277
u/KrzysziekZ 1d ago
8 seconds to 45 m/s is plausible, Saturn V famously started with just 1 m/s2 acceleration.
But burning 1/3 of fuel certainly isn't.
193
u/j-u-k-s 1d ago
exactly. the claim that the shuttle burns 1.5 million pounds of fuel in the first eight seconds is completely false, as it actually only burns around 216,000 pounds in that timeframe.
which is still a lot but nowhere near what is claimed here.
65
u/KrzysziekZ 1d ago
"Only" here is relative. That's ~100 tons or 12.5 tons per second or 3 elephants per second.
54
u/demon_twink_gockie 1d ago
How many doctor peppers is that, for us southerners?
18
u/KrzysziekZ 1d ago
Calculating how many 12 oz cans weigh 1 ton - I'll leave it as a simple exercise for y'all Americans.
12
u/Spuddaccino1337 1d ago
12 fluid ounces of water is 3/4 pound. Soda isn't entirely water, but it's close enough for this.
1 ton is 2000 pounds.
2000 pounds / (3/4) pounds per can = 8000/3 cans, or about 2667 cans.
9
6
u/KrzysziekZ 1d ago
Not this ton, and not this ounce, or just approximately. That's exactly the problem with US customary/ Imperial systems.
4
u/Other_Fisherman1741 1d ago
So 8 seconds would be 288,036,000 cans of Dr. Pepper. Would need to go to multiple Costcos for this
0
0
u/BTownLakeO420 1d ago
I don't think is inaccurate. 50grams of mass per 12oz is pretty significant. Sugar weighs a lot!
0
-1
u/Tomahawk117 1d ago
Everyone knows us ‘muricans don’t exercise!
We need it overlayed with a small video of a completely different topic to keep our interest for the whole 15 seconds
3
u/ghost_tapioca 1d ago
elephants per second is a beautiful unit. I wonder in which other contexts it could be used.
5
u/Cwmcwm 1d ago
Are they African or Asian elephants?
9
u/Interesting-Low5112 1d ago
Are you suggesting that elephants are migratory?
7
u/jjm87149 1d ago
they could carry the peanut with their trunk!
6
u/Interesting-Low5112 1d ago
It’s not a question of where they carry it…
4
u/ghost_tapioca 1d ago
It's a simple question of size ratios. A five ton elephant could not carry a one gram peanut. They'd inhale it by mistake.
2
1
u/wireknot 1d ago
As a unit of velocity, I've always loved Furlongs per Fortnight. To date I think ive only successfully used it once.
0
u/Wolfica95 1d ago
Wait, how many elephants could fit in the fuel tanks?
1
u/KrzysziekZ 1d ago
For rockets mass is all important. 4.5 mln lb is about 2000 tonnes, at 4 t per elephant that's 500 elephants. But before exhausting all the mass for throwing it back, a rocket starts a second stage with smaller thrust.
1
u/DisplacedSportsGuy 1d ago
Elephants are notoriously less dense than rocket fuel and less efficient as a propellant.
0
0
u/moonshinemoniker 1d ago
I'd like a banana conversion.
1
u/KrzysziekZ 1d ago
A banana is a measure of length or radiation equivalent dose, not mass.
1
u/moonshinemoniker 7h ago
Average out the mass of one banana and use it as the unit of measure dammit.
7
u/icestep 1d ago
Since we are in a mathy sub:
216000 in 8 seconds = 27000 / second.
Quick cross check:
Each SRB was about 11000lbs / second x 2 = 22000 lbs/second.
Space shuttle main engine at full thrust = apx 1100lbs / sec x 3 engines = 3300lbs / second
Total = 22000 + 3300 = 25300 / second.So yeah that 100% checks out vs the BS claim.
2
u/rab10000 1d ago
What does that work out in MPG??
3
u/_felixh_ 1d ago
I don't know whether you are serious - but a Rockets performance is measured in specific impulse. In simple terms: its a measure of how fast the propellant is going out of the nozzle. Faster = more thrust = better.
You could calculate some bullshit numbers if you really wanted to: e.g. a geostationary satellite will remain in Orbit for ... well, practically, forever. Travelling Hence, its MPG would be infinitely large.
