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Dr. Stanley provides an overview of space activity in the last month, at the regular meetings of the NSS North Houston chapter, as reported in the topic for that organization.
He posts videos about space and other subjects on YouTube, and articles on LinkedIn.
This topic is available for postings of material by Dr. Stanley that might not otherwise make it's way into the forum.
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This update is on feedback on the presentation about a possible inertial separation of Starship stages, first given at the May meeting of the North Houston chapter of the National Space Society. Video's about the presentation are available on YouTube.
Sorry for the delay. I'm busy now researching Saturday's talk. I'll eventually get to your request to post something about the signup process for your site, although I may have already forgotten the details.
On your site, I looked the discussions involving the comments you passed back & forth between Gary and me, but couldn't find much. I guess a word for the site is "sprawling" and I haven't spent enough time on it to really figure out how to find things of interest yet.
One thing that may interest you is that the kind of comments Gary made also showed up in the comments in that YouTube video "Starship's innovative stage separation ... " at
. I was surprised at how much interest that video got. It's normally unusual if I get more than a few hundred views, maybe a few comments, and a few "likes". But so far that one registered over 69,000 views, 2,221 "likes" and 237 comments !
There were a lot of comments about how stage separation for an abort with a crew would work, how violent or long lasting the rotation will be, ullage (need for thrusters to ensure that the stage 2 engines will get liquid to start after separation), the destruction of the concrete pad, my mistake in saying the booster would do a "belly flop", my mistaken comment that the angular velocities of the individual stages would differ from the original velocity of the combined ship, my mistake in noticing that the Falcon 9 uses detonation cord for their "extended fairing", but somehow skipping over that they use the helium-powered pistons to push the standard fairing apart, etc.
To answer questions about the feasibility of that stage separation method, I started a spreadsheet to work through the centers of mass, moments of inertia, kinetic energy, angular velocity and linear velocity. The distribution of mass still needs work before really using it, but it's enough of a start that I think the method will be workable without involving any vomiting by astronauts. There's details in the comments, but here's a summary, pulled from several comments, of my start on the calculations:
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re: Angular velocity of stages matches that of ship
Yes, the angular velocity of the separated stages matches the initial angular velocity of the ship, as I realized when I finally worked through the calculations. No matter what sort of variations apply (stage lengths, stage weights, distribution of weight within a stage due to payload or propellant), the end result is always the same angular velocity for the separate stages, matching the initial angular velocity. I'll have to look into how easy it is to modify videos already out on YouTube to fix this.FYI, I worked through the calculations with a finite element approach, dividing the ship into 200 segments to place the mass based on dividing up the empty stage masses, the propellant masses, and the payload according to locations in the ship diagrams. Once that's in place, the spreadsheet calculates (before and after separation) centers of mass, moments of inertia, rotational and translational energy, linear velocities of the centers of mass, and verifies angular velocity for the separated stages given an initial angular velocity for the ship as a whole.
Based on dimensions and weights from Wikipedia, and eyeballing the rough locations of propellant tanks, etc., it looks like the center of mass (CM) of the ship prior to clamp release is around 255 ft from the bottom, which places it 25 feet up into stage 2. Stage 1 CM is at 101 ft. and stage 2 CM is at about 281, all out of a total height of 394 feet. Exact numbers depend on assumptions on stage 1 propellants remaining (I assumed 2% for my base case, and the CM for stage 1 is sensitive to that assumption), exact locations of the propellants in either stage which I only roughly approximated, and payload weight and distribution. This is just "ball park", and doesn't account for the heavier dry mass at the bottoms of stages due to the engines, or the differences in mass between LOX and methane (yet). This will be improved later.
People have been speculating on how violent and lengthy the rotation needs to be. From the test timeline at
https://www.space.com/spacex-starship-f … -explainer ,
there's only 5 seconds between clamp release on 2nd stage engine start.If I assume an initial 0.1 radian/sec rotation (about 5.7 degrees/sec) about the ship center of mass, someone sitting at 80% of the height of stage 2 would experience that as about 10 - 11 ft/sec (right angles to the longitudinal axis). If the ship takes, say, 2 seconds to get to the desired angular velocity before MECO, that would imply a (lateral) acceleration of about 5 ft./sec^2, way less than normal gravity. The centers of mass of the stages after separation would move apart at about 18 ft./sec. So in 5 seconds, the stages center of masses drift apart by about 5*18 = 90 feet. In those same 5 seconds, their stage would have rotated 5*5.7 degrees = 28.5 degrees. And, as pointed out elsewhere, the stage 2 exhaust would be rotated, not pointing directly at stage 1. Are these acceptable sorts of numbers to not be considered "violent", and safe to avoid damage to the booster? Another commenter, @hendrik1745 , has suggested doubling that initial velocity, but has concerns about how the geometry really works out. This all needs to be refined, but suggests that the method won't be that stressful for an astronaut compared to all the other stresses from normal acceleration, going supersonic, decelerating before maxQ, maxQ, re-accelerating after maxQ, and then main engine cutoff going from high acceleration to weightlessness for the first time. We will probably just have to wait to see what happens when the method is tried, hopefully the next test.
