totm nov 2023 SpaceX Discussion Thread

[Elthy]

After playing with the simulator im wondering about the actual accuracy of the Dragon 2 when it comes to velocity changes. A Draco thruster delivers 400N, the whole Dragon 2 including cargo has a mass of about 15t. I guess for translation you need two thrusters at once, so 800N/15000kg = 0,05m/s^2. Im sure a burst can be shorter than 2 seconds, so IRL they will have smaller increments than 0,1m/s as in the simulator. Does anyone know whats actualy the shortest burst of such RCS thrusters and thus the smalles possible speed change?

[RCgothic]

About 17t including up to 6t cargo.

For precise pointing yaw roll and pitch are probably reaction wheel controlled (yes yes, not as strong as in ksp. Strong enough.)

[Guest]

32 minutes ago, RCgothic said:

For precise pointing yaw roll and pitch are probably reaction wheel controlled (yes yes, not as strong as in ksp. Strong enough.)

It's not. Reaction wheels are ridiculously weak IRL. They're useless for anything but space telescopes, very much not strong enough. The Dragon has no CMGs, either, because these need a lot of room (these are used on space stations). All they use is RCS thrusters.

45 minutes ago, Elthy said:

After playing with the simulator im wondering about the actual accuracy of the Dragon 2 when it comes to velocity changes. A Draco thruster delivers 400N, the whole Dragon 2 including cargo has a mass of about 15t. I guess for translation you need two thrusters at once, so 800N/15000kg = 0,05m/s^2. Im sure a burst can be shorter than 2 seconds, so IRL they will have smaller increments than 0,1m/s as in the simulator. Does anyone know whats actualy the shortest burst of such RCS thrusters and thus the smalles possible speed change?

I suspect that automatic docking system has access to smaller rates. Manual docking, of a sort that is simulated there, has to contend, among other things, with being controlled by a touchscreen. The way I see it, it looks OK in the sim. '"Fine" rates could use an "even finer" setting, but they're usable if you don't mind wasting a few meters per second.

[sevenperforce]

53 minutes ago, RCgothic said:

About 17t including up to 6t cargo.

For precise pointing yaw roll and pitch are probably reaction wheel controlled (yes yes, not as strong as in ksp. Strong enough.)

Dragon 2 absolutely does not have reaction wheels. Reaction wheels are pointless on a craft that large and that mobile.

1 hour ago, Elthy said:

After playing with the simulator im wondering about the actual accuracy of the Dragon 2 when it comes to velocity changes. A Draco thruster delivers 400N, the whole Dragon 2 including cargo has a mass of about 15t. I guess for translation you need two thrusters at once, so 800N/15000kg = 0,05m/s^2. Im sure a burst can be shorter than 2 seconds, so IRL they will have smaller increments than 0,1m/s as in the simulator. Does anyone know whats actualy the shortest burst of such RCS thrusters and thus the smalles possible speed change?

Fractions of a second:

 

[mikegarrison]

2 hours ago, Elthy said:

After playing with the simulator im wondering about the actual accuracy of the Dragon 2 when it comes to velocity changes. A Draco thruster delivers 400N, the whole Dragon 2 including cargo has a mass of about 15t. I guess for translation you need two thrusters at once, so 800N/15000kg = 0,05m/s^2. Im sure a burst can be shorter than 2 seconds, so IRL they will have smaller increments than 0,1m/s as in the simulator. Does anyone know whats actualy the shortest burst of such RCS thrusters and thus the smalles possible speed change?

I'm gonna guess that it's all "fly-by-wire" and the computer is set up so that you click a button one time and you get a 0.1m/s change.

[kerbiloid]

4 hours ago, mikegarrison said:

I'm gonna guess that it's all "fly-by-wire" and the computer is set up so that you click a button one time and you get a 0.1m/s change.

Returning to the issue of the far future star ship (not the pretentious name of SpaceX rocket Starship) command interface and its crew...

There will be no buttons, and nobody will remember that "0.1 m/s".
Just "Hey, Ship. Dock it." or even just snap your fingers and point at the station.

Doesn't AI know itself what to do. Why should the humans stop their philosophical discussion and bother?

[mikegarrison]

1 minute ago, kerbiloid said:

Returning to the issue of the far future star ship (not the pretentious name of SpaceX rocket Starship) command interface and its crew...

There will be no buttons, and nobody will remember that "0.1 m/s".
Just "Hey, Ship. Dock it." or even just snap your fingers and point at the station.

Doesn't AI know itself what to do. Why should the humans stop their philosophical discussion and bother?

