# Basic space physics question: propulsion speed limit

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#### mvandiermen

##### Guest
I don’t know where to look for my answer so I hope you don’t mind me asking here.

IF the speed of the object does not slow down AND IF the same amount of propulsion energy is used to get a craft from 100KPH to 1100KPH as it is from 1100KPH to 2100KPH (meaning the propulsion is not effected by space) e.g. if “the thing that in front of it” has the initial spend of that it comes out of?; then there would be no limit to speed ?
If you were at the speed of light what would stop you from going faster?

Light or a wave; only one propulsion of an energy wave… unstopped throughout space because there is no matter. Unlike with matter on earth absorbing it. ?

off subject question:
It seems satellites can be given great speed in orbit, I figure the propulsion must be very effective, or what trick do they use.

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#### theridane

##### Guest
In a Newtonian universe there really is no limit. You can accelerate as long as you want, and reach any velocity desired.

Sadly, that's not the universe we live in right now in this universe, the faster you go, the higher your relativistic mass is, and since acceleration is a function of mass and thrust, the heavier you seem to be, the less acceleration your engines give. As you approach the speed of light, your relativistic mass increases to near infinity, so your acceleration drops down to near zero. That's why you're not gonna beat the light speed record

Sats aren't going fast at all, at least when talking relativity. Some 10-ish km/s is nothing compared to 299 792.458 km/s (speed of light). The only trick they use is a rocket motor and a load of cash.

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#### mvandiermen

##### Guest
theridane":tris1p8q said:
the higher your relativistic mass is, and since acceleration is a function of mass and thrust, the heavier you seem to be, the less acceleration your engines give.
I suppose physicist know but I cant imagin they tested this in space

theridane":tris1p8q said:
Sats aren't going fast at all, at least when talking relativity. 10-ish km/s
Just that I thought I saw some almost stationary satellites and some that go around quite fast, maybe I was mistaken. [difference between would take allot of propulsion]

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#### mvandiermen

##### Guest
p.s.
I just edited my stupied question in my responce to you in my last post

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#### mvandiermen

##### Guest
does the same amount of propulsion energy used to get a craft from 100KPH to 1100KPH as it does from 1100KPH to 2100KPH (meaning the propulsion is not effected by space)
Does additional propulsion work on the current speed of the craft ?
if the speed of the propulsion/propellant was 200kph would the craft continue to gather speed. ?
I really should read a bit more I think.

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#### theridane

##### Guest
I'm unaware of any measurements taken in space, on a spacecraft scale. But the concept of relativistic mass is well described, well measured and routinely worked with on particle accelerators.

Those sats that were going really fast were closer to Earth - those were going up to some 7.5 km/s. The slower ones were further away, going as slow as 3 km/s (geostationary satellites). Also, the further away they are, the lower is their apparent (angular) velocity from your point of view.

The amount of energy to get you from 100 km/h to 1100 km/h is not the same as 1100 to 2100, even though the delta-v is the same. It's because kinetic energy grows with the square of velocity and not linearly. But that doesn't mean a rocket has to do more work at higher speeds - a one-second burn of your engine will always increase your velocity by the same amount (assuming constant mass). The only difference is that when accelerating from 100 to 1100 km/h the rocket will have lower efficiency than when going from 1100 to 2100 km/h.

Edit: yea, you should read more I suggest you get yourself a copy of Orbiter and practice a little.

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#### mvandiermen

##### Guest
theridane":3llcsqsd said:
the higher your relativistic mass is, and since acceleration is a function of mass and thrust, the heavier you seem to be, the less acceleration your engines give.
mvandiermen":3llcsqsd said:
I suppose physicist know but I can’t imagine they tested this in space
I intended to say: It would be interesting if they have tested this in space, I cant imagine they would have reached a zero point yet
speed/fuel data would be recorded from flights

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#### theridane

##### Guest
That would be neat, but we're centuries away from even approaching the speed of light (say 0.9 c). The energies involved are ridiculous.

