totm nov 2023 SpaceX Discussion Thread

[tater]

landed, nominal orbit

 

[kerbiloid]

7 hours ago, Minmus Taster said:

it will work as a heavy lift vehicle

There was a reusable shuttle, with 30 t of cargo. It was to replace all other rockets.

But it was never flying with payload even close to its capability.

The problem was not in the shuttle, it was in the presence of payload.

5 hours ago, mikegarrison said:

There are payloads that a single SLS can launch but no number of F9s can launch.

And SLS has a great advantage, it's a foot-in-the-door for combat missile manufacturers, so it won't get gone.

6 hours ago, SunlitZelkova said:

Would something like KORD help SH?

The N1 engines weren't gimballed.

3 hours ago, AckSed said:

Hey, do you like spaghetti? I like spaghetti. (Internal structure of the Super Heavy's propellant system.):

https://ringwatchers.com/article/booster-prop-distribution

Looks Lovecraftian, like N1 inside in descriptions.

[Ultimate Steve]

23 hours ago, Exoscientist said:

Two separate, independent methods suggest SpaceX throttled down the booster engines < 75%, while the Starship engines fired at ~90% thrust:

Did SpaceX throttle down the booster engines on the IFT-2 test launch to prevent engine failures?
http://exoscientist.blogspot.com/2023/12/did-spacex-throttle-down-booster.html

 This is important to know because if the engines need to operate at < 75% to be reliable, then I estimate the reusable payload would be lowered from 150 tons to ~100 tons. Then instead of needing perhaps 16 refueling flights for the Artemis landing missions there would need to be perhaps 24.

  Robert Clark

I had a big long thing typed out but then realized I had multiplied by the wrong number somewhere and had to restart.

In short, I have a large number of problems with the assumptions you made, but Raptor is indeed not operating at 100 percent of its advertised thrust. I have more problems with your conclusions, but let's just walk through the re analysis for now. This analysis only covers stage 1.

I initially went to the raw video but then downloaded the data used to make the graphs you used after I had to restart after I was confident they matched well enough.

I added columns for acceleration, smoothed acceleration, and expected mass at that point given the linear decrease you would expect at full throttle (this is not a valid assumption beyond the first 30 seconds or so of flight as any off nominal throttle would cause the mass to diverge). This analysis also assumes vertical flight by adding 9.81 to the acceleration, this is also not a good assumption outside of the initial 30 seconds. I then made a column for the current thrust, and another column for the fraction of the expected thrust (throttle).

From my own double checking, Raptor is producing at or above 90 percent of its advertised thrust for at least the first 30 seconds of flight .

Spikiness is probably down to inconsistencies in the source video but I am not sure. Due to the assumptions mentioned earlier, ignore everything after 30 seconds or so, it is not valid. The ramp up at the beginning may either be due to my implementation of the smoothing filter, or possibly the original source data has had a smoothing filter applied to it. Telemetry might also have a lag of a few seconds.

 

EDIT: This section has some good points, but argues against a strawman. I somehow misread his stage 2 analysis as a stage 1 analysis, this a monumentally stupid error and I have no idea how I didn't catch it. Keeping it in for the good bits, but Exoscientist did NOT calculate stage 1 as having burned from 80% to 5% of its fuel.

 

You used this graph to estimate fuel flow rate:

Which has the starting fuel capacities at 80 percent, which does not seem right at all. Per official SpaceX telemetry, this is the stage 1 fuel load at liftoff:

That's not quite full, but that looks like a lot more than 80 percent to me. Plus measuring fuel levels can be tricky, I'm not 100% convinced that sensor is fully accurate, as we have no idea what type of sensor they are using, and measuring exact fullness of a fluid container that big probably doesn't have an easy solution.

The person who made the plot you used also had this pointed out to them, realized their mistake and uploaded a new version on the reddit thread you linked (update upon further inspection, this is in your blog post, but you used the numbers from the false one):

Your math in the expected fuel consumption of 23.1 tons appears correct given the assumptions you made.

However, just simply by inspection, it cannot be running at this throttle level for the entire 150 second burn, as it would consume more than the 3400 tons of estimated propellant.

