Re: [FT] Solar Thrust: Strategic spaceflight

On Thu, 11 Mar 1999, Matthew Seidl wrote:

> I was playing arround with some calculations about travel time between
> planets last night, assuming the FT example on the page.  So D-T
> engines, 15 minute turns, etc.  It seemed to me that using these rules
> it would take about 25 months to travel from the earth to Mars,
> assuming you wanted to stop at mars and then get home later.	Very
> rough math.

For a Hohmann transfer (minimum-energy, longest time) orbit, it takes
about 8 months to get to Mars from Earth. For any other orbit the energy
will be higher.  But also by increasing the energy you can get there
faster, so there are two variables in operation here.  The program will
let you play with both.  I should give a quick definition of the orbital
elements, I think:

*Semi-major axis (a) = half the length of the elliptical orbit

*Eccentricity (e)    = the shape of the orbit, 0=circle, 1=parabola

*Longitude of Perihelion (Pi) = the orientation of the orbit, this is
the
angle between the semi-major axis and a reference line (for the Solar
System, that's the poition of the Earth on the spring equinox)

*Rotational velocity (omega) = how fast the planet revolves about the
Sun

True Anomaly (nu) = where the planet currently is on its orbit at lauch
(note: for both target and origin), 0=perihelion, 180=aphelion

Arrival time (t2) = the number of years of flight time you want between
origin and target

The quantities marked with a * are constants for a given body, the
program
has values for all the planets stored in it.

So, for example, on 9 March 2063, the true anomaly of the Earth is about
190 degrees and the true anomaly of Mars is about 224 degrees.	If I
want
to arrive in 3 months (0.25 years) I need a delta-V of 24.877 km/s (for
a
DT-fusion drive that means 68% of my rocket needs to be fuel; for a
He3-D
fusion drive it's only 0.3%).  If I don't mind a little over 8 months of
travel time (0.7 years), I'll only need a delta-V of 5.78 km/s.  This is
very close to a Hohmann orbit.

Hmm.. there actually may be a bug in the fuel calculations, I just
noticed
an answer I didn't expect.  I'll have to check that.  The delta-V is
okay
though.

> So, when talking about SolarThrust, what kind of time delay are you
> thinking about for inter planatary fights?

I'm looking at strategic turns of 88 days (Mercury's orbital period,
call
it 3 months).  That seems to make things work fairly well.

I'll let you know about the fuel calculations when I get a chance to
check
it.  That was a last-minute add - I should know better.

TTYL..
Keith

kwatt@astro.umd.edu
Univ. of Maryland Astronomy