I think we'd be testing FTL ages before reaching any significant fraction of the speed of light. :lol:

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#### dangineer

##### Guest
Hope this info helps: as far as testing relativistic mass, it has been done in particle accelerators. Scientists have accelerated particles to very near the speed of light (>99 percent c) and have measured their masses. More precisely, they have measured their energy (E=mc^2, remember). Thus they have found that as they put in more energy, the particle accelerates less and less according to relativity.

On the subject of propulsion systems, taking a look at the following equation, you can see that for a given increase in velocity (deltaV), you need exponentially more propellant.

Mp=Mv*(e^(deltaV/Ve)-1)

Equations aren't always helpful, so here's an example:

Say you have a rocket that has a mass (Mv) of 100 kg, and the engine has an exhaust velocity (Ve) of 4,000 m/s. Your rocket is in low Earth orbit (V~7.5 km/s). You want to go 8.5 km/s, so your deltaV is 1km/s or 1000 m/s. According to the above equation, you need about 28.4 kg of fuel. Not so bad. Now say you want to go 9.5 km/s, so deltaV=2000 m/s. now you need 65 kg of fuel, more than double. With a deltaV of 4000 m/s your propellant goes up to 172 kg. So your speed limit depends on how much fuel you can carry. And carrying more fuel means you need more structural mass for the larger fuel tanks, so the amount of fuel you need goes up even faster.

Okay, so lets disregard relativity and see how much fuel it would take just to get to 0.1 percent the speed of light given a 100,000 spacecraft (just to be a tiny bit more realistic). If we used the same engine (Ve=4000 m/s), then it would take about 3.7*10^37 kg of fuel. Thats orders of magnitude heavier than the Earth!

Hope that helps.

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#### neilsox

##### Guest
Newtonian physics give reasonably accurate results as long as you think of the space craft as the frame of reference. The problem arises if you think with respect to the starting point or the destination or a star about 13.7 billion light years away which is receding at 0.999c To reach 0.9991 c (with respect to that star) takes extreme amounts of fuel even if you are at zero speed with respect to Earth's poles.
The other problem is, if you need one ton of fuel to increase your speed by one kilometer per second, you need 100,000 tons of fuel to reach 1/3 the speed of light. Since you have to carry the fuel from the beginning, the fuel better be antimatter, or the one ton was not realistic. Neil

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#### SpaceTas

##### Guest
For those up for it ..... a deep point about mass.

Mass has 2 effects:
first: the more mass the harder it is to accelerate (increase/decrease or change direction of speed ie a change in motion). This is the inertial mass.

second: the greater the masses the greater the gravitational force.

In this Universe these two affects are exactly the same. This equivalence is encountered but rarely pointed out in early on in physics courses but is still unexplained after 200+ years. The Higgs boson and the LHC are an attempt to explain this!

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#### SpaceTas

##### Guest
There is a simple way to estimate the amount of energy required to accelerate a space-craft from one speed to another.
Assume non special relativity ( less than 0.1 C 10% speed of light), assume a 100% engine that does not use reaction mass .... say a photon drive or interstellar ramjet ... so the mass stays the same.

So starting kinetic energy is zero
Kinetic energy (motion energy) is = 1/2 Mass * speed * speed

Energy required = difference in kinetic energy = 1/2 mass * final speed * final speed - zero assume start at zero speed

take 100 metric ton (roughly a shuttle = 100,000 kg), final speed = 0.1 * 300,000,000 m/s

energy required = 0.5 * 100,000 * 0.1 * 300,000,000 * 0.1*300,000,000
= 450,000,000,000,000,000,000 J
= 4.5*10^20 J
This is just over a 450 KiloTons of TNT ie (Little Boy dropped on Hiroshima was 15 KiloTon) so 30 Hiroshima bombs.
This is doable as the USA/Russia/France+England/Israel(??) each have more than that amount in their nuclear arsenals.

Look up Project Orion:

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#### MeteorWayne

##### Guest
I split this thread from where it started using imaginary physics to the Unexplained.

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#### Last89er

##### Guest
Actually relativity works at all speeds up to light. We don't live in a Newtonian universe because of motion. The effects are not easily observable at lower velocities.

This explains the reason the speed of light is the limit. The difference between a wave and a particle is charge.

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