Using your numbers of 80% and 5%, that is 75% of 3400 tons burned in 150 seconds, or 17 tons of propellant per second, as you found.

Using the corrected graph, with what appears to be 95 percent to 10 percent drop, we get instead ~19.25 tons per second, implying 83% throttle average if we assume a linear relationship between thrust and mass flow rate.

I will return to my discussion about the accuracy of this data and whatever sensors are providing it. If it is some sort of distance sensor, which measures the height of the fuel in the tanks, it is possible they printed the raw value instead of accounting for the tank geometry, and this value could also be affected by slosh and possibly pitch if drag is significant enough to cause an offset in the fuel level from horizontal. If the sensor is of that type and the raw value is used, the first and last 5 percent or so would hold much less fuel than the rest. There also might be zeroing differences. A "full" tank with 3400 tons of propellant needs some volume free for pressurant, and will not actually be 100% propellant, so on this type of sensor, a full tank might show up as 95% full or so. I also don't know the methodology of how this data was extracted from the X livestream, the small rounded bars at low resolution don't seem conducive to precise analysis. Is the true value at the tip of the )? Or at the base? Middle? This could give us an offset as well.

If the bottom 5 percent holds half of what a normal 5 percent would, and the top 5 percent is pressurant by design, it then comes out to 21 tons per second  or 91% throttle, but I wouldn't put any stock in this number either, there are simply too many things we don't know.

But either way from this graph, if I am reading your blog post right, you saw a constant slope and assumed a constant throttle, and also assumed a linear relationship between mass flow rate and thrust, where when throttling engines in real life, you can only do that to a point, you can also vary the mixture ratio to increase or decrease exhaust velocity and throttle that way.

 

But why are we even using that graph anyway when you posted this far better one on Reddit a while back?

From this graph it is very clear that there are at least four, possibly five separate throttle regimes, and that the mixture ratio varies throughout the flight. The slopes are, well, not visibly different, but they are with a ruler up to the screen.

I was unable to find how you got this graph, but I am going to assume it is from the same raw data as the other graph, just properly plotted. In that case, the same sensor biases could apply if they exist.

In either case, we can see by the variations in slopes and differential slopes, we can see that Raptor is altering its throttle by altering both mass flow rate and mixture ratio, which makes this a very complicated problem to figure out its thrust at any point in flight from this information alone. The simple F = mdot(u9-u0) formula cannot be used here as we do not know the isp of Raptor at all of its different mixture ratios and mass flow rates.

 

Fortunately, we also have the acceleration data which you also used:

The first 30 seconds or so seems to more or less match what I found, so it can probably be trusted, although I'd love to double check how the pitch data was extracted given that we only had the low resolution indicator on the livestream. Especially for the tail end of stage 2, it could skew the results if that indicator is inertial to the vehicle or fixed to local up/down.

I do see pretty obvious throttle changes on that graph, though, which correlate to the earlier fuel graph.

So, what I see in general is a gradual throttle down to maintain a constant acceleration near max Q, and then either a gradual throttle up or holding at that throttle level (or some combination thereof). After this the throttle tapers off towards the end of the burn.

Redoing your analysis at the marked yellow bars, horizontal acceleration is 11m/s^2 (at the marked point it does not look like 10 to me) and vertical acceleration is about 6m/s^2. Plus the 9.8, vertical becomes 15.8. This is at about 95 seconds, and the pitch of the vehicle at this point in the flight was about 57 degrees above the horizon. (although as I said earlier the accuracy of this pitch data is questionable)

Doing some simple trig, we can estimate vertical from horizontal as 16.94m/s^2 or horizontal from vertical as 11.26m/s^2, so there's a bit of inaccuracy somewhere, as expected when rounding to whole numbers. Net Starship acceleration is either:

• Via pythagoras: 19.25
• Trig given horizontal: 20.2
• Trig given vertical: 18.84

Which are similar to, but generally higher than what you found.

