[Petrochemicals (OP)]

I was just wondering why we do not build an experimental light speed ship? Apparently if we accelerated at 1g for about a year we would achieve almost light speed, that is from the point of the observer of course. To achieve 1g of acceleration for 365 days would not be that unachievable.

1 tonne requires 7 tonnes of oil joules equivalent to achieve this.

10ms x1000kgx=10000 Newtons

10000Nx3600x24x365= 315,000,000,000 joules, or about 7 tonnes of oil

A gradient of 7 to 1 which I think is about 28 tonnes? To decelerate will obviously take a lot more, but we could transmit signals back to earth.

[Kryptid]

Are you including the oxidizing agent with those mass calculations? You also can't count the kinetic energy going into the ship alone, as the propellant is going to contain considerable kinetic energy as well.

Specific impulse is very important when trying to calculate a spacecraft's maximum velocity. Chemical fuels like oil have a disappointingly low specific impulse. Someone did the math using the Tsiolkovsky rocket equation, and found that, using all of the oil reserves on Earth, you could accelerate a 15 metric ton ship only up to 0.038% the speed of light: https://www.quora.com/What-is-the-maximum-speed-a-chemical-rocket-can-achieve

[CliffordK]

I think I did some calculations on this a while ago.

E=mc²
Kinetic Energy:
KE=½mv²

I have to look back at momentum to see if it plays a role.. 
p=mv

Ok, so set the mass to the mass of your ship.  Momentum is in Energy.  And, of course, V is the speed of light (your target velocity

So, KE = ½mc²

Somehow I thought it meant using all of the mass of the ship.  But since the mass of the ship decreases with burning fuel, it may not be as bad as I thought.  I need to look back at my earlier calculations.

Nonetheless, one would have to convert somewhere on order of half of the ship into pure energy.  I.E.  burning fossil fuels won't cut it.  Fission/fusion won't cut it.

The only way we currently know of generating that type of energy is matter annihilation.  I.E.  Antimatter. 

We are now able to produce a few antimatter protons, hydrogen atoms, and even helium atoms. 

But, really, we may need anti-lithium to make a stable form of antimatter that could be transported.  And,it would need a lot of it.  Think going from a few hundred short-lived atoms to Avagadro's Number: 6.022 x10²³ atoms per mole.  That is a lot of atoms to produce and store.

Now, once one has the anti-lithium stored, one has to contain it, and devise an engine to slowly inject it to produce power...  and all at near 100% efficiency. 

We have a lot of research to get to that point.

[CliffordK]

I was just wondering why we do not build an experimental light speed ship? Apparently if we accelerated at 1g for about a year we would achieve almost light speed, that is from the point of the observer of course. To achieve 1g of acceleration for 365 days would not be that unachievable.

Whew...  I need to brush up on my physics a bit.

Ok, so we know that:
F=ma  (force = mass x acceleration).

And KE = ½mv²  (kinetic energy = ½ times mass times velocity squared).

Seemingly different. 

The problem comes that Energy isn't a measure of force, but rather force through a distance.

And, the faster one goes, the more distance one travels in a given amount of time.

I.E.  it takes increasing energy to achieve constant acceleration.

One can use the Kinetic Energy formula to calculate the energy required to achieve a velocity. 

So, for accelerating 1 kg from 0 to 100m/s, one gets:
Energy = KE₁₀₀ - KE₀ = 1kg * 100² - 1kg * 0² = 10,000 - 0 = 10,000 = 10,000 Watt Seconds.

Now, to accelerate that same 1kg from 100m/s to 200m/s, one gets:
Energy = KE₂₀₀ - KE₁₀₀ = 1kg * 200² - 1kg * 100² = 40,000 - 10,000 = 30,000 = 30,000 Watt Seconds.

I.E.

You'll need a bunch more energy to maintain constant acceleration.

[Origin]

I think I did some calculations on this a while ago.

E=mc2
Kinetic Energy:
KE=½mv2

I have to look back at momentum to see if it plays a role..
p=mv

Ok, so set the mass to the mass of your ship.  Momentum is in Energy.  And, of course, V is the speed of light (your target velocity

So, KE = ½mc2

That equation would be the KE of a mass moving at the speed of light in Newtonian physics.  We know it is not possible to move at c and at relativistic speeds Newtonian physics is not applicable, so this equation is not correct.

This is the equation for relativistic KE:

[Janus]

The KE needed to get 1 kg up to just 90% of c is ~ 1e17 joules

But, that it only part of the problem. Since this would have to be achieved via rocket engine, you need to use the Rocket Equation ( as per Kryptid's post),  and it gets even more difficult.