Elsewhere, I did some look at the lateral acceleration an astronaut would experience from the rotation. It was pretty mild, way less than a G, and not lasting very long.
Another guy suggested 0.2 radians/sec might be a better choice, but I think we're in the ball park.
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And another exchange,
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re: ullage, subcooled liquid and autogenous pressurization helping, etc. :
@tedarcher9120 • 2 weeks ago
How would you light up the engines after that maneuver though? There is going to be a lot of bubbles in the tanks everywhere, you need starter motors anyway, why not just use them to separate?gstanley0
• 2 weeks ago
This is just speculation, but I could think of a few possible reasons.
(1) Because Starship uses autogenous pressurization in the tanks (same pressurizing gas as the propellant) rather than a gas like helium, and significantly subcools both propellants (CH4 and LOX), it's likely that bubbles are less of a problem in the first place. Bubbles somehow forming in the liquid would just collapse and get absorbed into the liquid. That's unlike the case with, say, the Falcon 9 kerosene and helium, or LOX and helium. Although full autogenous pressurization won't have started yet in stage 2 (because of no heat from Stage 2 engines for vaporization), again, because there is so much subcooling in the loaded propellants, there probably hasn't been much time for much vapor to form. (And, really pure speculation) There might be some electrical heaters available for pressurization prior to engine firing, and maybe they even increase the pressure just prior to firing the engines to suppress bubble formation?
(2) The second stage is full just prior to separation and lighting up, so there is little vapor in the first place. The only vapor that would form would be because of heat passing into the tank from outside and vaporizing propellant. I realize that's not a completely convincing argument because you could say any second stage will initially be full, but with the subcooled liquid, until main (booster) engine cutoff, all the vapor would be up high, with little time to move down, and bubbles won't form spontaneously in subcooled liquid.
(3) The rotation must help, just like in a lab centrifuge used to separate things by density. Any gas bubble is lighter, and would tend to get displaced by the heavier liquid flowing to the bottom(outside), analogous to the case on earth where steady gravity keeps the liquid on the bottom. Unlike the case with linear acceleration, where things immediately start floating around as soon as the external force ( propulsion) stops, with rotation, the heavy liquid stuff would always flow towards the bottom.(4) Lighting up stage 2 might be fairly quick, before there's much time for bubbles to make their way down to the bottom of the tank
(5) Starship has those small "header tanks", which are better insulated than the main tanks. They are always kept completely full of liquid, to ensure that even after a long time (unlike the case of initial launch), engines can be restarted using them before switching over to main tanks. They are there to handle landings without worrying about vapor, because by then subcooling would be lost and also the tanks are no longer full. They're only billed as being used for landings, but perhaps they could be used in the initial firing as well? My guess is they don't need to do that because of the other reasons above.@tedarcher9120 • 2 weeks ago
@gstanley0 that's exactly my point. As starship rotates, heavy liquid will be pushed forward to the nose, and vapour at the top will flow towards the engines, snuffing them out. You'd need to stop the rotation and settle the liquid before lighting engines@gstanley0
• 2 weeks ago (edited)
After separation, rotation of stage 2 will be around the center of mass of stage 2. Heavier stuff (liquid) will flow outwards from there. I don't know exactly where the center of mass is, and it would depend on how heavy the payload is. But, looking at a Starship diagram, e.g., at
https://everydayastronaut.com/definitiv … -starship/
it looks like the main LOX tank, the lower of the propellant tanks, is most likely all below the center of mass.
(I'm using "top" and "bottom" to refer to the configuration when standing vertically at launch time, with the engines at the bottom).