Now, as for the expected acceleration at this point in flight (T+95), there's about 42% propellant remaining. Taken naively as a fraction of the booster's 3400 tons of propellant, the stack has a mass of about 2928 tons at this point in the flight, and the expected acceleration is 25.43m/s^2 using the sea level thrust. As for a vacuum, you have fed the vacuum isp back through the F=mdot(u9-u0) equation assuming constant mdot, which may not be the case, as Raptor may prioritize to maximize isp in some situations and thrust in others, by varying mixture ratio and mass flow rate (as we saw above, it does do this), so this assumption is a bit flawed.

The source for the 363s number is also a NSF article from 2014, so I would not take stock in that either. That was the 4.5MN version of Raptor, which had 321s at sea level.

I've looked and I cannot find an up to date vacuum isp or vacuum thrust value for non vacuum raptor. If you do find one, please let me know.

Assuming your number of ~30m/s^2 as the worst case, Raptor is producing somewhere between 63 and 80 percent thrust at this point in the mission.

This is not really a good faith analysis though without good numbers for Raptor's vacuum stats, so a wide range is the best I can do. At its worst, though, this would indeed mean that Raptor is throttling down to possibly below 2/3 at this point in the mission.

 

So, from the data, your analysis appears to say that Raptor operates at below 75% throttle for most or all of the flight, the wording is unclear. My analysis shows that Raptor does operate at at least 90 percent of advertised thrust for a significant amount of time, and throttles down a lot throughout the rest of the flight, including to values in the rough range of what you were saying.

From that, you conclude:

Quote

 

" Note that throttling down to 75% also correspondingly drops the combustion chamber pressure from 300 bar to about 225 bar, allowing the Raptor to operate without leaks.

 But this reduced thrust would also mean the SuperHeavy/Starship could carry less payload. I estimate a drop in payload to ca. 100 tons reusable. In such a scenario, the 16 refueling launches needed for a Starship HLS would be increased to 24 launches."

 

Which is quite the leap. In two mini paragraphs you take that, assume that throttle has a linear relationship between pressure ratio (I just passed a class over these exact equations, a  clean obvious linear relationship is anything but the case), and conclude that:

• Raptor has a leaking problem big enough to warrant throttling down
• Throttling down would solve the leaking problems
• The observed throttling down is to solve the leaking problems
• This throttling down would drop the payload by 1/3
• This change would increase refueling launches from 16 to 24

With no calculations. Especially the 1/3 payload drop, I want to know how you got that number.

I do not understand how you can conclude this so confidently. I don't think we have enough information to conclude the reason  behind the throttling, but off the top of my head, it could be any of these, likely some combination of many of them:

