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On 9/30/2022 at 9:59 PM, magnemoe said:

Why is half of the height of the trunk off limit and coned

That is the PAF and upper bulkhead of the second stage propellant tank.

Note that while this is the cutaway/schematic for the v1 cargo dragon, which had berthing rather than docking, the dimensions are approximately the same for Dragon 2.

On 9/30/2022 at 10:06 PM, Rutabaga22 said:

What is the difference between the draco engine and the superdraco? Is one for abort and one for another thing?

The Draco engine is a very small bipropellant engine used for RCS and in-space maneuvers. It has a vacuum-expanded nozzle and gets about 300 seconds of specific impulse in short bursts. With each burst you get a short puff at about 90 pounds of thrust, which is enough to rotate or realign the capsule but otherwise isn't particularly significant.

The SuperDracos are much, much larger bipropellant engines. They run on the same propellant as the Dracos but they are intended only for abort and they produce an immense amount of thrust for their size: up to eight short tons of thrust per engine. They are significantly more powerful than the Kestrel engine used on Falcon 1's upper stage. With all eight SuperDracos at full throttle, the abort thrust of Crew Dragon is 30% greater than the full thrust of the SLS Block 1B Exploration Upper Stage and more than half as powerful as the J-2 vacuum engine on the Saturn V third stage that propelled the Apollo astronauts to the moon.

Although the SuperDracos and the Dracos both draw from the same propellant tanks, the SuperDracos are single-use and are activated with burst disks rather than standard valves. They also have nozzles sized for sea level so their specific impulse is extremely low. They are not at all suitable for use in space. 

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On 10/1/2022 at 5:49 PM, wumpus said:

Note that Draco is optimized for vacuum, Superdraco  needs to operate in the atmosphere (and at low pressures as well), this may be a big reason in the difference in Isp.

SuperDraco doesn't **need** to operate in the atmosphere; it operates just fine in a vacuum. It is just wildly overpowered for use in space. For additional reference, a single SuperDraco is three times as powerful as a RocketLab Rutherford engine.

It doesn't operate at particularly low pressures, either. The difference in specific impulse is because the SuperDraco has a truncated nozzle to allow deep throttling since it was originally intended to support hovering landings on Earth or Mars.

On 10/1/2022 at 5:49 PM, wumpus said:

I'm guessing that it has turbopumps (couldn't find data), as it appears considerably more complicated than you would expect from a pressure fed engine.

It does not have a turbopump; it is entirely pressure-fed just like the SuperDraco. Because Dragon doesn't need to carry a bunch of dV, it can afford to have very beefy high-pressure tanks. SuperDraco's increased complexity is partly from the fact that it is regeneratively cooled using the hypergolic fuel, allowing sustained firing (which, again, really wasn't as necessary as originally planned). It is 3D-printed, though, which helps make the regenerative cooling a little simpler.

On 10/1/2022 at 9:58 PM, mikegarrison said:

SpaceX eventually got rid of the check valves by putting in burst panels, IIRC, so that really I think they have no way of shutting off the SuperDracos, much less restarting them. They are kind of one-time-use items.

The SuperDracos can still be throttled despite using burst discs for startup, and since they can be throttled they can also be shut down. They have to be shut down because the capsule can still use the remaining propellant for maneuvering/pointing via the Dracos. I believe the difference (with respect to the burst disc) is that the SuperDracos receive the full tank pressure rather than the step-down pressure that the Dracos use.

On 10/2/2022 at 10:35 AM, Beccab said:

Some info from Jared:

So, no additional fuel needed for the mission but the one in the Dragon tanks and the docking will happen backwards, with the tank. This allows the deorbit thrusters to be used to boost it and the crew of the capsule to come out of it from the usual docking hatch instead of modifying the ingress hatch for it. 

**performs brief "of course I was right" dance**.

23 hours ago, Beccab said:

The forward translational thrusters can't be fired with the Dragon nose cone closed. Obviously.

I confess I was taken aback by how large Dragon is compared to Hubble. For some reason I had it in my head that Hubble was bigger (e.g., Skylab sized) but I guess if it was carried in the Shuttle docking bay then this makes sense.

