1G Acceleration

[Ishimura_]

Starting off with 1G acceleration would it be possible for a spacecraft to reach 0.1c and does it take an ever growning amount of thrust or could you provide enough thrust to reach 1g acceleration and keep it at this level? Would the limitation to achieving 0.1c be the amount of fuel that would need to be carried? I'm at a loss on why high speeds cannot be achieved in the vacuum of space, basically is it a fuel issue or something else? Thanks.

 

[theridane]

(...) is it a fuel issue or something else?

Pretty much, yes.

If you want to have it running for a long time, you need it to consume as little propellant as possible. We're already doing this - with ion drives. These can run for years, providing very little acceleration, but high propellant efficiency.

However, if you want more acceleration, say 1 g, then there's a problem. The problem with rockets is that the exhaust energy increases with the square of the exhaust velocity. So in order to have high thrust (and therefore acceleration) you need to either dump large amounts of propellant into the engine (which means extremely large tanks) or increase your exhaust velocity.

More precisely, in order to have the same change of momentum over time (which is mass times velocity) but using only half the propellant consumption rate you'd need to increase its speed twice, and the energy required to do that four times. So if you want to drop the propellant flow rate from say 200 kg/s to just 2 kg/s, your power source would need to be 100[super]2[/super] times more powerful.

A 1 g drive (pretty much the holy grail of intrasystem travel) would be capable of reaching 0.1c no problem. The drive itself would be a problem however. Not unsolvable, but still pretty darn difficult.

 

[kelvinzero]

And in practice because you have to carry your propellent along with you, it is even worse than squared, the mass requirements go up exponentially!

http://en.wikipedia.org/wiki/Tsiolkovsk ... t_equation

 

[theridane]

From a point, yes. If your delta-v requirements are lower than your effective exhaust velocity, then it's all daisies. Once you want delta-v greated than your v[sub]e[/sub], things go bad (and exponentially).

 

[theridane]

To expand on that - the problem is that with a non-energetic fuel (i.e. low exhaust velocity, low specific impulse) you need to carry lots of it. The amount you need grows exponentially as kelvinzero said.

So naturally you'd want to use a fuel that has a high exhaust velocity. And that's the problem, because the larger the exhaust velocity is, the lower the energy efficiency of the motor is (kinetic energy is 1/2 * m * v[super]2[/super]).

You don't even need to have ridiculous requirements (think Star Trek) to reach petawatt numbers in your equations. Even if we could build a ship with such a powerful reactor, if a mere one percent of its energy is lost as waste heat (and it's pretty obvious that it's gonna be way more than one percent) the ship would just vaporize itself. Gone.

So it's either ludicrous powerplants and even more preposterous radiators to cool them down, or a huge amount of propellant. And you have to pick at least one.

 

[kelvinzero]

There is some speculation of methods where you do not carry your own propellant. The most famous example is the light sail powered by massive lasers near the sun. I suppose this would give you constant acceleration for the craft, limited only by what the lightsail could bear, but the power requirements back home would have to keep being built up over the trip. The sorts of acceleration mentioned never seem to be near 1g though.

Getting speculative now, if you could somehow trap light between two perfect mirrors, one on a moon, one on the craft, in theory nearly all its energy could end up in the lighter object.

Perhaps you could beam something with a better push than light, such as plasma, or even mass driven propellant capsules which could be strung out like a beaded necklace along your course before you even begin. If you disregard the set up time, during the trip you could have one-g acceleration.

 

[Ishimura_]

So, given the current level of technology, what is the best speed (man or unmanned) we could travel in outer space (assuming the will power and finances exist to make it happen)? I'm trying to comprehend how far we've come and how far we still have to go (on the technical, not political side of things)

 

[MeteorWayne]

Where we are:

The fastest spacecraft ever launched, New Horizons, is now between the orbits of Saturn and Uranus. It weighs only 478 kg, or 1050 lbs. It is traveling at 16.36 km/sec or 0.000055 c.

 

[kelvinzero]

Though since these missions take years, I guess we have already surpassed this with current technology.

Anyone got a good estimate of where we are at right now? There have been lots of claims about VASIMR.