• Increasing isp at the expense of thrust
  • For a reusable vehicle thrust is very important in the early stages of first phase flight.
    • Raptor's design has been changed a lot for maximum thrust at the cost of isp in the past.
  • The optimal engine changes with a lot of factors.
  • It may be mathematically optimal to throttle in a certain way throughout the flight to alter isp and thrust to whatever is optimal at that time.
  • Raptor has been observed making many changes to mixture ratio and mass flow rate throughout the flight
  • Basically, the idea here is that Raptor operates in an inefficient but very powerful way early on, and then powers down somewhat to increase efficiency later on
• Maybe the numbers we have are wrong (in which case our baseline for 100% thrust is also wrong)
  • Most mass numbers we have are conveniently rounded to the nearest 100 tons, this is not a lot of precision. Fuel and dry masses could be off by a lot.
    • Has subcooling changed since the propellant numbers were first given?
  • Raptor isp is always changing, there's no guarantee that 327s is the correct value, or that the raptors on the most recent booster were the version that had 327s
  • Likewise, Raptor thrust is always changing. No guarantee that 230 tons is the correct value, or that the Raptors used were the 230 version.
    • 230 could have been a dev value from when they were pushing it and not necessarily what they would set it to on any given launch.
    • 230 could also be an "Emergency thrust" value or something like that.
  • These numbers could also vary from engine to engine.
• Differences between non gimbaling and gimbaling raptors
  • I have not seen this talked about, maybe they have different thrusts. Are they the same, just one without gimballing hardware? Or has the non gimballing version been pushed further due to less complexity?
    • The non gimbaling one is I think called Raptor Boost, maybe those have the 230 thrust and the gimbaling ones have less.
  • Maybe they have different isp.
  • Maybe they have different other characteristics.
  • Maybe they are throttled separately to optimize for any given time of flight.
• Could telemetry have been inaccurate?
• Later in flight, does telemetry analysis take into account reduced gravity both from altitude and centripetal acceleration from increased orbital velocity?
• Does drag play much of a role in reducing apparent acceleration?
• Throttling down for vibrational reasons
  • Combination of engine vibrations and atmosphere based vibrations could cause problems in some areas
• Throttling down for structural reasons
  • Does not explain why throttle is so low so much earlier
  • Does explain further throttling towards the end of the flight
• Throttling down for aerodynamic reasons
  • Starship is pretty aerodynamically weird with the grid fins, chines, and body flaps, it may have different requirements than normal vehicles
  • Max Q throttling as per usual
• Is this normal? Is this even a throttle down?
  • Well, obviously the throttle is decreasing for some reason
  • This could be the correct throttle profile for whatever reason, the one they think they can get 150 tons from
  • This throttle down is not necessarily associated with a payload cut and likewise an increase in refueling launch count
• If this is not normal, is this permanent?
  • There are many prototypes until Artemis, the throttle cut cannot be assumed to be a permanent feature (or a temporary one)
  • The design is constantly evolving, SpaceX is not the type of company that sees a problem and just lets it sit there, they will keep tweaking and changing Raptor until they are happy with it.
  • NASA still seems to be happy with progress from what I've gleaned in recent statements, although this is the public facing side of things and should be taken with a grain of salt
• Engine problems
  • Engines could leak
  • Engines could have been de rated mid flight by the flight computer due to off nominal signals
  • Systemic engine problems could merit a reduction in target thrust by 10% for early flights
  • And I suppose that it is possible that despite working fine for 30 seconds at 90% throttle, 60-70% was deemed the safe maximum for the rest of the flight, and that 100% all the time was the baseline for 150 tons to orbit.

I don't think it is a good idea to blame any one thing (let alone the worst thing) and then extrapolate that to the program as a whole. Most of those bullet points are easily more effort to analyze than I put in to this semester entirely, and many we don't have data for.

Edited December 19, 2023 by Ultimate Steve

[RCgothic]

Eloquently put.

[Exoscientist]

 I was using the starting propellant load for the booster at the ~90% range, not 80%, and for the Starship at the ~95% range in my calculations. The authors of those graphs are at the links I provided in my updated blog post. I realized I had not credited them properly so I updated my blog post to give the links where the authors described their calculations.

 By the way the authors provide links to the actual data not just the graphs so you can calculate with them more accurately. 

 About determining the acceleration provided by the engines, to determine thrust, when you take into account gravity you do need to be able provide separately the horizontal and vertical acceleration. This needs to be calculated from the speed and altitude information provided by the launch video. But since the separate coordinate info is not provided directly, errors are introduced when you derive them numerically. 

 There is an alternative method by using the graphic in the video that shows the angle of flight of the vehicle. From that you can determine the x-and y-velocities, and then the x- and y-accelerations.

  Robert Clark

[tater]

 

[Ultimate Steve]

4 minutes ago, Exoscientist said:

I was using the starting propellant load for the booster at the ~90% range, not 80%, and for the Starship at the ~95% range in my calculations. The authors of those graphs are at the links I provided in my updated blog post. I realized I had not credited them properly so I updated my blog post to give the links where the authors described their calculations.

Good catch, I somehow saw the stage 2 analysis and misread it as stage 1 in my head, my bad. A mistake that bad is inexcusable, and I do indeed have no excuse, and it calls into question the rest of my analysis.

4 minutes ago, Exoscientist said:

 

About determining the acceleration provided by the engines, to determine thrust, when you take into account gravity you do need to be able provide separately the horizontal and vertical acceleration. This needs to be calculated from the speed and altitude information provided by the launch video. But since the separate coordinate info is not provided directly, errors are introduced when you derive them numerically. 