Not so far future, really. Both CST-100 and Dragon have autodock. Manual docking is only backup.

[kerbiloid]

2 minutes ago, mikegarrison said:

Not so far future, really. Both CST-100 and Dragon have autodock. Manual docking is only backup.

First they killed romantics of Saturn flight in leather helmets and glasses, now we see they lied us about Second Astronavigator and Lead Cybernetist in the crew of the ship landing on Pandora... :(

[tater]

This will be a hoot if it happens:

(article with this image is about other things: https://www.nasaspaceflight.com/2020/05/heo-tdrs-replacement-improved-artemis-testing/)

The interesting thing is that Lunar SS could easily do TLI and LOI with Orion attached. SS could return to a frozen polar orbit, and Orion could go home.

This would allow a Orion mission with no SLS—fly Orion on NG to LEO. For Lunar SS to be reused, a tanker would have to go to lunar orbit anyway, and taking the puny mass of Orion along for the ride would be trivial.

[CatastrophicFailure]

1 hour ago, tater said:

The interesting thing is that Lunar SS could easily do TLI and LOI with Orion attached. SS could return to a frozen polar orbit, and Orion could go home.

 

Good Kerm, that just looks ridiculous. 

Maybe once it actually happens some congresscritters will realize the absurdity of the whole situation and nix that titanic fund drain called SLS.

I’m not holding my breath, but...

[magnemoe]

9 hours ago, sevenperforce said:

Dragon 2 absolutely does not have reaction wheels. Reaction wheels are pointless on a craft that large and that mobile.

Fractions of a second:

Yes, they have the puffs they uses for fine control. This was also an thing back during Apollo. 

[magnemoe]

 

4 hours ago, tater said:

This will be a hoot if it happens:

(article with this image is about other things: https://www.nasaspaceflight.com/2020/05/heo-tdrs-replacement-improved-artemis-testing/)

The interesting thing is that Lunar SS could easily do TLI and LOI with Orion attached. SS could return to a frozen polar orbit, and Orion could go home.

This would allow a Orion mission with no SLS—fly Orion on NG to LEO. For Lunar SS to be reused, a tanker would have to go to lunar orbit anyway, and taking the puny mass of Orion along for the ride would be trivial.

Problem is that the moon starship can not aerobrake back into LEO, now it they fueled it up in moon orbit they could probably do an circulation burn into LEO.
Yes they might be able to aerobrake it over many orbits as crew went home in the orion anyway. You could even do the return burn with the starship and just enter orion a day or so before returning to earth, Orion lines up for landing while starship for areobrake
 

Edited May 14, 2020 by magnemoe

[RealKerbal3x]

23 minutes ago, magnemoe said:

Problem is that the moon starship can not aerobrake back into LEO, now it they fueled it up in moon orbit they could probably do an circulation burn into LEO.
Yes they might be able to aerobrake it over many orbits as crew went home in the orion anyway. You could even do the return burn with the starship and just enter orion a day or so before returning to earth, Orion lines up for landing while starship for areobrake

Starship doesn't need to head back to LEO. It would remain in high lunar orbit while Orion used the propellant in its service module to return to Earth. Though I doubt aerobraking would be too much for Starship HLS to handle. It's still made from stainless steel, so even without a heatshield it can take a respectable thermal load. 

And also, there's a way humans could be put back on the Moon without using SLS or Orion at all. Put Starship HLS in LEO, and refuel it with a couple of tanker flights. Then launch a Crew Dragon on a Falcon 9, dock with the Starship in LEO, and have the whole assembly continue to lunar orbit. Undock Starship, land, do your stuff on the Moon, and return to the Dragon in orbit. Starship can then remain in orbit while the Dragon returns to Earth, perhaps with the help of an auxiliary propulsion unit in the trunk. It's possible that this auxiliary propulsion unit could be used for an entire mission, so that a Falcon Heavy could deliver Dragon to TLI, and have this extra engine perform lunar orbit insertion, rendezvous maneuvers, and trans-Earth injection. I'm just guessing here, I'm not sure how much dV that would need, and whether it would be small and light enough to fly to the Moon with Dragon on FH.

Edited May 14, 2020 by RealKerbal3x

[tater]

2 hours ago, magnemoe said:

Problem is that the moon starship can not aerobrake back into LEO, now it they fueled it up in moon orbit they could probably do an circulation burn into LEO.
Yes they might be able to aerobrake it over many orbits as crew went home in the orion anyway. You could even do the return burn with the starship and just enter orion a day or so before returning to earth, Orion lines up for landing while starship for areobrake

I said lunar SS would go back to a lunar orbit. It never leaves lunar orbit.

