SpaceX Falcon 9 / Dragon 2 : SpX-DM2 : May 30, 2020 : DISCUSSION

[karanfildavut]

From the same article, Koenigsmann says mission duration may be 6-16 weeks. However he expects it to be a longer mission.

Also mentions that solar cell power generation for return trip are limiting factor, but doesnt expect there to be any problems related to their function.

Translated by me.

[sanman]

Here Doug (02:30) says it was like going through a gravel road when 2nd stage kick in, not unpleasant but the feeling was there.

So why would the upper stage portion of the ascent be bumpier than the lower stage portion?
Shouldn't it be smoother, since there's no air?

Could there likely be some corrective changes in the future, to smooth out that part of the ride? Or not worth it?

[Kabloona]

Here Doug (02:30) says it was like going through a gravel road when 2nd stage kick in, not unpleasant but the feeling was there.

So why would the upper stage portion of the ascent be bumpier than the lower stage portion?
Shouldn't it be smoother, since there's no air?

Could there likely be some corrective changes in the future, to smooth out that part of the ride? Or not worth it?

It's not about the airflow or lack thereof, it's about engine vibration/acoustics and vehicle dynamics.

As mentioned upthread, the stage 1 engines are much farther aft of Dragon than the stage 2 engine, with a large mass of rocket structure/propellant between them, which can act as an energy absorber/damper for vibration from the engines. By contrast, the stage 2 engine is much closer to Dragon, so engine vibrations have a much shorter path to Dragon, with less inert mass to absorb/damp it en route.

Also, the 9 engines on stage 1 may be somewhat self-canceling when you add their vibro-acoustic waveforms together (think noise-cancelling headphones), vs. the lone stage 2 engine that's generating its own waveform with no others engines to partially cancel its vibrations out.

And the stage 2 engine is also throttled to lower thrust levels in order to keep the G level low. Throttling the engine can induce additional vibration by increasing the "roughness" of the combustion process inside the engine.

But SpaceX has plenty of data on the vibration/acoustic environment inside the capsule from cargo Dragon  missions, so they know the environments are well within acceptable levels for human flight.

All that happened on DM-2 was humans rode it for the first time and didn't really know what to expect. Now they know.

[Eer]

Any further word on the hardwire communication connection from ISS to DM-2? I’m curious whether it turned out to be interference or something else.

Edit: clarity question is about the communication link.

[sanman]

It's not about the airflow or lack thereof, it's about engine vibration/acoustics and vehicle dynamics.

As mentioned upthread, the stage 1 engines are much farther aft of Dragon than the stage 2 engine, with a large mass of rocket structure/propellant between them, which can act as an energy absorber/damper for vibration from the engines. By contrast, the stage 2 engine is much closer to Dragon, so engine vibrations have a much shorter path to Dragon, with less inert mass to absorb/damp it en route.

Also, the 9 engines on stage 1 may be somewhat self-canceling when you add their vibro-acoustic waveforms together (think noise-cancelling headphones), vs. the lone stage 2 engine that's generating its own waveform with no others engines to partially cancel its vibrations out.

And the stage 2 engine is also throttled to lower thrust levels in order to keep the G level low. Throttling the engine can induce additional vibration by increasing the "roughness" of the combustion process inside the engine.

But SpaceX has plenty of data on the vibration/acoustic environment inside the capsule from cargo Dragon  missions, so they know the environments are well within acceptable levels for human flight.

All that happened on DM-2 was humans rode it for the first time and didn't really know what to expect. Now they know.

Thanks for the informative response. So the Merlin Vacuum Engine (Mvac) wasn't necessarily designed ab initio to be optimized for space, it's just a modified version of the Merlin sea-level engine. Is there a justification for coming up with a more dedicated vacuum engine whose performance is more optimized for space? Or would the current performance be seen as satisfactory for human spaceflight needs?

[NaN]

Thanks for the informative response. So the Merlin Vacuum Engine (Mvac) wasn't necessarily designed ab initio to be optimized for space, it's just a modified version of the Merlin sea-level engine. Is there a justification for coming up with a more dedicated vacuum engine whose performance is more optimized for space? Or would the current performance be seen as satisfactory for human spaceflight needs?

The astros described it as "not unpleasant", and making significant changes for something which is not a problem is a bad idea. Also, remember that Bob and Doug were comparing and contrasting it to their experiences on Shuttle, which has far more mass entering LEO while driven by three sustainer engines. It is not surprising that F9S2 would be a bit "gravelly" by comparison; they were just contrasting experiences as they were answering the question.

