SpaceX Dragon XL

[baldusi]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

Do you have a link to that document?

[rakaydos]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

It puts all the heavy complicated bits in one place, to be joined with a simple tank and an unpressurized payload collar on the other end.

[DistantTemple]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

It puts all the heavy complicated bits in one place, to be joined with a simple tank and an unpressurized payload collar on the other end.

This is a way out suggestion.... but one possible benefit for future alternative uses of DXL .... If thee is a tube or tunnel to traverse the service section, this could later be redesigned as an airlock. However I think that's wishful thinking.

[Robotbeat]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

It puts all the heavy complicated bits in one place, to be joined with a simple tank and an unpressurized payload collar on the other end.

Also means they don't have to redo as many things if they want to make it longer.

[su27k]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

I think it's because Dragon XL is using thruster/docking port arrangement similar to Dragon 2, where the 4 inline thrusters are placed around the docking port. Since the thrusters are already near the docking port, it makes sense to place the tanks and other stuff there too.

[TheRadicalModerate]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

I'll speculate:  Because then they can remove the dock and the pressure vessel, stretch the tanks, and turn DXL into a tug for taking Starship payloads direct to GEO.

Starship can get 21 t to GTO and still de-orbit and land.  But it can get zero tonnes to GEO, even if it were expendable, and it takes 700 tonnes of tanker prop (7 launches, more or less) to get both to and from GEO reusably.

An expendable tug version of DXL would almost certainly be cheaper than the tanker launches needed to go all the way to GEO with Starship.  If you arm-wave the early flights of Starship at $20M (which I think is optimistic, until we get to a mature version of Starship), then a direct GEO launch costs $160M and likely takes three days to get on station from when they start launching prop for the mission.  I'm guessing that an expendable DXL tug (essentially a third stage for Starship) costs less than $50M, so the mission costs $70M, and it'll get you on station within a couple of days.

Weasel words:  Dracos are significantly underpowered to do this job quickly, and I'm not sure that they have the design life to do it.  You might need a pair of SuperDracos attached to the back of the trunk for this tug.

[TheRadicalModerate]

Where does the PAF attach to the DXL?

[woods170]

Where does the PAF attach to the DXL?

Ring around the docking collar.

[dror]

Weasel words:  Dracos are significantly underpowered to do this job quickly, and I'm not sure that they have the design life to do it.  You might need a pair of SuperDracos attached to the back of the trunk for this tug.

Or maybe that could fit in the docking tunnel

[baldusi]

Where does the PAF attach to the DXL?

Ring around the docking collar.

The source selection document criticized Dragon XL because the SV/LV was not well defined. I think it was the only technical "weakness" on Dragon (apparently they consider Falcon Heavy performance "unclear", too).

[Alexphysics]

Where does the PAF attach to the DXL?

Ring around the docking collar.

To be more specific: you mean the actual docking system or the part around it?

[Alexphysics]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

This is literally what SpaceX already does with Dragon 1 and Dragon 2, I don't know what's the difference in here.

[woods170]

Where does the PAF attach to the DXL?

Ring around the docking collar.

To be more specific: you mean the actual docking system or the part around it?

Not the actual docking system, which is unsuited to link Dragon XL to the FH S2 PAF.

Dragon XL attaches to FH S2 PAF via the larger ring around the docking system. Within this ring are also mounted four Dracos.

[woods170]

From source selection statement:

Its design approach would locate the service section of the Dragon XL between
the pressurized volume and the Gateway, meaning crew would have to translate through the
service section, which is mechanically active.

Any speculation regarding why SpaceX would dock through the service section? This seems to be unique among cargo spacecraft.

This is literally what SpaceX already does with Dragon 1 and Dragon 2, I don't know what's the difference in here.

Uh no, it is not for Dragon 1 and Dragon 2.
On Dragon 1 and Dragon 2 the service section (housing the propellant tanks, ECLSS tanks and most of the Draco thrusters) is located around the bottom of the pressure vessel, near the primary heatshield. At the top of the Dragon 1 / Dragon 2 pressure vessels are the berthing/docking tunnels.

On Dragon XL the service section is moved from the bottom of the pressure vessel toward the top. The service section is literally situated around the Dragon XL docking tunnel. The propellant tanks and ECLSS tanks are all arranged in a cicle around the docking tunnel. Draco thrusters are no longer integrated flush into the outer moldline but are now mounted in external pods, much like the thruster clusters on the Apollo service module.