It gets refilled with a tanker. The tanker can aerobrake.

[tater]

Cool (from an NSF post):

3d viewers of SS

construction facility:

https://p3d.in/4j9Pg

 

Pad area:

https://p3d.in/dGEWg

[RealKerbal3x]

It says somewhere in the LabPadre livestream chat that today's testing window is 1400-0200 UTC.

 

Edited May 14, 2020 by RealKerbal3x

[cubinator]

Does anyone know of a precise and detailed timeline for the Demo-2 launch profile?

[sevenperforce]

2 hours ago, RealKerbal3x said:

There's a way humans could be put back on the Moon without using SLS or Orion at all. Put Starship HLS in LEO, and refuel it with a couple of tanker flights. Then launch a Crew Dragon on a Falcon 9, dock with the Starship in LEO, and have the whole assembly continue to lunar orbit. Undock Starship, land, do your stuff on the Moon, and return to the Dragon in orbit. Starship can then remain in orbit while the Dragon returns to Earth, perhaps with the help of an auxiliary propulsion unit in the trunk. It's possible that this auxiliary propulsion unit could be used for an entire mission, so that a Falcon Heavy could deliver Dragon to TLI, and have this extra engine perform lunar orbit insertion, rendezvous maneuvers, and trans-Earth injection. I'm just guessing here, I'm not sure how much dV that would need, and whether it would be small and light enough to fly to the Moon with Dragon on FH.

Crew Dragon would need 410 m/s to get from NRHO to earth entry interface. Not sure how much dV it takes for rendezvous and docking, which Crew Dragon would need to do twice...let's say 100 m/s total. If Crew Dragon still carries the same prop load as its pad abort test, then it's 1.4 tonnes. At 300 s isp, that's a maximum wet mass of 8.3 tonnes which I am pretty sure is less than a lunar Crew Dragon.

An aux propulsion unit in the trunk would work but it would likely take FH out of the running for direct TLI. 

Starship can't do the Earth return burn; NASA would want independent abort capability via Dragon at any time. Honestly I don't blame them.

This is what Orion was originally designed for during Constellation.

9 hours ago, tater said:

This would allow a Orion mission with no SLS—fly Orion on NG to LEO. For Lunar SS to be reused, a tanker would have to go to lunar orbit anyway, and taking the puny mass of Orion along for the ride would be trivial.

It is pretty heavy. Still manageable, BOE, but not trivial.

[tater]

9 minutes ago, sevenperforce said:

It is pretty heavy. Still manageable, BOE, but not trivial.

Lunar SS is nominally 100-something tons empty. They claim an additional 100-150t to the surface.

Tanker SS has 100-150t cargo, but they fly it empty as a tanker. Alternately, make the orbital depot tanker a ferry. Small nose cone flips open for docking. Rest of cargo area is tankage, say 100t. Fill that in LEO, dock Orion. Fly to Moon. (so tanker has 1300t props vs 1200t)

What's the LEO/NRHO rule of thumb, something like 4kg props per kg of payload? So SS would lose ~100t of props to take Orion?

[RCgothic]

19 minutes ago, sevenperforce said:

Crew Dragon would need 410 m/s to get from NRHO to earth entry interface. Not sure how much dV it takes for rendezvous and docking, which Crew Dragon would need to do twice...let's say 100 m/s total. If Crew Dragon still carries the same prop load as its pad abort test, then it's 1.4 tonnes. At 300 s isp, that's a maximum wet mass of 8.3 tonnes which I am pretty sure is less than a lunar Crew Dragon.

An aux propulsion unit in the trunk would work but it would likely take FH out of the running for direct TLI. 

Starship can't do the Earth return burn; NASA would want independent abort capability via Dragon at any time. Honestly I don't blame them.

This is what Orion was originally designed for during Constellation.

It is pretty heavy. Still manageable, BOE, but not trivial.

Dragon is 9.5t empty with 1.3t propellant and up to 6t of cargo. Superdraco is 235s ISP, and the nozzle angle is about 20°. Dragon2 therefore has between 173 and 275 m/s in the superdracos.

I'm unclear if the Draco RCS thrusters are sourced from a different tank.

Dragon2 would need to use that 6t additional cargo for an auxiliary propulsion module, extended life support, and most likely extended RCS for all the docking manoeuvres.

