The care and feeding of the rocket equation
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- Опубликовано: 8 сен 2024
- The rocket equation seems complicated and mysterious, but it's really based on some very simple concepts.
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Congratulations! After way too many hours in KSP and researching orbital mechanics, this is the best explanation I ever found about the rocket equation, mass ratio, exaust velocity and Isp. You got a new subscriber and fan. I also like the way you deliver it: simple, minimal words/graphs/equations and slow pace, to give us a chance to grasp it. 🙂
Thanks.
Dude, it seems to me you are doing something to anger the algorithm, because with your high quality content you should have at least 100k subs by now😢
If I had to guess, I think it's punishing you for not changing the default logo
You may be right.
Maybe it's because I used to work for a large Redmond-based software company...
The algorithm loves:
1. At least 1 video per week on the same day and time. Preferably a time that makes sense for where most of your audience is;
2. Lengths of 10, 15 and 20 minutes;
3. Good thumbnails, first impressions are very important. You already have a thumbnail style of your own which is a good thing and halfway there. Real Engineering, The efficient Engineer and Economics Explained have good thumbnails imo.
Why is it so underrated? this actually EXPLAINS the Isp
Incredible video! really shocked that this channel doest have 100k+ subscribers!
one thing i noticed... 9:12 it seems you are referring to 9.8m/s^2 as in earths gravitational acceleration, but in the video you call it the Gravitational Constant (which is equal to 6.67*10^-11 m^3/kg*s^2)
all in all amazing video and I have subscribed to see more!
Excellent explanation!
Incredible video! But 8:20 genuinely hurt to see you adding all the numbers up manually instead of just taking an integral at first, I breathed a sigh of relief when you mentioned it
I don't know what my viewer demographic, but I'm already worried when I use a natural log or e^x in my talks. If I went straight to the integral I think some of their brains would have exploded.
Not to mention that calc class was a *long* time ago for a lot of people, including me.
Fantastic as usual!
The "weird" isp definition is awkwardly refering to the fact that a rocket with the weight of its propellant equal to the thrust of its engines will be able to burn them for the duration of its isp.
Yes.
I just don't find any use in that definition - I haven't seen any calculations that make use of it.
@@EagerSpace me neither now that i think of it... only use i found for it is that when i'm building rockets in KSP and i see the engine's rated burn time is higher than its isp then i know i need to add boosters to make full use of the burn time. 😅
But I'm not a rocket engineer so there's that...
@@EagerSpace though now that i think of it, engine Isp does therefore give you the theoretical maximum burn time of any rocket stage that intends to/uses said engine (without other boosters). Whether this is a metric that's needed/practical for any actual rocket engineers, that i have no clue of.
Edit: theoretical maximum burn time of the first stage, since higher stages don't need TWR's > 1. And since first stages need a substantially higher TWR than 1 and since dry mass is never 0 i have to concur with your assesment. I see no real use for it, but this got me curious and i will try to look for one.
Extraordinary as always!
Can’t help but feel like this motivated in part by your recent conversation with Tory Bruno. I’m curious if you would be willing to address his points in video form, specifically the different criteria he uses to justify ULA’s business cost. Why does coast time / number of burns matter? What capabilities does Atlas V / Vulcan have that Falcon 9 / Heavy do not?
I wrote most of this back in January but lost interest in finishing it then, only to come back to it recently.
It does relate to the discussion with Tory Bruno. I think Tory Bruno is a good CEO and I appreciate how it interacts compared to some other CEOs out there, but people lose sight of the fact that his job as CEO is not one of education but one of advocacy. When he interacts he is mostly trying to advance his company's business. And you do that by trying to differentiate your company from your competitors.
The "SMART reuse" paper is a great example of this, as are the frequent tweets about the the accuracy of their orbital deployment. The "rocket architecture" article is just another attempt at the same thing. It's a clever piece of marketing - appealing to "efficiency" and ignoring price - but not that convincing. And it's frankly aimed more at Starship than at Falcon 9, and to be fair, Starship is an unproven approach, especially for the kind of missions Vulcan will fly.
To answer your question, I'll think about a video, but the short answer is that the high energy market is pretty much wholly defined by NSSL launches and NASA planetary launches, and since F9/FH are doing all of those, the answer is that there is no functional difference in terms of capability.
A video on how ULA - and other launch companies - market and differentiate themselves might be interesting...
I really love your content and the way you produce it.
I'm confused. Why is every number larger in space? How is there more mass coming out of an engine as soon as the rocket leaves the atmosphere?
To oversimplify, when you run the engine in atmosphere you have the atmosphere pressing against the exhaust and that slows it down. Get out of the atmosphere and there's no atmosphere and that increases the exhaust velocity and therefore the specific impulse.
underrated
Thanks.
At @7:57 Shouldnt that be 7460m/s^2 since you added all the accelerations together so it wouldnt be delta v, it would be delta a. Because adding acceleration is not cancelling any units. However, if you multipled acceleration by the time step, then it would be delta v
Yeah, that's a little sloppy of me.
