A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Space Science » Science
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

Alternative to Rockets



 
 
Thread Tools Display Modes
  #51  
Old March 26th 04, 12:23 AM
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

In article , (Greg) writes:
wrote in message
and using this, the light generated force on the sail is simly

F = K*a/r^2

Note that this has the same form as gravity, only repulsive instead of
attractive, so you can associate with it an 1/r potential energy (with
positive, instead of negative, sign). Thus, startin tracking the sail
from some moment when it has a velocity v and is at distance r from
the sun, you can write for its energy

E = m( v^2/2 - GM/r + K/r*(a/m))

where m is the mass of the sail (and all that's attached to it).


This is not correct because the *direction* of the force is *not* away
from the sun in general. In fact its *not* constant in general.


Indeed, and when it is not exactly away from the Sun, the force
*decreases*.

It's not a conservative "feild",


Again, the "conservative field" part is a charitable assumption. You
can always make things worse. You cannot make them much better.

that is the path the sail takes and its
relative angle history all effect the final velocity.

The only extra gain you can have beyond the numbers I provided is by
getting the sail tilted around the closest approach point, to get a
force component along the trajectory. This only yields an advantage
along a short fraction of the trajectory in the case described. The
overall result doesn't chage appreciably. You're welcome to try and
calculate.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
  #52  
Old March 26th 04, 04:51 PM
Ian Woollard
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

Henry Spencer wrote:

The idea is not ridiculous, but advanced propulsion in general is very
poorly funded. The laser rocket suffers from needing a very large laser,
which is a big up-front capital expense.


Yes. The rule of thumb is that a laser to launch 1kg payload to orbit
needs 1 MW. Another rule of thumb is that currently lasers cost order
$1/watt and are getting rapidly cheaper.

However a laser rocket might currently make more sense as a second
stage- the power of the laser is proportional to the thrust required and
the thrust required goes down with burn time, and after staging; this
means that less than the 1MW rule of thumb is needed.

In addition, fuel for laser rockets is very lightweight (pun not
intended, often hydrogen is proposed), and so the first stage would
benefit from not having to carry so much mass.

For near-term applications, it
is not obvious that the money needed for that wouldn't be better spent on
improved application of conventional rocket technology.


Possibly- but it's not completely clear. In particular laser launch
usually deals with smaller orbital payload sizes than conventional
rocketry sensibly can address. Whether there is a market for small
payloads isn't all that clear however.
  #54  
Old March 31st 04, 02:45 AM
external usenet poster
 
Posts: n/a
Default Alternative to Rockets

In article , (Greg) writes:
wrote in message
The only extra gain you can have beyond the numbers I provided is by
getting the sail tilted around the closest approach point, to get a
force component along the trajectory. This only yields an advantage
along a short fraction of the trajectory in the case described. The
overall result doesn't chage appreciably. You're welcome to try and
calculate.


my point is that your *proof* was flawed. Your derived bound is not
indeed a bound.


It is close enough to a bound to not make any significant difference.

Example: Tether pair of solar sails with accurate independant tilt
control (also rapid), there speeds of rotation can simply continue to
incress untill the teather breaks...

You can play all the games you want, it doesn't matter. The scenario
I described is one in which you already played *all the possible
games*, within the system, using if needed multiple orbits to get
yourself to the optimal *final pass*. And the optimal final pass is
one where the excentricity of your orbit is as close to 1 as possible
(i.e. sum of kinetic and gravitational energy close to 0) and your
final perihelion is as close as survivable. And at this point you
unfold your sail and go out. Remeber, that's the finite pass.

So, what more you can do at this point? Well, the scenario I
calculated for is one where you let the sail point straight away from
the Sun. The optimal scenario (energy transfer wise) is one where the
sail points halfway between the "straight away" direction and the
"tangent to the orbit" direction. The difference between the two is
negligible except in the immediate vicinity of the Sun. And, the
contribution of this difference can be estimated as well. In the
limit where the force on the sail is just barely larger than gravity,
you can, by maintaining optimal orientation, improve on the final
energy by a tad less than 30% and on the finite velocity by about 15%.
But, as I said, that's for a poor sail. The better the sail (in the
sense of area/mass ratio) the smaller the improvement since the better
the sail the faster do the tangential and straight away directions
approach each other. For a realisticall good sail you won't gain more
than few extra percent above the previous estimate. And there are no
more gains available.

Using the reverse angular momentum manover speeds of up to 50 AU per
year are possable with moddest tech addvance in solar sails.


50 AU per year is about the number I got (and no, we're talking
serious, not modest advance). 210 km/s is 44 AU per year. And at
this rate it'll take you a very long time to get anywhere.

Sure you
need to wait a long time to get to the nearest star. But it *is*
doable. This kind of space exploration will need missions lasting many
10's of years and even lasting gererations.


Can you calculate? No, it is not "10s of years" or "generations".
We're talking below 0.001c. At this rate it'll take upwards of 4000
years to get to the nearest stars. That's comparable to the time
since the construction of the pyramids till today. Even the
assumption that the civilization which launched the probe will still
be around when the probe gets somewhere is quite optimistic (based on
prior data).

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Titan 4s costly AllanStern Space Shuttle 9 February 17th 04 06:02 AM
Von Braun rockets on Encyclopedia Astronautica Pat Flannery Space Science Misc 41 November 11th 03 09:10 AM
Rockets George Kinley Science 29 August 1st 03 06:06 AM
"Why I won't invest in rockets for space tourism ... yet" RAILROAD SPIKE Space Station 0 July 30th 03 12:06 AM


All times are GMT +1. The time now is 02:23 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.