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/sci/ - Science & Math

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11028593 No.11028593 [Reply] [Original]

So I had an argument the other day with someone at work. We were debating the mechanics of thrust in space.

I have always thought that throwing any sort of reaction mass out the back (opposite direction of travel) of a ship will add to (forward) velocity. A more powerful thruster will accelerate you faster, but in the end as long as I'm bolted or stuck to the ship with a magnet, I could throw pebbles from the rear of the ship and very slowly add velocity with each pebble toss.

My coworker (a LAAAAAAAAADY) contended that thrusters have a "max speed limit" and that (assuming the vacuum of space and no gravity) a thruster cannot accelerate anything beyond this speed limit (presumably the speed at which it throws reaction mass out).

Which one of us is correct?

>> No.11028609

>>11028593
she's correct. for example, if you are travelling near the speed of light relative to an observer on earth, and you throw a pebble backwards to speed your ship up, from your perspective that will cause you to speed up just the same amount that any pebble would speed you up, but from the observer on earth's perspective, you only get a tiny bit closer to the speed of light but never pass it. this is because of relativity and length contraction / time dilation

>> No.11028612

>>11028593

https://www.youtube.com/watch?v=ua0MMXJl3FM&t=

>> No.11028617

>>11028593
Assuming you had a bottomless fuel tank that weighed nothing you could accelerate forever. As you approach c you need more and more energy to accelerate, but since youve got your magic gas tank you can push through until you hit light speed and your trip would be subjectively instantaneous.

>> No.11028618

>>11028593
You're both retarded.
What happens when you throw the pebble is that the whole Universe accelerates around you.

>> No.11028622

>>11028609
I've aware of the ACTUAL speed limit of the universe, ie the speed of light. We were arguing whether a sub-light thruster has a limit to how fast it can accelerate a ship. So for example, if I want to get to some measurable fraction of c but my thruster throws mass out the cone much slower than my desired speed, can we reach that fraction of c?

>> No.11028626

>>11028617
This is my understanding. Obviously we are assuming unlimited reaction mass, thank you for intuiting that.

>> No.11028646

>>11028617

Put your fuel tank inside a bag-of-holding. Boom! Problem solved

>> No.11028657

>>11028593
How can you affect thrust or velocity in a vacuum once you've already left an atmosphere/body of gravity?

There is no resistance to push against?

>> No.11028670

>>11028622
yes, in an idealized scenario yes. by conservation of momentum you always get a force and a nonzero acceleration no matter how slowly you eject the pebble backwards. it always accelerates you no matter how weakly you throw it. in the real world though, there is nonzero drag in space since it's not a perfect vacuum, so you reach a terminal velocity a finite amount below c thanks to that

>> No.11028681

>>11028657
Newton's Third Law

>> No.11028702

>>11028593
>she thinks max speed is limited by max force inherently

This is a common intuition of lay people based on their "real life" experiences with friction and drag. I assume her specialization isn't physics?

>>11028670
Has it.

>> No.11028703

>>11028593
Sounds like she played a video game once where that's how it worked. I can't imagine another reason a woman would claim to be an expert in rocket science, of all thigs

>> No.11028729

>>11028670
>>11028702
Thank you sci

>>11028703
Yeah idk, I cannot imagine she is being retarded on purpose; perhaps she oversold her knowledge of rocketry to me.

>> No.11028730

>>11028593
She's being smug and referring to the point where you run out of fuel without actually saying so. Yes thrusters do have a "built in" maximum speed limit - the point you run out of shit to eject.

>> No.11028760

>>11028593
lmao, you think a girl understands physics?

>> No.11028774

>>11028593
Wait, so you can reach near lightspeed in space by throwing pebbles???

>> No.11028782

>>11028593
>My coworker (a LAAAAAAAAADY)

bill burr is a cuck

>> No.11028787

>>11028774
you could even just shine a flashlight

>> No.11028797

>>11028787
M8, how tf have we not reached other galaxies yet
Wtf are scientists doing??? Why don't they just strap a bunch of flashlights together and we'd be reaching the edge of space by 2030

>> No.11028800

>>11028797
i mean, you'd need a flashlight with the power of like a billion hydrogen bombs to make it there that fast

>> No.11028803

>>11028800
Easy, just get all the flashlights together

>> No.11028814

>>11028787
fleshlights generate more thrust

>> No.11028827

>>11028622
Yes, because you are still expending reaction mass, of course the closer you get to C the more diminishing returns will set in as you still aren't traveling in a perfect vacuum and thus like a hypersonic aircraft or a ship moving through the water you will at some point begin experiencing significant resistance in the form of collision with material ahead of your course, at near C even the hyper-rarefied material of interstellar or intergalactic space will rapidly exceed in resistance what either your hull can protect you from or what your reaction drive can produce in thrust.