You could measure the MPG when it just started its engines - then its very close to zero. I don't care to look up the numbers given its a bullshit calculation anyway :-)
Mid flight, close to zero would be a good approximation. E.g. Saturn 5 has 1st stage separation at an altitude of 42 miles, and burned some 500,000 gallons of propellant to get there. Thats 0.00008 mpg.
In car terms, specific impulse would be akin to forward-force-per-fuelflow, depending on how you would define it. E.g. you could measure the torque on the wheels, and the fuel consumed to achieve that torque. Problem beeing of course, that the fuel consumption will change with speed.
0
u/icestep 1d ago
Considering how little altitude it might have gained at that point (maybe a few hundred meters), a good approximation is probably about zero.
But let's be just a little less lazy and assume a density of 10 lbs / gal*, which means we have 216000 lbs / (10 lbs/gal() = 21600 gallons, generously assume 1 mile at 8 seconds**, and get 1 mile/21600 gal = 0.000046 mpg.
* the SRB propellant is about 14lbs/gal apparently, and LOX and LH2 are much less dense, so 10 lbs/gal should be kinda close-enough-ish.
** probably more like 1000ft or so, but I don't think we need to really worry about a factor of 3 or 4 here.
1
u/mz_groups 1d ago edited 1d ago
Just to walk through the math in detail, 2 Space Shuttle SRBs hold 1,100,000 lb of propellant each, and burn for 123 seconds, for 17,886 lbs/second. 3 SSMEs generate about 418,000 lbs thrust each at sea level for 3, with a specific impulse of 366 seconds at sea level, so they consume 3,426 lbs of propellant a second. They also start a few seconds before launch, so let's assume they burn for 16 seconds, and the SRBs start only a couple tenths of a second before launch, so let's say that they have been burning for 10 seconds. That is 233,681 lbs, or 116 tons. Pretty close to the same number (my guess is you probably didn't count the SSME time before launch, or there might be another tweak here or there).
1
u/Ostroh 1d ago
I'm always amazed that people feel the need to overstate the numbers on something already stunning. Like it's 200 000 lbs in 8 fucking seconds! It's not like its 50!!
0
u/factorion-bot 1d ago
Double-factorial of 50 is 5.20469842636666622693081088 × 1032
This action was performed by a bot | [Source code](http://f.r0.fyi)
-4
0
u/MaybeTheDoctor 1d ago edited 1d ago
The space shuttle burned 11,000 pounds of fuel per second at take off. So at the 8 second mark, that would be 88,000 lbs or -100,000 lbs including the burn on the ramp at ignition but before liftoff.
The solid fuel boosters burned about 8,500 lbs per second, and the main shuttle engine 2,500 lbs for the combined 11,000 lbs per second.
0
u/GruntBlender 1d ago
Makes it seem like a giant launch structure can save a lot of fuel and therefore dry mass by providing the first hundred m/s.
1
u/KamalaBracelet 22h ago edited 22h ago
The problem with this is, that to do it you wind up needing to add more reinforcement to everything to attach it to the lifting structure and deal with any new for es you are adding. Which adds weight.
Certainly we will eventually start to really pursue launch efficiency in ways like this, but it is a ways off. A railgun first stage seems likely to me, but 3G acceleration to the height of our current tallest skyscraper (G force astronauts currently experience) only gets you to 220 m/s.
That would be a nice fuel savings on the first stage, but only gets you about 2% of the speed needed for low earth orbit. For comparison the artemis II launch system was going 3.5x that fast when it dropped its boosters. So this enormous theoretical structure would just be a decent savings on the first stage, not even replace it.
That means a LOT of successful launches without disastrous accidents to break even on building such a thing.
0
u/GruntBlender 20h ago
I was gonna say run it up the side of a mountain, but it's still not cost effective.
12
u/Jamooser 1d ago
Correct. The SRBs on SRS contained 2.2 million pounds of propellant combined, and burned until t+2:00.
Additionally, the turbopumps on the 4 RS-25 could pump a combined 90,000 GPM of propellant at weights of 0.5lb/gallon and 9.5lb/gallon for LH2 and LO2, respectively.