The staging process sounded much more intense, but that's inherent to true multi-stage vehicles.

[Kabloona]

Thanks for the informative response. So the Merlin Vacuum Engine (Mvac) wasn't necessarily designed ab initio to be optimized for space, it's just a modified version of the Merlin sea-level engine. Is there a justification for coming up with a more dedicated vacuum engine whose performance is more optimized for space? Or would the current performance be seen as satisfactory for human spaceflight needs?

"Performance" of an engine usually means figures of merit like Isp, etc, not vibro-acoustic characteristics, so let's be clear about that. Mvac is in fact highly optimized for space (for a kerolox engine) in terms of performance (Isp).

In terms of vibration, there too it's probably already optimized as well as can be expected. As I mentioned, the main issue there is probably due to the wide range of throttling the engine was designed for. It's hard enough to get a liquid engine to operate stably at one fixed chamber pressure, and harder still to make it stable over a wide range of pressures/throttle settings. And the need to make the engine throttleable in the first place is a result of overall vehicle design considerations that make it desirable to have high intial thrust levels on stage 2, with lower thrust later in the burn so you don't overstress the payload or astros as the propellant is expended and the stage mass gets significantly less.

So the derivation of Mvac from sea level Merlin isn't really the issue. Mvac is as optimized as a kerolox engine probably can be for space,  and the resulting vibration environment is just a result of the physics of combustion, fluid dynamics, and turbomachinery dynamics that have all been  tamed as well as the best engineering minds can reasonably accomplish with a non-infinite budget.

[OneSpeed]

"Performance" of an engine usually means figures of merit like Isp, etc, not vibro-acoustic characteristics, so let's be clear about that. Mvac is in fact highly optimized for space (for a kerolox engine) in terms of performance (Isp).

In terms of vibration, there too it's probably already optimized as well as can be expected. As I mentioned, the main issue there is probably due to the wide range of throttling the engine was designed for. It's hard enough to get a liquid engine to operate stably at one fixed chamber pressure, and harder still to make it stable over a wide range of pressures/throttle settings. And the need to make the engine throttleable in the first place is a result of overall vehicle design considerations that make it desirable to have high intial thrust levels on stage 2, with lower thrust later in the burn so you don't overstress the payload or astros as the propellant is expended and the stage mass gets significantly less.

So the derivation of Mvac from sea level Merlin isn't really the issue. Mvac is as optimized as a kerolox engine probably can be for space,  and the resulting vibration environment is just a result of the physics of combustion, fluid dynamics, and turbomachinery dynamics that have all been  tamed as well as the best engineering minds can reasonably accomplish with a non-infinite budget.

I agree with everything Kabloona has said here. Just to add, it might help to consider the difference in the vibration of a single cylinder internal combustion engine and a nine cylinder radial engine.  The radial is not as smooth as a turbine, but certainly better than the single pot. A single cylinder engine can be improved somewhat by the addition of a counter-rotating balance wheel, just as some dampening could be added to the MVac, but additional mass is the enemy of rockets.

[daedalus1]

Here Doug (02:30) says it was like going through a gravel road when 2nd stage kick in, not unpleasant but the feeling was there.

So why would the upper stage portion of the ascent be bumpier than the lower stage portion?
Shouldn't it be smoother, since there's no air?

Could there likely be some corrective changes in the future, to smooth out that part of the ride? Or not worth it?

Don't forget it's a lot closer to the astronauts than the first stage engines.

[Earendil]

To the above I would add up that probably the hyddrogen combustion might be smoother then the kerolox?

Also.. Hope some of the future Dragon riding astronauts will have experience to the Souyz, so they could compare to it

[Oersted]

Just seeing the pics of the historic crewed flight first stage coming into port.

Koenigsmann said in his Spiegel-interview that it would fly commercial missions from now on.

I think that's so wrong! It looks fantastic with the singed surface, with the NASA worm and meatball, it should be preserved at the Smithsonian together with Crew Dragon and a 2nd stage.

I would be very sad to see the NASA symbols scrubbed off and replaced with a company logo for another flight. Save the stage I say!

[cuddihy]

Another minor factor on stage 2 bumpiness could be gimballing/guidance related. If you’ve watched second stage flight for a lot of F9 launches, the “digital,” very discrete movements of the gimbals for MVac are noticeable. It sometimes looks like it’s a bit jerky.

Add in the single nozzle extension & it no doubt causes some bumps up front.