In the meantime, look at all these worlds to explore in just our solar system.

http://en.wikipedia.org/wiki/List_of_So ... ts_by_size

I often inject this link when people worry about how daunting interstellar travel is. There is a lot to do right here before worrying about that. Arguably these small icy worlds towards the bottom of that list are better than earthlike ones for colonization, because they probably have all the chemical elements we need and are much easier to land on and leave.

 

[MeteorWayne]

Please read what I posted. The best we've been able to do so far is 0.000055 c. with a spacecraft far smaller than to even permit one human to survive in. We are decades or centuries away from interstellar missions.

 

[theridane]

Atlas V (New Horizon's launcher) is hardly state of the art propulsive technology.

The best (with regard to delta-v) we have flight tested so far are ion thrusters. As a real-life example, NASA's Dawn spacecraft is expected to perform a velocity change of over 10 km/s entirely on its own over the course of its mission.

Assuming a one-ton spacecraft frame with two tons of Xenon propellant, using Deep Space 1 heritage electrostatic thrusters (de facto obsolete by now, but let's use them as a worst-case scenario demonstration). When you feed these figures into Tsiolkovsky's equation (dv = ve * ln((m_frame + m_propellant)/m_frame)) you'll get a delta-v capability of over 33 km/s.

A larger amount of propellant would of course increase this number, for example to 55 km/s with 5 tons of Xenon. This shows that there really isn't any upper limit on the achievable velocity (apart from special relativity of course). Only that at some point it becomes impractical to increase it, because the achievable delta-v grows logarithmically with respect to propellant mass fraction (i.e. even slower than linearly).

To find this threshold we need to define practicality. Let's assume that it's still practical for us (mainly economically) to have a spacecraft with a propellant mass fraction of 8 - that is 8 parts of propellant mass per 1 part of spacecraft and payload mass. Assuming we use the most propellant-efficient engine available today (that's actually being built, not one that is theoretically researched but not ready for deployment), Ad Astra's VASIMR with up to 30 000 seconds of Isp, we get (again, Tsiolkovsky) a delta-v capability of almost 650 km/s, or 0.22% c, or about 1850 years to reach the nearest star system. So that's what our current, state of the art and ready to fly practical velocity is, assuming the will power and finances exist to make it happen.

It has one drawback though - piss poor thrust levels. The 0.22% c VASIMR ship would have only 5 newtons of thrust available, which is perfectly OK for a deep space probe, but completely inadequate for intrasystem travel, or even manned flight.

 

[kelvinzero]

Thanks theridane,
And I guess that was something like what I was imagining, ie that current ion propulsion technology had improved by some significant factor over the old ion drives in current missions.

Still a long time to other stars, which is why I keep pointing out what an interesting solar system we have right here

 

[SpaceTas]

You can't do it if you take your fuel with you.
But if either power is beamed light-sailing
or if you get your fuel on the way interstellar ramscoop
it may be doable.

 

[neilsox]

With present technology, interstellar travel is extremely impractical. Some ideas in this thread might work if we spend incredible amounts of money, annually, for centuries. Typically we accelerate at several g for seconds to minutes to get to about 30 kilometers per second max. 0.1 c is about 30,000 kilometers per second. 100 kilometers per second may be possible with a 5 stage rocket and multiple sling shot maneuvers = gravity assist maneuvers. If we use a bunch of ion engines to accelerate at 0.001 g for a thousand times a million seconds. We will need casches of reaction mass and replacement parts placed along the route. v = at = 32.2 million feet per second = 22 million miles per hour = 6100 miles per second = 0.033 c = 10 light years at this speed takes 300 years, but you flash past the destination as there is no practical way to slow down. Worse it took a billion seconds = 277,777 hours = 11,574 days = 32 years to reach that speed, and we don't have ion engines or equivelent, yet that can accelerate a practical craft at 0.001 g yet, soon perhaps.
Collisions per second with sub atomic particles increase slowly up to about 0.033 c. Faster, the number of collisions per second is approximately proportional to the speed. Worse the speed of average collisions is also roughly equal to the speed, so the radiation from the collisions become a major problem. Avoidance of collisions with grain of sand size particles (and larger) also becomes increasingly challenging and costly. Neil