[vansig]

these are great ideas. structural forms for the ship should be multi-purpose. the skin can double as a whipple shield and radiator, if the vascular structure for the refrigerant has lots of redundancy and failsafe valves. we should note that micro-meteors at .084 c will conceivably do a tremendous amount of damage; and everything has to operate for 50 – 100 years.

now, is there any way to make thrust scale better than linearly with mass?

in terms of scaling, 7500 five MW electrodes, that have to be replaced every 90 days, is about the same as 2500 fifteen MW electrodes, that have to be replaced every month, unless worn parts can be repaired/refurbished aboard ship.

[vansig]

a google search, for the names listed on their board of directors, reveals a little more character

[vansig]

The current alternative is VASIMR, which already consumes lots of electricity; can a below-unity FF reactor achieve greater specific impulse than VASIMR on the same input power? maybe it can.

[vansig]

I’d so-much like it to be true, but those laws of thermodynamics do get in the way, sometimes.

Even with next-generation technologies, we could probably only reduce the radiator’s mass to 22g per m². It would be a magic black cloth, having .999 emissivity, strong, flexible, lightweight, and double sided, and having a vascular structure, inflated with refrigerant. but it doesn’t radiate better than a perfect black body. So there must be 1 km² of it.

Laser cooling is a nice idea, but unless the lasers are separate from the ship, (strewn like breadcrumbs along the path), they add to the overall heat more than they take away.

[vansig]

Lungs are great! but as heat exchangers, they’re convective, not radiative.

Best emissivity for radiators cannot exceed 1 (a perfect blackbody), so I still get 1 km² area for a 37 GWt radiator at 900 kelvin.
at the thickness and density of household aluminum foil, 16 µm, double-sided, that’s 8 m³ x 3500 kg/m³ = 28t.

To make it useful for interstellar travel for the Icarus project requires bringing the combined mass of the radiators, 37 GW engines, fuel tank, and payload down to, say, 1t. That’ll be quite a trick, even with no radiator.

So once again, the only way to reject the excess heat is to vent plasma.

[vansig]

we’re close, but oh! so far..

the figure of 37GW was to provide 2016 N thrust at effective velocity of 0.028 c,
(which was the thrust needed per tonne of mass of the ship!)

at 50% efficiency, that’s 37GW useful work, and an additional 37 GWt to reject.

the radiator is way too massive, so the only way to reject heat is to send it off as additional propellant,
which increases thrust per engine, but reduces the specific impulse.

[vansig]

i’m calculating that a large radiator, 1 km² area, could keep a 37 GWt heat source below about 900 kelvin.

in terms of 5 MW generators, that’s about 135 m² area each.
still quite onerous to keep the total mass really low.

[vansig]

right, so that may lead to a fundamental limit.
what’s the lower bound on mass, of a ship with a 37 GW focus fusion power source,
including radiators and fuel tank, but empty of fuel?

[vansig]

actually i think the 400:1 ratio does account for slowing down. but, there are still problems.
i calculate the thrust needed to meet this as:

Thrust = ( delta-m/delta-t ) x Ve
— http://en.wikipedia.org/wiki/Specific_impulse

if m1 = mass of the ship, without fuel, then delta-m on the last leg (slowing down) is 19 m1. and
delta-m on the first leg is 380 m1.

mass flow rate = 380 m1 / 1.57788e9 s = 2.4e-7 m1 ( kg/s );
for a 1t stage, = 1e3 kg, then this yields 2.4e-4 kg/s, = 0.24 g/s flow rate.

then thrust needed = 2.4e-4 kg/s x 8.4e6 m/s = 2016 N.

but that seems unrealistic.. too high for FF, which consumes ~ 1 kg/year,
= 3.17e-8 kg/s, yielding 0.27 N

ability to use magnetic breaking would improve the situation; as would ability to run a million pinches per second

[vansig]

er, my bad.
i gave total distance of 1 lt yr, in error. correcting this, acceleration has to be 0.016 m/s². top speed will need to be .084 c,
and m0 / m1 = 20, for each leg.

so mass of fuel will end up having to be 400 x the mass of the ship.

[vansig]

Let’s throw some numbers in…

Distance to a star 4.2 light years away,
= 365.25 days/year x 86400 s/day x c = 9.467e15 m;

.5 of the journey is acceleration, and .5 is deceleration, each taking at most 50 years, to meet the project’s goal,
= 1.57788e9 s each;

Continuous acceleration required to travel .5 of the total distance in that time,
.5 d = .5 a t²;
9.467e15 m = a (1.57788e9 s)²;
a = 3.8e-3 m/s² (=.00039 g);
not too bad, so far.
So long as average acceleration is at least this through the whole journey, you can get there.

Top speed will need to be, at least
a t = 3.8e-3 m/s² x 1.57788e9 s
= 6000 km/s
= .02 c;

So, you’ll want calculate drag on a craft travelling through interstellar space at .02 c, which will yield important info about the maximum speed of the ship.

But also, unless/until reactionless drive is invented (a.k.a. star trek impulse drive), or unless this is a Bussard ramjet, you’ll have to carry sufficient reaction mass for the journey, and apply the rocket equation. — http://en.wikipedia.org/wiki/Rocket_equation

delta-V = Ve ln ( m0 / m1 );

Ve is the effective exhaust velocity, which could be as great as .028 c for Focus Fusion. delta-V = .02 c; so,
neglecting drag, m0/m1 ~= 2.0, for each leg of the journey.

[vansig]

dennisp wrote: Being that focus fusion is already a rocket, I’m wondering: what is the exhaust velocity?

Beyond that, has anyone figured specific impulse and thrust? Would it make sense to simply mix additional reaction mass into the exhaust stream for a higher-thrust application?

I asked a similar question awhile back, which was “what is the speed of the alphas?”, and got the reply ~0.028 c, (8400 km/s).
http://answers.yahoo.com/question/index?qid=20090402091850AAAnih4

exit velocity, and therefore specific impulse, will depend on what these carry with them as they leave the reaction chamber. yes, you can mix additional reaction mass for a higher thrust application.

thrust will be very low, since amount of material expended per pinch is very low, but FF becomes a viable form of propulsion even before it reaches break-even.

[vansig]

i bet that a pinch near atmospheric pressure is possible
(but probably at much higher voltage).

[vansig]

Aeronaut wrote:
Now it’s starting to sound like an unbalanced wheel is helping it past the magnetic lockup zone. 🙂

well, i could be wrong, but Minato has sold and delivered many thousands of units, so if there is any doubt about its operation, it’s possible to obtain one for detailed analysis.

it was a pretty obvious match from Muammer Yildiz’s youtube video.

[vansig]

Aeronaut wrote:
The YT vid of the Minato motor looks very similar to one I built last year from a lazy susan and a bunch of tiny N-42 magnets, but was trying to power mechanically instead of inductively. If the article I just read is accurate, Minato has an inductive motor running on 20% of the power that it should be. What am I missing here?

afaik it uses a mechanical trick: by physically moving each magnet at particular parts of the cycle, it allows gravity to assist. the result is, for low overall torque, an efficiency boost. it ends up making a good fan motor.

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