[CorvusCorax]

I think the ride experience of Dragon on F9 second stage becomes quite obvious if you just look at the vehicle configuration.

The thing is, we rarely get an outside view of this outside of renderings, because the only cameras are on board. It's too far for the tracking cams.

But there are a few nice renderings, this one is from SpaceX rendered DM-2 preview video:

Just look at the proportions:

1/3 of the vehicle: Dragon and its trunk
1/3 of the vehicle: Propellant
1/3 of the vehicle: Big F...alcon... Engine with a nozzle extension for more boost.

It doesn't really take more than one glance at this and realize this is going to be a sporty ride.

[CorvusCorax]

Just seeing the pics of the historic crewed flight first stage coming into port.

Koenigsmann said in his Spiegel-interview that it would fly commercial missions from now on.

I think that's so wrong! It looks fantastic with the singed surface, with the NASA worm and meatball, it should be preserved at the Smithsonian together with Crew Dragon and a 2nd stage.

I would be very sad to see the NASA symbols scrubbed off and replaced with a company logo for another flight. Save the stage I say!

Company logo? The only part on commercial flights that typically bears company logos (aside from SpaceX's) is the fairing.

I don't think SpaceX will do anything to the worm or meatball. In the past they haven't even washed the stages anymore for reflight. It will proudly fly again and again, as long as the stage manages successful landings.

What I could see SpaceX do is reserve the stage for flights with a comparably high chance of landing success as opposed to edge-of-the-envelope ones.

[ugordan]

I don't think SpaceX will do anything to the worm or meatball.

It may not be as simple as that, "legally" speaking. Apparently NASA was reluctant in the past to allow their insignia on the F9 for certain reasons (like being associated with SpaceX if the rocket should fail). The same argument could be applied to seeing NASA logos on either Starlink or other commercial missions. It could be construed as NASA sponsoring those launches.

[Oersted]

....For all of those reasons.. Save the stage!

How deep are the pockets of the Smithsonian and its sponsors? Perhaps they could chip in. ...And get Hopper as well: it deserves a better fate than glorified radar and surveillance video pole.

[Welsh Dragon]

The best thing the stage could do as far as I'm concerned is have a long and productive life launching payloads. Maybe see if it could be the first one to get to ten.

[eriblo]

To the above I would add up that probably the hyddrogen combustion might be smoother then the kerolox?

Also.. Hope some of the future Dragon riding astronauts will have experience to the Souyz, so they could compare to it

Everyone but Glover on Crew-1 has flown on the Soyuz so there is hope for some comparative comments regarding both launch and splash down vs. landing.

[woods170]

Just seeing the pics of the historic crewed flight first stage coming into port.

Koenigsmann said in his Spiegel-interview that it would fly commercial missions from now on.

I think that's so wrong! It looks fantastic with the singed surface, with the NASA worm and meatball, it should be preserved at the Smithsonian together with Crew Dragon and a 2nd stage.

I would be very sad to see the NASA symbols scrubbed off and replaced with a company logo for another flight. Save the stage I say!

Say all you want, but the decision was made to refly this stage, like the other stages, for other missions. This decision was made long ago.
You seem to be forgetting that, when properly executed, a single F9 booster stage can provide SpaceX with a very healthy profit over its life time (north of $150 million). SpaceX needs that profit to ultimately go to Mars.
Sacrificing a stage to some museum, just because it launched DM-2, is the dumb thing to do (from a SpaceX standpoint).

Quite frankly I don't think there is any museum out there willing to reimburse SpaceX for lost profit in the order of $150 million.

So, the NASA worm will be erased and the stage will be refurbished to fly another day. The same is likely to happen to at least 3 other F9 booster stages, given that it will take SpaceX at least 3 operational missions to convince NASA to refly booster stages on CCP missions.








Edit: June 3, 2020. Only one day after my post it became clear that NASA is allowing SpaceX to start flying reused boosters and reused Crew Dragons from the second operational crew mission forward. Huge win for SpaceX and I'm glad my initial assessment was off by two flights.

[woods170]

....For all of those reasons.. Save the stage!

How deep are the pockets of the Smithsonian and its sponsors? Perhaps they could chip in. ...And get Hopper as well: it deserves a better fate than glorified radar and surveillance video pole.

Emphasis mine.

Because it flew a single Grasshopper style mission? In case you had not noticed: Starhopper didn't do anything that hadn't already been done by Grasshopper. And the latter is still sitting pretty in Texas instead of in the Smithsonian.