It isn't enough to work out just how much energy it would take to get the payload up to velocity, but since you are carrying your own fuel, during the earlier parts of the acceleration you have to accelerate the fuel you are going to need for later parts of the acceleration.
As you begin to get nearer to c, relativistic effects become more prevalent and you have to account for them as well.
The Rocket equation that takes this into account gives:
M0/M1 = e^(atanh(v/c) c/Ve)
M0 is the starting  mass of the ship (ship + fuel),
M1 is the final mass ( ship)
v is the final velocity
Ve is the exhaust velocity of your rocket.

An important factor to take away is that the amount of fuel needed to reach a given velocity is heavily dependent on the exhaust velocity of the rocket.

The best exhaust velocity for any engine presently under development is for the DS4G*, with a Ve of 193,000 m/s (compared to the ~4500 m/s for a contemporary chemical rocket)

Using that in the equation above, it works out that for your ship to reach even 4% of c, it would have to use roughly 1/2 the mass of Jupiter in fuel per kilogram of ship mass. To push that up to 5% of c would increase that to 3000 times the mass of the Sun. ( For practical purposes, since v is such a small fraction of c, you don't actually need to use the Relativistic form of the Rocket equation to get a reasonably accurate answer in these examples.)

But let's say that we could push the exhaust speed up to 10% of c. Then it would take over 90 million kg of fuel per kg of ship to reach just 95% of c and 3 times as much to get up to 96% of c. ( and if you want to slow back down afterwards, this jumps to 8.1 x 10 ^15 kg and 7.29 x 10^16 kg respectively.  )

As far as times go:
If you could maintain a 1g acceleration ( as measured in the ship), then:

in 1 yrs ship time and 1.19 yrs Earth time you would achieve 0.77c and have traveled 0.56 ly.
in 2 yrs ship time and 3.75 yrs Earth time you would achieve 0.97c and have traveled 2.90 ly (not yet even as far as the nearest star system to our own)
in 3 yrs ship time, and 11.8 yrs Earth time you would achieve 0.9969c and have traveled 9.7 light years ( There are 7 star systems within this distance; most of the stars are red dwarfs .)

If you want to stop at your destination, you would need to accelerate for half the trip and decelerate for the second half.
Doing so would get you to, for example, Barnard's star ( a red dwarf with ~14% the mass of the Sun) in ~4.25
 yrs ship time and and bit over 9 years Earth time.



* and while the VASIMR has a high exhaust velocity, it doesn't produce a high thrust, and wouldn't be able to achieve an acceleration of 1g.    This, unfortunately, is a common feature with rockets,  You tend to trade high exhaust velocity (thus efficiency) for  thrust.   ION engines for example are very efficient, but have very low thrust, thus low acceleration.  The type of energy sources needed to get around this just aren't achievable/practical at this time.

[CliffordK]

If your plan was to get to say 1% of the speed of light, then one should be able to reach Proxima Centauri in 400 to 500 years. It might not matter if one had very slow acceleration. 

A basic CRT can apparently get electrons up to about 10% of the speed of light.  Our big super-colliders can do better, even with larger particles, but wouldn't be very energy efficient as an engine.

Ideally, whatever power source one was using would create high energy decay particles that could be directed to provide propulsion.

Could one recover at least some of the energy of ejected particles?  For example ejecting a stream of electrons and a stream of positrons which would then interact sending some energy back towards the ship that could be picked up with solar cells.

[Bored chemist]

Why do we not build a light speed ship?
Because we can't.

What I can't understand is why that wasn't obvious to the OP.

[CliffordK]

Why do we not build a light speed ship?
Because we can't.

What I can't understand is why that wasn't obvious to the OP.

I think part of it is reply #3.

While our current rockets use short bursts of fuel, getting constant 1G acceleration is extremely difficult (except with gravity for a few moments).

Aerobraking is good for low speed rockets to slow down further, but would be bad for dumping speed from close to the speed of light.

[Bored chemist]

Aerobraking i... would be bad for dumping speed from close to the speed of light.

Another problem with very fast travel is that if you hit a speck of dust, you are in trouble.
Once you are going very very fast, hitting a single atom is a problem.
If you go fast enough, hitting the CMBR will kill you.

[Kryptid]

What I can't understand is why that wasn't obvious to the OP.

I can admit that the simple kinetic energy calculation that he did is deceptive. It doesn't seem like a lot of energy, and I suppose it isn't (relatively speaking). It isn't necessarily intuitive that the amount of kinetic energy needed to accelerate the propellant to produce a constant 1G acceleration until you reach relativistic speed is colossal by comparison. So I can understand his misconception.

[CliffordK]

Another problem with very fast travel is that if you hit a speck of dust, you are in trouble.
Once you are going very very fast, hitting a single atom is a problem.
If you go fast enough, hitting the CMBR will kill you.