So, with liquids flowing outwards from the center of mass, that means most or all liquid oxygen will go to the bottom, keeping the engines fed with liquid. It's not as clear in the case of the methane tank. The main methane tank center looks to be somewhat below the geometric center of the rocket. As long as some portion of the methane tank is in fact below the stage 2 center of mass, then liquid methane flowing outward will keep the bottom of methane tank liquid. If that center of mass is indeed somewhere in the methane tank, then some liquid will also flow to the top of the tank. That would mean that any methane vapor would move towards that center of mass, which would be between the top and bottom of the methane tank. Eventually the vapor would all go there, but the stage 2 engine would start long before could happen... (see next comment)...@gstanley0
• 2 weeks ago (edited)
(continuing...) I found a some details of the originally-planned timeline at https://www.space.com/spacex-starship-f … -explainer
Booster Engine cutoff is at 169 seconds into the flight, separation is at 172 seconds, and upper stage engine start is at 177 seconds. Up until engine cutoff, the liquid in both propellant tanks would be solidly at the bottom because of the linear acceleration. Then, there would be 3 seconds between booster cutoff and separation, when the rotation is about the center of mass of the combined ship, and the methane vapor could go down, as you were describing. It's not as clear for the LOX, because the center of mass of the entire combined ship might be in the LOX tank, or maybe not. But then there are 5 more seconds when the rotation just around the second stage center of mass would be helping to get propellant liquids back closer to the bottom. Given that there would be very little vapor in the first place (and starting with none in the wrong place), the SpaceX engineers must have worked all this out, and concluded it was unlikely that vapor would make it into the lines to the engines.@hamjudo • 10 days ago
@gstanley0 some more supporting points. The methane downcomers are large pipes and thus represent a significant volume. The volume in the pipes below the center of mass is more than sufficient to start 3 engines.SpaceX has tested a number of engines to destruction. They must know how well the engines recover from "small" gas bubbles. Where the upper end of "small" is defined as the largest bubble that is reliably recoverable.
The start up sequence itself involves running high pressure gas through the turbo pumps to get them up to speed. This gas is expelled out of the bottom of the rocket and will act just like a cold gas thruster.
-----------------------------------------------And another exchange on human-crewed aborts with the new separation :
Several people have commented on this. For now, the focus is on getting the rocket ready for cargo only, following the usual SpaceX approach of doing that first, just like they did for the Falcon series. There's plenty to do for that as the top priority. That's a faster way to get to a minimum sellable product.
For the longer term, I'm sure they thought about that. It doesn't seem that unsolvable. For instance, as one other commenter pointed out, in an emergency, simply start the stage 2 engines and blast away - saving the booster isn't crucial in that case.It's probably better than that. If the booster has burned out, then ullage (ensuring that only liquid propellant reaches the engine after some free fall) and some push-off from the booster can be provided by the cold gas thrusters at the bottom, fed from the propellant vapors (available because of the autogenous pressurization) and also because of the startup of the turbopumps at engine start. If the booster is still firing, then you already have the acceleration needed to keep the propellants liquid and still can blast away.
Emergency escape does depend on the unclamping working. But that approach has already been accepted as safe in the Falcon 9 series (and probably other rockets -- there's probably something like clamps to hold stages together). In the Falcon 9 case, they use pneumatic pistons to release the clamps. I don't know what they're using on Starship, but if it's pneumatic, the principles are the same. The difference might just be that there's no pressurized helium available like they use on Falcon 9. If they're still using pneumatics, maybe they use oxygen or methane gas, the only gases available -- I don't know. That's something to explore.
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- regards,
Greg
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Yes, we know that something happened at the point of stage separation but the question is did the stage intersection just fold and keep it from doing the stage departure not work.
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For SpaceNut re #3
Thanks for giving this new topic a boost!
With any luck, we may get to see further development of the Inertial Stage Separation analysis in days ahead.
As a reminder to all ...
Dr. Stanley will present his report on space activities today, at the NSS North Houston meeting.
The meeting is free and open to the public, on site or via Zoom.
Please see the North Houston topic for details.
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GW Johnson requested a link to one of Dr. Stanley's YouTube videos ...
The link above points to a movie / recording of his presentation to NSS on November 11th.
While the presentation was about multiple topics, Asteroid Bennu is one of the topics presented.
I think the link will work for everyone, but if it does not and if someone would like to see the video, please post a request here.
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As an FYI, Dr Greg Stanley has his own YouTube channel. Anyone with a YouTube account can subscribe to Gregs channel (or Google account, Google owns YouTube). The channel is under the name "gstanley0". Here is a list of some of his more recent videos (Including the one Tom listed in post #5)
MonthlySpaceNews 2023/11: Asteroid prospecting Psyche, Bennu; space manufacturing; regulatory crisis
47m 7s
Industrial revolution in space (focusing on Low Earth Orbit) ... and regulation slowing it down
27m 45s
Asteroids: visiting metal-rich Psyche, and returning samples from Bennu
18m 30s
MonthlySpaceNews 2023/09: Moon landing, crash, launch. Kuiper. Orbital debris. LEO/GEO. Starship
37m 0s
MonthlySpaceNews 2023/08: 2 Moon launches, Chinese win methalox orbit race, Euclid update, launches,
19m 28s
Starship design: simplify, simplify, simplify. Eliminating consumables, also new hot staging.
28m 10s
Monthly Space News 2023/07: Starship hot staging; Euclid Space Telescope; Virgin Galactic; launches
18m 24s
Monthly Space News 2023/06: Blue Origin as 2nd Moon lander; Virgin Orbit gone; recent launches
13m 25s
Wood Satellites! Finland vs Japan! Why?
7m 14s
Fault management overview (emphasizing aerospace)
57m 26s
Hakuto-R lunar lander crash: A failure in fault management
24m 40s
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