What I'm looking for is the total change in velocity that I get due to the acceleration, so I'm adding up the changes in velocity for each second.
In getting all those numbers, I approximate that by taking the average of the acceleration at the start of the second and the end of the second. And then I'm looking that value for one second, so the change is in m/s.
@@EagerSpace ok, great thanks. Im not being picky i promise haha, im just writing everything down so that i can see how it all works :) thanks
I appreciate the comment.
Hi I have a question at 5:55 you got 348 seconds. Where did you get that number?
That's the most common number I found for the Merlin 1D Vacuum. It's considerably higher than the vacuum specific impulse for the sea level Merlin 1D because of the much larger nozzle.
ok, last comment. At the end, how did you convert the 7.6 mass ratio to percentages? thats what i want to know please : )
The stage full of propellant would be 100%, so to get the 7.6 mass ratio we would need the spacecraft mass to be 100/7.6 or 12%.
Well, actually 13%, as I apparently did the math wrong.
@@EagerSpace ok, i see how that works now. My calculus is great but it has destroyed my basic arithmetic hahaha.
It took me about 10 seconds to answer your questions and I'm the one who wrote the slide...
@@EagerSpace i feel that. There were times in my calculus classes that after a while, i couldnt do basic fractions for the life of me. I was too used to other forms of math that i forgot the basics.
Thanks for all the help, im writing everything down in a notebook thats all on propulsion and rocket design..
@@EagerSpace I have trouble to understand this. Let have X the fuel mass and Y the "empty" rocket + payload etc, we do have X/Y = 7.6 and X+Y =1 (100%) right ? If I am right then we have fuel mass = 88.4% and rocket = 11.6% of the total mass.
I don't want to critisize and point an error (well in fact the video has the same values than me thanks to the approximation), i am just making sure that i correctly understood what the mass ratio is.
If anyone can correct me or confirm my maths, that would be awesome
(sorry for my bad english)
I am not sure if you correctly explained the deltaV.
Okay. What part do you think is wrong?
@@EagerSpaceat 6:55min the propellant mass is increasing from 287kg to 1435kg. That’s not possible. It should be decreasing because rocket threw mass to create thrust.
Why the natural log?
In math, if you want to figure out the sum of a value that is changing, you do it through integration. It turns out that if the value that is changing is of the form 1/x, the result is ln(x).
Explaining why is a math question.
@@EagerSpace thanks a lot, I've always been confused by the natural log and the natural number e. Just like П, e is constantly used in math but while the former indicates a precise thing (the ratio between a circumference and its diameter) I've never understood the reason for the latter.
It is a bit weird. Hey, there's this magic number that we just use because math.
If they build the hydrogen upper stage tanks out of some kinda carbon composite material it will take care of this supposed mass ratio problem.
The biggest factor for the mass ratio is simply the density of hydrogen; compared to liquid methane, it takes a tank 6 times the volume to hold a similar mass of liquid hydrogen. Tank dry mass isn't a big factor.
The general estimates are that composite tanks might be 20-40% lighter than aluminum/lithium tanks. If you need a tank 6 times the volume that's clearly not enough.
There are issues with carbon composites and cryogenic materials. Rocketlab obviously does it with LOX on Electron and plan on doing both LOX and liquid methane for Neutrton, but those are stored at much warmer temps than hydrogen and are bigger molecules so they are easier to keep confined.
@@EagerSpace The overall tank size has the LOX averaged in there and with hydrogen, they run it oxygen rich for the most performance per tank mass. The size of tanks seems like a non issue if you know how to build a cylinder 5 meters or bigger.. I mean how big is the first stage? There would only be a volume problem for the 2nd stage if you built a rocket in the shape of a pencil(falcon 9). The composite materials issue for hydrogen can be solved.
@@sirmicro >The overall tank size has the LOX averaged in there and with hydrogen, they run it oxygen rich for the most performance per tank mass.
"oxygen rich" or "fuel rich" is typically is used to compare the actual mixture ratio to the stoichiometric ratio. The stoichiometric ratio for hydrogen is 8:1, so the typical hydrolox mixture ratio is fuel rich, somewhere around 6:1. Methalox and kerolox also run fuel rich.
There are a couple of reasons. If your fuel is lighter than your oxidizer, you get an isp boost as the average exhaust particle size is smaller and therefore the exhaust velocity is higher.
The second reason is that gaseous oxygen at 3500 K is really, really nasty and wants to eat everything, so you get "engine-rich combustion".
The only oxygen-rich combustion in rocketry is preburners in staged combustion engines like the RD-180 and the Raptor.
It is true that mixture ratios burn significantly more oxygen than fuel by mass.
For the rest, nobody has flown big composite hydrogen tanks.
@@EagerSpace yeah I got tank fuel masses mixed up with the combustion fuel ratios forgive me but thanks for clarifying it up a bit.
@@sirmicro That's what I figured. The "fuel rich" verbiage is confusing.
why am i the first comment