Our current rockets are bombarded by cosmic rays which travel at close to the speed of light.  They may do some damage, but each one doesn't blow the things up.

But yes, small cosmic dust particles and micro-asteroids could be huge problems.

[Petrochemicals (OP)]

What I can't understand is why that wasn't obvious to the OP.

I can admit that the simple kinetic energy calculation that he did is deceptive. It doesn't seem like a lot of energy, and I suppose it isn't (relatively speaking). It isn't necessarily intuitive that the amount of kinetic energy needed to accelerate the propellant to produce a constant 1G acceleration until you reach relativistic speed is colossal by comparison. So I can understand his misconception.

That does not answer the point of why if we can accelerate at 1g or 10ms (f=ma 1 tonne 10000 Newton's or joules) why we cannot continue for 365 days. Even though as pointed out oxydiser would have to be included, there is nothing that prohibits the scale we are talking about.

[Janus]

A basic CRT can apparently get electrons up to about 10% of the speed of light.  Our big super-colliders can do better, even with larger particles, but wouldn't be very energy efficient as an engine.

Ideally, whatever power source one was using would create high energy decay particles that could be directed to provide propulsion.

An electron gun also produces almost no thrust.
Here's the problem: while thrust produced is directly proportional  to the exhaust velocity, ( double the velocity, double the thrust), the energy required to reach a given exhaust velocity increases by the square of the velocity.  ( to double the exhaust velocity, takes 4 times the energy.)
So say, for example, you have a fixed power source. You want to increase the rocket efficiency by doubling the exhaust velocity (meaning you can end up with twice the final velocity with the same amount of fuel). To do this with your available power, you have to decrease the amount of mass you eject per sec by a factor of 4, which in turn, decreases your thrust by a factor of 4. If you want to increase the final velocity by a factor 10 ten, you have to decrease the thrust by a  factor of 100, etc.
We simply do not have the types of energy sources that could produce the levels needed to have both high exhaust velocity and even a reasonably high thrust.
Take the ION engine I mentioned with the 19,300 m/s exhaust speed.  It only can produce about 2.5 Newtons of thrust and requires at 250 kw power supply, which in of itself adds mass and reduces the thrust to mass ratio.   
Electron guns can achieve high electron speeds simply because they accelerate a very, very small amount of electrons in terms of mass.
 

Could one recover at least some of the energy of ejected particles?  For example ejecting a stream of electrons and a stream of positrons which would then interact sending some energy back towards the ship that could be picked up with solar cells.

If you had positrons and electrons, why in the world would you have them come together outside of the ship where the vast majority of the energy released would be wasted?  Use them to generate energy in a chamber and then use that energy to accelerate the reaction mass.

[Janus]

A basic CRT can apparently get electrons up to about 10% of the speed of light.  Our big super-colliders can do better, even with larger particles, but wouldn't be very energy efficient as an engine.

Ideally, whatever power source one was using would create high energy decay particles that could be directed to provide propulsion.

An electron gun also produces almost no thrust.
Here's the problem: while thrust produced is directly proportional  to the exhaust velocity, ( double the velocity, double the thrust), the energy required to reach a given exhaust velocity increases by the square of the velocity.  ( to double the exhaust velocity, takes 4 times the energy.)
So say, for example, you have a fixed power source. You want to increase the rocket efficiency by doubling the exhaust velocity (meaning you can end up with twice the final velocity with the same amount of fuel). To do this with your available power, you have to decrease the amount of mass you eject per sec by a factor of 4, which in turn, decreases your thrust by a factor of 4. If you want to increase the final velocity by a factor 10 ten, you have to decrease the thrust by a  factor of 100, etc.
We simply do not have the types of energy sources that could produce the levels needed to have both high exhaust velocity and even a reasonably high thrust.
Take the ION engine I mentioned with the 19,300 m/s exhaust speed.  It only can produce about 2.5 Newtons of thrust and requires at 250 kw power supply, which in of itself adds mass and reduces the thrust to mass ratio.   
Electron guns can achieve high electron speeds simply because they accelerate a very, very small amount of electrons in terms of mass.
 

Could one recover at least some of the energy of ejected particles?  For example ejecting a stream of electrons and a stream of positrons which would then interact sending some energy back towards the ship that could be picked up with solar cells.

If you had positrons and electrons, why in the world would you have them come together outside of the ship where the vast majority of the energy released would be wasted?  Use them to generate energy in a chamber and then use that energy to accelerate the reaction mass.

Another problem with very fast travel is that if you hit a speck of dust, you are in trouble.
Once you are going very very fast, hitting a single atom is a problem.
If you go fast enough, hitting the CMBR will kill you.