22 hours ago, CatastrophicFailure said:

I think maybe that burst disc is just a replacement for the “master valve” that allows fuel into the system, there’s other valve down the line for actually controlling the engines.

You are correct.

21 hours ago, magnemoe said:

Hubble has an docking adapter, not sure its compatible with the dragon one, as in did not understood, but the docking adapter can be swapped out. 

The docking adapter soft capture ring on Hubble is not at all compatible with the standard forward docking adapter on Dragon. They will need to do a telescoping bespoke adapter in the trunk.

1 hour ago, Beccab said:

Oh, with "airlock in the trunk" you shouldn't think it as something like Spacelab with shuttle, but more as the Apollo Soyuz docking tunnel: Dragon would deploy it from the trunk, make a 180 degrees rotation and dock it with the docking port in the nose where it can be accessed. A problem with this is that the weight constraints in the trunk are much more severe in crew missions due to the aerodynamic forces during abort, but that is something that could be solved by attaching the payload to the ascent stage instead of the trunk (again like Apollo Soyuz and every crewed Saturn V launch)

There would be a pretty nasty limitation on the physical dimensions of any such trunk airlock. There's just not that much room in the trunk. Doing an extending trunk or fairing would require re-rating the whole vehicle due to the OML changes.

And they'd still be left unable to perform a boost burn.

Here's a better render of what the reboost would look like in real-time, from an NSF member:


The soft capture ring on Hubble can't be used directly for the official "grip" because it's, well, soft. Hubble has hard "towel bars" that can be gripped by a bespoke system for hard capture.

However, if the adapter is extensible already, then it's possible that they wouldn't need the towel bars at all. They could extend the adapter, execute soft capture, and then retract the adapter, pulling the base of Hubble flush against the trunk. The trunk handles all gee-loading on Dragon during launch compressively, so the trunk will easily be able to transfer the comparatively much lower forces directly into Hubble's structure.

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6 minutes ago, sevenperforce said:

I confess I was taken aback by how large Dragon is compared to Hubble. For some reason I had it in my head that Hubble was bigger (e.g., Skylab sized) but I guess if it was carried in the Shuttle docking bay then this makes sense.

You aren't the first person I saw making this comment, interesting. And yeah, dragon is only a bit smaller than the Shuttle cargo bay (iirc, the fins actually make it bigger than it), so even if that's with KSP scaling it should be very similar to reality

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1 hour ago, Rutabaga22 said:

Does dragon deorbit with the dracos?

As @Beccab said, yes it does. There are twelve "canted" Draco engines which are used to control pitch/roll/yaw as well as perform short-distance translational maneuvers for docking, and then there are four main Draco engines under the nose cone which perform all major burns. The deorbit burn takes about 10 minutes, if I recall correctly, and expends about 100 m/s worth of dV.

When I say that the other 12 engines are "canted" I mean that they point at odd angles. This allows them to be used in pulses to rotate the vehicle around its center of mass. But it also means that firing them together is not an efficient way of giving the spacecraft more forward momentum because even though their thrust CAN balance, it ends up losing efficiency.

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1 hour ago, magnemoe said:

What does people think of Scott Manley video about the dragon Hubble mission. No eva but leaving behind an support module with reaction wheels in addition to boost orbit.  

I'm unsure whether Dragon's trunk can hold something large enough to do that.

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Feels so strange to see that shiny white new booster.

We haven't had a brand spankin' new booster launch since Starlink 4-15 in May.

Vehicle is on internal power.

And we've lit that glorious candle! Go Endurance!

Through Max-Q and throttling back up.

That nice white-and-black booster sure is purty.


Separation and second-engine start confirmed! Endurance is well on its way to orbit.

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Entry burn start. Nice pretty Eye of Sauron.

I love seeing that hard-over body lift turn into the wind at the end of the entry burn. 

SECO -- we have Endurance in orbit!

Landing burn, and down!

Very shaky coming in, though.

And a nice clean separation from the second stage. Endurance is flying free.

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