I am aware of that, which is why I said that my first analysis was not valid past the first 30 seconds or so. In the second part of the analysis, the x and y accelerations are broken out and trigged together separately according to that graph and the current pitch angle. But yes, I did not really question the validity of that set of data, as the method I think they used to separate horizontal and vertical (finding vertical velocity by numerically deriving the altitude telemetry, using that and the total velocity to find horizontal velocity, and numerically deriving both to find both accelerations) is only as good as the available telemetry and timestep. Since we can see the accelerations change more or less in line with the mass flow and mixture ratio changes, I'm inclined to believe that the current numerical way is good enough on these timescales, but if the telemetry we have can't be trusted, then any conclusions based on it are little better than speculation.

I also thought about the pitch angle method, I would have done that analysis above for the whole flight with a spreadsheet if that data was available, but I don't think pitch angle data is available unfortunately.

[RCgothic]

I'm a lot more worried about the future of SpaceX thanks to Musk's behaviour and contagion from other businesses of his than I am about technical aspects of Starship.

Technical difficulties can be solved, but there's no recent evidence Musk can keep his erratic behaviour from impacting his companies. Twitter's on a death spiral, and the debt is heavily leveraged against Tesla.

[Meecrob]

44 minutes ago, RCgothic said:

I'm a lot more worried about the future of SpaceX thanks to Musk's behaviour and contagion from other businesses of his than I am about technical aspects of Starship.

Technical difficulties can be solved, but there's no recent evidence Musk can keep his erratic behaviour from impacting his companies. Twitter's on a death spiral, and the debt is heavily leveraged against Tesla.

At the risk of sounding like an Elon supporter, if you dig a bit beneath the headlines, you will see that Elon did not go from Tony Stark to some deranged hard-right wing megalomaniac. When someone is on top, they become a target, and everyone who is jealous starts talking trash. You can disagree with my take on this, but you cannot deny the fact that it is hot to dump on Elon right now.  John Oliver just sold out and spent a half hour ripping on Elon instead of his usual gig of bringing to light social issues that are hidden away by the media.

My overall point is "take what the media says about Elon with a grain of salt because there is money to be made trying to take him down."

Edited December 19, 2023 by Meecrob

[NFUN]

bro you can read his posts they speak for themselves

 

regardless, I don't think his other businesses will affect SpaceX. as far as I knew they're completely isolated

[mikegarrison]

14 minutes ago, Meecrob said:

At the risk of sounding like an Elon supporter, if you dig a bit beneath the headlines, you will see that Elon did not go from Tony Stark to some deranged hard-right wing megalomaniac.

Elon Musk is not, and never was, "Tony Stark".

[Ricktoberfest]

21 minutes ago, NFUN said:

bro you can read his posts they speak for themselves

 

regardless, I don't think his other businesses will affect SpaceX. as far as I knew they're completely isolated

His posts have always been a little crazy. Oftentimes it’s misunderstanding the situation, or humor that gets taken wrong.   There are things he has said and done that are disappointing, but he’s human.  People change over time so what he believed 10 years ago isn’t the same as now. Media does like to try and take down whoever happens to be popular at that moment.

[Meecrob]

21 minutes ago, mikegarrison said:

Elon Musk is not, and never was, "Tony Stark".

Of course not. My point is that 5 years ago, you couldn't turn around without being bashed in the head with headlines about how "Elon Musk is the IRL Tony Stark!!" then the honeymoon period wears off, he turns twitter from a cesspool to a slightly worse cesspool and everyone wants his head. I'm absolutely sick of the sensationalism around him. There is more than enough about the man to study for lifetimes before uninformed talking heads go making stuff up.

[RCgothic]

I was a prodigious twitter user until it became X and witnessed firsthand his descent into echo-chamber and buying into his own trollish cult of personality. It has not been at all edifying, and his decisions related to the site have been almost purely destructive and he's now on the hook for billions a year in debt repayments.

Something like 73% of his Tesla stock is leveraged, so if he has to sell any significant amount to cover his twitter debt or default on that debt then that could both crash the value of Tesla and cause banks to call in his collateral and he'll lose the company.