[Technical Ben]

5 hours ago, magnemoe said:

 

Problem is that the moon starship can not aerobrake back into LEO, now it they fueled it up in moon orbit they could probably do an circulation burn into LEO.
Yes they might be able to aerobrake it over many orbits as crew went home in the orion anyway. You could even do the return burn with the starship and just enter orion a day or so before returning to earth, Orion lines up for landing while starship for areobrake
 

Just use lasers in orbit/on the ground to provide ablative propulsion from some moon rocks you pick up on the way. I mean, this is Elon, right? No idea is too strange. XD

Edited May 14, 2020 by Technical Ben

[sevenperforce]

35 minutes ago, RCgothic said:

Dragon is 9.5t empty with 1.3t propellant and up to 6t of cargo. Superdraco is 235s ISP, and the nozzle angle is about 20°. Dragon2 therefore has between 173 and 275 m/s in the superdracos.

I'm unclear if the Draco RCS thrusters are sourced from a different tank.

Dragon2 would need to use that 6t additional cargo for an auxiliary propulsion module, extended life support, and most likely extended RCS for all the docking manoeuvres.

Superdracos are not used for on-orbit propulsion for numerous reasons:

• Waaaay too much thrust
• Low isp, underexpanded
• Cosine losses
• Cannot be re-fired

The Draco thrusters pull from the same tanks as the Superdracos. There are four Dracos around the docking port, under the nosecone, that are used for all major orbital maneuvers since they are the only ones which have no cosine losses. Their isp is 300 s and their combined thrust is 1.4 kN; it takes about ten minutes of continuous burning just to deorbit from the ISS.

I am not sure if that 9.5 tonnes empty mass is accurate. That might be mass exclusive of cargo, which would include propellant. 

[RCgothic]

53 minutes ago, sevenperforce said:

Superdracos are not used for on-orbit propulsion for numerous reasons:

• Waaaay too much thrust
• Low isp, underexpanded
• Cosine losses
• Cannot be re-fired

The Draco thrusters pull from the same tanks as the Superdracos. There are four Dracos around the docking port, under the nosecone, that are used for all major orbital maneuvers since they are the only ones which have no cosine losses. Their isp is 300 s and their combined thrust is 1.4 kN; it takes about ten minutes of continuous burning just to deorbit from the ISS.

I am not sure if that 9.5 tonnes empty mass is accurate. That might be mass exclusive of cargo, which would include propellant. 

I've seen 9.5t specifically referred to several times as dry mass, and 11t wet plus 6t of payload is 17t, the limit of payload on F9 LEO ASDS landings so far, so it does make some sort of internal logic.

 

In Flight Abort pics:

 

Edited May 14, 2020 by RCgothic

[sevenperforce]

20 minutes ago, RCgothic said:

I've seen 9.5t specifically referred to several times as dry mass, and 11t wet plus 6t of payload is 17t, the limit of payload on F9 LEO ASDS landings so far, so it does make some sort of internal logic.

I just dug a little and came across this, from discussion of the pad abort test:

"The instrumented Dragon crew module—whose payload includes a crash test dummy, named 'Buster'—and the unpressurized trunk will be mounted atop a simulated upper segment of the Falcon 9 v1.1 rocket and a steel truss framework, which has been installed directly over the flame trench aperture on SLC-40. The overall weight of the stack will be in excess of 21,000 pounds (9,525 kg), plus around 3,500 pounds (1,590 kg) of propellant."

So this actually gives us a better picture of things. They say the payload of the crew module "includes" Buster, and it wouldn't make sense to launch with less payload than a normal flight, so I think we can say that it included enough mass to represent an actual ISS mission. There wasn't anything in the trunk, though.

Using these numbers, Dragon 2 ordinarily packs 453 m/s of dV. Enough to return from NRHO but only 43 m/s for repeated rendezvous and docking maneuvers, which obviously is not enough.

20 minutes ago, RCgothic said:

In Flight Abort pics:

 

Beautiful shots.

[RCgothic]

1 hour ago, sevenperforce said:

The Draco thrusters pull from the same tanks as the Superdracos.

Their ISP is 300 s and their combined thrust is 1.4 kN; 

Shared RCS and abort propellant is efficient, I like! This would have been awkward for landing burns though.

Ok, so 300s ISP and 1290kg propellant is 235 to 375m/s. Should be sufficient for all docking manoeuvres on a lunar mission.

*BUT* not enough for NRHO injection or TEI. Dragon would have to devote up to 2.2t of its 6t payload to get to TEI from NRHO. How much DV for NRHOI?