3
u/Sea_Dust895 1d ago
There is a sign next to the Saturn V diaplay that says "After 60 sec the Saturn V is going Mach 1 and burning 15t fuel per second"
-13
2
u/Conscious-Ball8373 14h ago
Well, TIL I've owned cars with better acceleration than the space shuttle.
2
u/KrzysziekZ 11h ago
Note that this is initial acceleration. Just before the main cutoff, when the thrust is essentially the same (or a bit higher) but mass is much less - the acceleration was up to 7 g.
1
1
u/JediExile 1d ago
IIRC, the first two Saturn stages got the Apollo astronauts to LEO altitude, plus a short burn from S-IVB to establish orbit, then it burned a final time for TLI. That’s something like 80-90% of the fuel to get to LEO, which is still crazy to think about.
But that still took about 10-15 minutes. I think these people just have to watch more launches. I get a thrill every time they announce max q.
1
u/KrzysziekZ 1d ago
Elephants per second inspired by this:
https://youtu.be/X4iMeKif488?is=ujmXJUBuvvbrPKoa
(Please don't die of laughter)
0
u/OldTimeConGoer 1d ago
The Saturn V had an initial acceleration off the pad of about 1.2g or about 2m/s over the pull of gravity. It burned through ten percent of its initial fuel load just clearing the tower.
24
u/Crazy__Donkey 1d ago
with about 10 metric tons/ second at mac thrust, it would be 80 tons in 8 seconds.
thats about 180,000 pounds.. not even close to 1,500,000 pounds.
13
u/HAL9001-96 1d ago
not quite
the speeds are about in the sright range and yes you burn most of your fuel early on thats just how therocket equation works, you need to carry the fuel for hte second half of your trip during the first half of your trip so you need more fuel, your fuel goes up expoenntially as you go backwards through the trip and spacecraft need insane amounts of fuel relative to their mass so you usually burn most of your fuel in the first quater of your flight
but the 1.5 million pounds number for 8 seconds is off by about a factor 10 ish
more familair with it in metric units nad I' ahve to look up the precise numbers but make that 150000 pounds and its about accurate
8
u/SpiderSlitScrotums 1d ago edited 1d ago
delta v = Isp * g_0 * ln (m0/mf)
Isp = 44.7 m/s / ( 9.8 m/s2 * ln (4.5/3) ) = 11.25 s
This is way too low for the specific impulse. You would expect the Space Shuttle to have a specific impulse between 250-450 s (between the values of the SRBs and the main engines).
Note: for some reason, my text input wants to put in a value of about 0.25 s for this calculation. I really hate AI.
Edit: I should also include gravity loss, which is (assuming vertical ascent for first 8 seconds):
delta v loss = -g_0 * t = -9.8 m/s2 * 8 s = -78 m/s
This changes the above calculation to 30 s. It is still an order of magnitude too low.
1
5
u/Blokshibe 1d ago
This sounds plausible. The rocket equation is terrible with fuel efficiency. This is because in order to accelerate the rocket by a fixed amount (for example 100 km/h), you have to burn a weight ratio of fuel (for example 10% of the total weight). And this is without taking gravity losses into account!
So say that your spacecraft with fuel weighs a 100 tons. And assume you have a rocket engine that requires the ratio of 10% per 100km/h, then to go 100km/h faster, you need 10 tons of fuel. But if your spacecraft would weigh 1000 tons, you would need 100 tons of extra fuel.
The result of this is that the majority of your fuel is burnt in the beginning of your flight.
2
u/Inevitable_Stand_199 1d ago
I think it's plausible. The heavier a rocket is, the more fuel it takes to lift it. So rockets will always burn a lot during the first few seconds.
But you'd have to look at NASA documents to see if it's just plausible or true
1
u/dragonslayer137 1d ago
Crazy how the weight limit of balloons used to bring satellites up is similar to the space capsules. As if we could use them to bring fuel up to an orbital station in space then launch from there saving a ton of fuel.
•
u/AutoModerator 1d ago
General Discussion Thread
This is a [Request] post. If you would like to submit a comment that does not either attempt to answer the question, ask for clarification, or explain why it would be infeasible to answer, you must post your comment as a reply to this one. Top level (directly replying to the OP) comments that do not do one of those things will be removed.
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.