>> No.11028834

>>11028797
Because the thrust-to-weight ratio of photon drives is utterly tiny, your maximum speed would be prodigious to be sure however it would be more like the year 2,000,030 for you to actually achieve it, then you'd have to turn around a little before halfway there and thrust against till the year 4,000,030 to avoid hurtling past your target at relativistic speeds.

>> No.11028836

>>11028814
Kek

>> No.11028879

>>11028593
So long as anything you throw is able to actually leave the shop entirely then yes it will add more momentum and increase your velocity. You could use your piss to rocket off if you really wanted to. You are not just moving further, you are moving faster by doing that. The instant you stop tossing turds out the back of your ship is the instant the ship stops increasing its velocity.

All this wishy washy shit ITT about speed of the universe is utter bollocks. That is why Newton's 3rd Law is a "LAW."

>> No.11030713

>>11028593
was she talking about delta V budget?

>> No.11031736

google the rocket equation, that's all you need

>> No.11031877

yes, there is particles in space, the cosmic background

this is essentially like the air on earth. There is a max falling speed for a given acceleration

>> No.11031910

>>11028593
Easy answer for this question :

We are basically talking about friction here.
Since we talk about space many peoples think that it's a perfect vacuum but this is not a thing.

>> No.11032081
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11032081

>>11028622
I think I know what you mean, and yes, a thruster can make you go faster than the speed at which the propellant is expelled

The answer is somewhere on this website
http://www.projectrho.com/public_html/rocket/engines.php
It's also quite good to get rid of misconception.
Even if you know everything on that list there's certainly many more idea you never bothered to question but is actually very wrong.
http://www.projectrho.com/public_html/rocket/misconceptions.php

>> No.11032098
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11032098

>>11028622
I think I know what you mean, and I'm almost certain she is right, a thruster CAN'T make you go faster than the speed at which the propellant is expelled.
If your fusion engine propel the reaction mass at 15% of the speed of light, then no matter how much reaction mass you have you'll never go above 15%
Other engines who look "weak" like ligth-sail can actually get close of light speed because they are propelled by photon or reaction mass that are expelled at far greater fraction of the speed of light.

The answer is somewhere on this website
http://www.projectrho.com/public_html/rocket/engines.php
It's also quite good to get rid of misconception.
Even if you know everything on that list there's certainly many more idea you never bothered to question but is actually very wrong.
http://www.projectrho.com/public_html/rocket/misconceptions.php

>> No.11032110
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11032110

>>11028593
Remember pushing a merry-go-round on the playground? And you'd time your pushes to match the passing of some kind of handle or bar? The merry-go-round would go faster and faster until it reached a limit. As your hand would try to make contact with the spinning merry-go-round, it was already going as fast as your arm could push. You couldn't keep accelerating it.

Your arm is your thruster, and the speed and strength of your arm, can only create so much thrust, and the merry-go-round can only accelerate to that limit. A larger, more athletic person's arm could move faster, and accelerate the merry-go-round to a higher speed than you could. This is because the thrusters are built different (like some rockets might have different propellants that exit at different speeds and energies).

>> No.11032601

>>11032098
>a thruster CAN'T make you go faster than the speed at which the propellant is expelled.

that's not accurate.

>Typical values of exhaust velocity for liquid propellant rockets range from 2000 to 4500 m/s

>The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s (28,000 km/h; 17,000 mph

>> No.11032789

>>11032098

From your link:

Δv = Ve * ln[ M / Me ]

Δv = ship's total deltaV capability (m/s)
Ve = exhaust velocity of propulsion system (m/s)
M = mass of rocket with full propellant tanks (kg)
Me = mass of rocket with empty propellant tanks (kg)

So if you have an arbitrarily large amount of mass to throw out, you can make ln[ M / Me ] arbitrarily large and reach any speed even if Ve is puny

>> No.11032812

>>11032110
This isn't how it works, a better metaphor would be of you were on a frictionless merry go round and you were constantly using one of your feet to "push" forward. The situation you described is misleading because

1) friction is slowing down the merry-go-round and

2) the reason you can't push it anymore is because you can't make contact with the handle (it's moving too fast to catch with the speed of your arm) which wouldn't happen if you were welded to it

>> No.11033657

>>11032601
>>11032789
Thanks for cross checking me, let it be known I was wrong here >>11032098.
No I'll have to track down where I got my error if it wasn't on that website.

Then I guess the only limit known is the speed of light.

>> No.11033692

Yes, thrusters have max speed limit - a speed of light
Anything below that will add to the speed thanks to the conservation of momentum