**Ask a monkey a physics question thread**

[Renton]

Would the fuel mass be so prohibitive if it weren't solid rocket fuel, but fissile material or some other dense fuel? I mean you're already going to need to provide the thrust required to make the craft get to cruising velocity, as well as slow the craft upon arrival, it's merely a matter of whether that thrust happens all at once at the beginning and end (which seems deadly to humans tbh) or gradually throughout the journey. This is especially the case if we're talking interstellar travel.

edit: I just realized how quick the rate of acceleration of 9.8m/s/s is. You would reach the speed of light in less than a year. If you wanted to travel at just 10% of c you would make it in like 35 days and only be like 0.25% of the way to Proxima Centauri. It seems like this method of artificial gravity would only work for very short distances.

[MadMojoMonkey]

Would the fuel mass be so prohibitive if it weren't solid rocket fuel, but fissile material or some other dense fuel? I mean you're already going to need to provide the thrust required to make the craft get to cruising velocity, as well as slow the craft upon arrival, it's merely a matter of whether that thrust happens all at once at the beginning and end (which seems deadly to humans tbh) or gradually throughout the journey. This is especially the case if we're talking interstellar travel.

I think all of your points here are worthwhile.

Fissile material has vastly higher energy density. There is really no problem with creating nuclear fallout in space. There are designs for spacecraft which gain thrust by launching what are basically nuclear warheads out the back. The entire back end of the ship is an enormous radiation shield and shock absorber. The materials and technology to build such a ship have been around since the 70s. The trouble is getting it to space w/o detonating a dozen nuclear bombs in the atmosphere.

The deadly to humans part is complicated. It kinda reminds me of this XKCD What If?

Most manned rockets need to remain under ~3.5 g's if the humans are expected to be able to reach switches and controls (with considerable effort). During extreme conditions, especially a launch abort sequence, the manned capsule can reach over 20 g's for a short burst. This is pushing the limits, as 25 g's is the standard accepted threshold of permanent damage. There are a few recorded examples of humans surviving much greater accelerations.

edit: I just realized how quick the rate of acceleration of 9.8m/s/s is. You would reach the speed of light in less than a year. If you wanted to travel at just 10% of c you would make it in like 35 days and only be like 0.25% of the way to Proxima Centauri. It seems like this method of artificial gravity would only work for very short distances.

@ bold: Hehe. No. You forgot about relativistic mass. The closer you are to the speed of light, the more massive you are. Meaning that it would take progressively more and more force to simply accelerate you the tiniest bit.

There is not enough energy in the universe to accelerate any massive object to the speed of light. Nor is there enough to slow any massive object even the tiniest fraction if it is already moving at the speed of light. Such an object would have infinite momentum.

Your estimate for 10% c is probably fine. Relativistic effects don't really show up until ~20 - 30% c. At 10% c, you'd expect your non-relativistic calculation to be off by ~0.5%.

Just to double check your calcs:

Spoiler: Show

Time to accelerate to 0.1 c at constant acceleration of 1 g:
Using c = 3(10)^8 m/s and g = 9.8 m/s^2
delta_t = 0.1 c / g = 3,061,224 s ~= 35.4 days

Distance covered during that time:
Using 4.37 ly as the distance to Proxima Centauri
x_final = 1/2 * (9.8 m/s^2) * (3,061,224 s)^2 = 4.58(10)^13 m ~= 0.005 ly
0.005 ly / 4.37 ly = 0.0011

After accelerating to 0.1 c at a constant 1 g, you would be 0.11% of the way to Proxima Centauri.
(This is the non-relativistic approximation.)

[Renton]

There is not enough energy in the universe to accelerate any massive object to the speed of light. Nor is there enough to slow any massive object even the tiniest fraction if it is already moving at the speed of light. Such an object would have infinite momentum.

My point was that you couldn't maintain a constant rate of acceleration of 1g for very long (within interstellar travel time frames). For some reason when I first approached this problem I just assumed it would take 500 years or whatever for 9.8 m/s/s to add up to relativistic speeds. Not so.


One thing is that it seems like you could achieve reasonably high speeds (say 0.1c for example) by applying a small thrust over a period of years that would only require a steady stream of a (relatively) small amount of energy. Could solar panels approach the needs of this, or at least make a dent so not so much fuel is needed?

[MadMojoMonkey]

My point was that you couldn't maintain a constant rate of acceleration of 1g for very long (within interstellar travel time frames). For some reason when I first approached this problem I just assumed it would take 500 years or whatever for 9.8 m/s/s to add up to relativistic speeds. Not so.

It's cool that you had the hypothesis and then tested it using theory.

The energy required to maintain the thrust would be the limiting factor.

No matter your speed, you can always accelerate. It's just that as your speed approaches that of light, your mass approaches infinity. So the amount of acceleration you get from the same force is ever decreasing as you go faster and faster. A constant force does not yield a constant acceleration when relativistic speeds are involved.

One thing is that it seems like you could achieve reasonably high speeds (say 0.1c for example) by applying a small thrust over a period of years that would only require a steady stream of a (relatively) small amount of energy. Could solar panels approach the needs of this, or at least make a dent so not so much fuel is needed?

I was going to point out that they are terribly inefficient, but that's only in comparison to other ground-based methods. Wow, double oops, 'cause I was thinking in terms of monetary cost efficiency instead of mass cost efficiency.

The ISS has ~ an acre of solar arrays on it, producing 75 - 90 kW of power. The solar arrays (panels with many cells) do rotate to orient the cells, but the cells may have great longevity for their operation time. I.e. there is probably a bonus in the fact that the solar cells do not have any moving parts.

Ongbonga has a good point about the amount of light diminishing as you move away from the sun. The utility of a solar panel would be pretty limited as you move toward the outer planets, and practically nothing as you move past the gas giants.

I thought about solar sails being a roughly similar concept. I know that we use solar sail concepts to assist in maneuvering satellites and probes, but I don't think there is a probe which uses solar sails as primary thrust. It faces the same problem with diminishing returns as it moves from the sun.