This arrangement has several advantages. Primary being that the main pressure vessel is now a straight cylinder: optimal shape for a pressure vessel. The outside of the blunt bottom end of this cylinder make for a great platform to mount externel cargo.
But it also meant that the stuff from the service section had to go elsewhere. And elsewhere in this case means the top of the pressure vessel. But, the docking system also needs to be there. So the service section equipment was arranged around the docking tunnel.

By the way, having a docking tunnel through the service section is nothing new. A similar setup exists on the aft end of the Russian Zvezda module of the ISS. Same for the Mir Core module and the core modules of Salyut 6 and Salyut 7.


I wouldn't be surprised at all to learn that SpaceX took inspiration from previous Russian/Soviet designs. They have done so before.

[eriblo]

I imagine a fresh intern sitting at Hawthorne late one night and Elon passing by on his way out, stoppning suddenly: "Hey, what do you think about sending an expendable Dragon the Gateway?" He throws a USB with some Dragon 2 schematics on the desk: "Take a look, see if you have any ideas. You know, cheap, minimum change, maximum cargo, that sort of thing... See you in the morning!"

Right... Delete hatch, windows and Superdraco nacelles with engines and plumbing. Extend pressure vessel forward slightly to increase volume. Hmm, now it almost looks like a cylinder... Make the upper conical section a cylinder. Delete heatshield with support structure - whoops, deleted the trunk as well. Copy the trunk back in as dawn approaches, accidentally start attaching it to the wrong end. Although that end now has the right diameter and much simpler structure... But then the docking port and those dracos have to move. Eye the now bare underside of the narrow bottom well of the pressure vessel and then the clock, grab the docking adapter and dracos and slap them on the bottom just as somebody walks up from behind.

"That actually looks quite good. I like your thinking with launching it upside down - although using the docking port as an adapter might be a bit too Kerbal. You've played, right?

"Yeah, right... If it works in KSP!" Eyes the screen where the copy/paste monstrosity now hangs upside down. Totally intentional. "Although you need to put a fairing on the tr-"

"Exactly! Launch it in a fairing and we can make it so much lighter. Great job, email me that file." Elon nods to himself and heads towards his office.

 

[Zed_Noir]

....
"Exactly! Launch it in a fairing and we can make it so much lighter. Great job, email me that file." Elon nods to himself and heads towards his office.

 

One minor nitpick. Musk does not have an office, just a desk at SpaceX AFAIK.

[DreamyPickle]

I guess it does vaguely resemble Zvezda or the Mir Core but those are the station modules with docking ports at both ends so there is no choice but to place equipment around the habitable area.

I wonder if DragonXL will ever get a second docking port and serve as a station module? This design does make it easy to add.

[baldusi]

[...]
By the way, having a docking tunnel through the service section is nothing new. A similar setup exists on the aft end of the Russian Zvezda module of the ISS. Same for the Mir Core module and the core modules of Salyut 6 and Salyut 7.

I wouldn't be surprised at all to learn that SpaceX took inspiration from previous Russian/Soviet designs. They have done so before.

Wasn't the original ESA freeflier arranged like this? I can't'recall if ATV was based on that concept or if it was derived from ATV, but they could connect in series because they had a docking port on aft section inside all the propulsion and ECLSS elements.

[ThePonjaX]

I guess it does vaguely resemble Zvezda or the Mir Core but those are the station modules with docking ports at both ends so there is no choice but to place equipment around the habitable area.

I wonder if DragonXL will ever get a second docking port and serve as a station module? This design does make it easy to add.

Please read the previous posts on the thread. These has been already discussed ... a lot.

[Paul451]

An expendable tug version of DXL would almost certainly be cheaper than the tanker launches needed to go all the way to GEO with Starship.

Would it have to be expendable? Fuel is cheap and customers don't need 20 tonnes to GEO, so recovering the tug would make launch cheaper, which customers do like. Customers who do want to go big or launch BEO can still pay more to expend the tug.

Weasel words:  Dracos are significantly underpowered to do this job quickly, and I'm not sure that they have the design life to do it.  You might need a pair of SuperDracos attached to the back of the trunk for this tug.

A single SuperDraco with its mount inside the tunnel where the docking collar currently is, fully-expanded bell nozzle hanging out the back. The theoretical maximum Isp for NTO/MMH is around 330s.

How does that change your payload numbers?