Our current rockets are bombarded by cosmic rays which travel at close to the speed of light.  They may do some damage, but each one doesn't blow the things up.

But yes, small cosmic dust particles and micro-asteroids could be huge problems.

It's not just the energy of the individual particles, its the total flux. 
The energy density of cosmic rays is about 1 electron volt per cubic centimeter.   
Interstellar space has about 1 proton per cc.  At 99% of c, that same 1 cc would have an energy density over 5 billion times that of cosmic rays, relative to the ship.   That's a pretty big difference.

[CliffordK]

If you had positrons and electrons, why in the world would you have them come together outside of the ship where the vast majority of the energy released would be wasted?  Use them to generate energy in a chamber and then use that energy to accelerate the reaction mass.

Pure light is the easiest high speed particle to generate, but has almost no motive force.
Electrons/Positrons are the next size larger.  Light, but high speed and easy to accelerate. 

As I understand it, pretty much everything needs to be electrically balanced, so every + particle must have a - particle.in one form or another.

The next size up would be protons and alpha particles.  Again light, but shooting out 200 protons would be the same as shooting out one lead particle, and potentially easier to accelerate at high speed. 

If one is using matter annihilation as an energy source, then one would likely choose light, high speed particles to expel assuming there was no other practical motive force.

Pure magnetic fields interacting with cosmic magnetic fields?

[Petrochemicals (OP)]

I was just wondering why we do not build an experimental light speed ship? Apparently if we accelerated at 1g for about a year we would achieve almost light speed, that is from the point of the observer of course. To achieve 1g of acceleration for 365 days would not be that unachievable.

1 tonne requires 7 tonnes of oil joules equivalent to achieve this.

10ms x1000kgx=10000 Newtons

10000Nx3600x24x365= 315,000,000,000 joules, or about 7 tonnes of oil

A gradient of 7 to 1 which I think is about 28 tonnes? To decelerate will obviously take a lot more, but we could transmit signals back to earth.

I'm afraid the correct answer you where all looking for was the ship would be in the future and therefore of no experimental use.

[Zer0]

' Gravity Assist ' would be of No use?
🤔
(Parker Solar Probe)

Solar Sails or Laser Sails?
🤔
(Sol 10%c & Laz 26%c)



P.S. - How much money would it cost someone to put their cell/mobil phone in a Stable orbit around Earth?
☎️
(sX falcon9 - 1kg - $2,720)

[yor_on]

There exist some projects for interstellar flights, but they build on miniaturization and are primarily thought to be used for explorations. You could possibly send some sort of incubator and sperms, maybe, with them to 'populate' some nice 'exoplanet'. That's something ' first and last men ' discuss, although in a even more advanced version as becoming earths last initiative. Then there is interstellar hydrogen but to collect it you need to be at a considerable speed before even starting to collect.

https://space.stackexchange.com/questions/5387/can-we-use-interstellar-hydrogen-as-a-fuel-for-interstellar-travel

One good thing with relative motion is that it doesn't stop, there's no 'friction' to space, sorry, empty vacuum, that I know of. But the most probable idea might be a combination of solar sails and hydrogen, with 'generation ships' if you want to have it as we we do it today.

This one is old, but interstellar space should be pretty cold.  https://forum.nasaspaceflight.com/index.php?topic=17490.0

Although 'hotter' the faster you go. Damn :) Then again, if you scope up the particles before they interact with your ship? Still hot though. It kind of f* it up, doesn't it?

Let's add this one.  https://www.discovermagazine.com/the-sciences/these-new-technologies-could-make-interstellar-travel-real
=

I forgot, Unruh and Rindler horizons. Virtual photons becoming 'real' in a constant acceleration   http://www.calphysics.org/rindler.html

Another 'local energy source' coming to exist, and that one do exist. I seem to remember it being tested tested at Lund's university (Sweden) involving a experiment using a rapidly oscillating 'squid'  creating 'light' out of 'nowhere'.

I'm not entirely sure if it was Unruh radiation it tested though. Can't seem to find the link. But as a concept it should relate to it anyway. But that one is very tricky as it involves a pair production in where you normally expect a annihilation, very close to Hawking radiation.

https://en.wikipedia.org/wiki/Superconducting_quantum_interference_device

Syntax.

[yor_on]

There is one way to reach infinitely close to 'c', and there our current physics stops. And you don't spend a jot of energy doing it. You will be 'weight less' (ignoring tidal forces) the whole time, being in a 'free fall' following a geodesic.

Passing the event horizon, of a schwarzschild black hole. It will take you 'out' of whatever sort of physics and universe we know.

It's called 'gravitational acceleration'.