amount that could be left off a cargo mission.

Am I missing something?

I have no idea how the trade-offs would work on using the older three-chute system vs the newer four-chute system.  I'm going to take a wild guess and predict they'll go with a standard four-chute.

[thirtyone]

Based off the SpaceX philosophy of testing as much as possible, I'd imagine they would go with the four-chute system if the details of the implementation say there's enough commonality with Crew Dragon. Using the same configuration as Crew Dragon would allow them to better detect unforseen failure modes in the system, and reduce the human safety risk of unknown failure modes in the parachute system (since some flights of the system would be without humans on board).

[woods170]

The point of using a cargo variant of the crew capsule is for maximum commonality. So I would be VERY surprised if it has a different parachute system.

Exactly.

Cargo Dragon 2 uses - as much as possible - the same systems as Crew Dragon (and that is actually an understatement). This includes the parachutes and also the propellant tanks. Even the reinforced "grates" which divide the service section into multiple bays are exactly the same on Crew Dragon and Cargo Dragon 2. Heck, even the load-carrying primary mounting points for the SuperDracos, which are integrated into those "grates", are present on Cargo Dragon 2. In stead of SuperDracos they carry bridge structures with mounting points for the faux SuperDraco aerodynamic fairings

Basically a Cargo Dragon 2 is a Crew Dragon, but with the SuperDracos (and associated plumbing) removed, modified aeroshell panels over the SuperDraco area, the windows plated-over and sporting a completely different interior in the pressure shell. But everything else, right down to the crew entry hatch, the docking hatch and autonomous docking system is identical.

[Comga]

None of those principals are in dispute but no one is answering (and perhaps no one here is able to answer) my question
Yes the Dracos and Super Dracos use the same propellants.
Yes they share propellant distribution and storage systems.
Yes  Cargo Dragon will use same parachute design as Crew Dragon.
But there is no reason to add three tons of propellant that Cargo Dragon cannot burn.
Edit: Yes darkenfast: “that amount COULD be left off the mission.”
One of the advantages of storable propellants ( that weighs against their toxicity) is that they can be metered and don’t boil off. It would be simple to stop the fueling 2500 kg or do short of full.
SpaceX might even be able to leave out a few of the redundant tanks, although that’s not fundamental to the question.
It is less clear whether or not they could pack the chute bag with dunnage instead of the expensive fourth chute.

[su27k]

I thought the previous consensus is that Cargo Dragon 2 has different pressure vessel from Crew Dragon 2? It was based on some comment from Jessica Jensen during a press event, and we're told this is the reason that they can't refurbish a Crew Dragon into a Cargo Dragon. Is this no longer the case? Can a Crew Dragon be modified into a Cargo Dragon now?

[dglow]

I thought the previous consensus is that Cargo Dragon 2 has different pressure vessel from Crew Dragon 2? It was based on some comment from Jessica Jensen during a press event, and we're told this is the reason that they can't refurbish a Crew Dragon into a Cargo Dragon. Is this no longer the case? Can a Crew Dragon be modified into a Cargo Dragon now?

That was my understanding, and I distinctly recall Jensen’s comment you reference.

Discussion here, transcript here. Not sure that her comments necessarily refer to difference in the pressure vessel itself, though.

Question:
"What I mean is they'll never be used for crew Dragon, they're just dedicated for cargo?"

Jessica Jensen, SpaceX:
"Correct, yes. As soon as we build the weldment there are slight differences. So while a lot of the subsystems are the same they really are – they will be different vehicles. We won't interchange between cargo and crew vehicles."

[cohberg]

I thought the previous consensus is that Cargo Dragon 2 has different pressure vessel from Crew Dragon 2? It was based on some comment from Jessica Jensen during a press event, and we're told this is the reason that they can't refurbish a Crew Dragon into a Cargo Dragon. Is this no longer the case? Can a Crew Dragon be modified into a Cargo Dragon now?

Jessica's comment for reference

I've always interpreted that as mainly mounting points inside the pressure vessel. Looking at the interior shots the support columns, middle shelf, and floor look substantial structurally and likely don't share common mounting points with crew dragon.

[Asteroza]

How much empty space is hiding under the Superdraco subsititue fairings? Are they nominally fixed? It would be an interesting place to hide a robot arm, assuming you could get it to open...

[gtae07]

How much empty space is hiding under the Superdraco subsititue fairings? Are they nominally fixed? It would be an interesting place to hide a robot arm, assuming you could get it to open...

You could maybe hide a small arm under there... but what would you do with it?