It's a small step from there to having to sell his stake in SpaceX to cover debts. He's already previously borrowed $1B from SpaceX (albeit he paid it back quickly on that occasion).

[CatastrophicFailure]

Sigh. I’ma say this just one, and probably regret it, and hopefully the mods just delete this whole tangent, but anyways…

X is objectively doing better now than Twitter ever was. Twitter was on a short path to bankruptcy, X is now moving strongly the other direction, and likely to break a profit next year. Certain people have been foretelling the impending dooms of Elon Musk’s various ventures for years, and they’ve been wrong every single time. Hate what X has become if you want (that’s your right, and it’s mine to say you are incorrect in thinking so), but it’s no more dying than Tesla is bankwupt or SpaceX will never fly again. 
 

/RantOff

[darthgently]

2 hours ago, RCgothic said:

I'm a lot more worried about the future of SpaceX thanks to Musk's behaviour and contagion from other businesses of his than I am about technical aspects of Starship.

Technical difficulties can be solved, but there's no recent evidence Musk can keep his erratic behaviour from impacting his companies. Twitter's on a death spiral, and the debt is heavily leveraged against Tesla.

Honestly, I think it would be a great idea if moderators considered discussions about Musk's personal life and views as "politics".  Because the venn diagram between political camps and opinions about Musk is a near perfect stack of pancakes.

[RCgothic]

1 hour ago, CatastrophicFailure said:

Sigh. I’ma say this just one, and probably regret it, and hopefully the mods just delete this whole tangent, but anyways…

X is objectively doing better now than Twitter ever was. Twitter was on a short path to bankruptcy, X is now moving strongly the other direction, and likely to break a profit next year. Certain people have been foretelling the impending dooms of Elon Musk’s various ventures for years, and they’ve been wrong every single time. Hate what X has become if you want (that’s your right, and it’s mine to say you are incorrect in thinking so), but it’s no more dying than Tesla is bankwupt or SpaceX will never fly again. 
 

/RantOff

I must be thinking of a different X because it is not even close to cash positive, not on a trajectory to be, and is in substantially worse shape than it was before his takeover.  Not only do the subscription model and staff firings not recoup even a fraction of the lost advertising revenue (lost for no good reason at all), but it has a heavy debt burden that's over a billion dollars per year and it's currently valued less than a third of what he paid for it.

The total mismanagement has completely exploded his aura of competence and poses a direct threat to his control of his other companies SpaceX included. If Shotwell retained control it could even be an improvement.

Edited December 19, 2023 by RCgothic

[Gargamel]

While this isn’t politics, it’s definitely not SpaceX either.    Let’s get back to the topic de jour and leave the owner’s other “interests” out of it.     Thanks.  

[CatastrophicFailure]

Got a nice peek up under the skirt here (scandalous!), everything is looking very tidy and finished. Now IIRC those “cans” around the outer engines are actually around all of them, specifically to contain a failure. 
 

 

 

[grawl]

24 minutes ago, CatastrophicFailure said:

[...]

And the Guinness World Record award for biggest Nerf gun goes to...

Edited December 19, 2023 by grawl
typo

[AckSed]

The cloth (?) covers are new to me. Have we seen this before?

[CatastrophicFailure]

Really kinda puts it into perspective… 

 

…wait fo’ it…

 

 

4 hours ago, AckSed said:

The cloth (?) covers are new to me. Have we seen this before?

Yes, think they started showing up around IFT-1, when everything started looking a lot more “finished” and a lot less “slapped together in a tent…”

[Exoscientist]

17 hours ago, Ultimate Steve said:

I also thought about the pitch angle method, I would have done that analysis above for the whole flight with a spreadsheet if that data was available, but I don't think pitch angle data is available unfortunately.

 The authors of those propellant burn and acceleration graphics deduced the data from the information on the SpaceX launch videos. The flight angle as a graphic is also provided on those videos:

 Bob Clark

[tater]

 

Road closed

[Geonovast]

There was a reminder a few posts up to keep this thread on topic and to keep Musk's other business out of this thread.  Some content has been removed.  Let's keep this thread on topic, please?