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Velocities etc. to be got by solar sails

Posted by: Ekkehard Augustin - Tue Jun 21, 2005 7:35 am
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Velocities etc. to be got by solar sails 
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Post Velocities etc. to be got by solar sails   Posted on: Tue Jun 21, 2005 7:35 am
This morning I read that by a solar sail the velocity of 16,000 km/h can be got in 100 days - and 160,000 km/h after three years. Would be 4.444 km/s and 44.444 km/s.

May be that these numbers are valid for Cosmos 1 only.

Are these numbers correct? What is the largest possible size of solar sails currently? And what velocities could manned vehicles get by them?



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Post    Posted on: Tue Jun 21, 2005 6:21 pm
wow not realy anseering anything here but ive just read from the BBC

"Over one day, its speed would reach 45 m/s (100mph); in 100 days its speed would be 10,000 mph, in three years 100,000 mph. At that speed, a craft would reach Pluto, the most distant planet in the Solar System, in less than five years."

WOW!!!

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Post    Posted on: Tue Jun 21, 2005 6:42 pm
I am guessing that assumes constant accelleration. However, the particle density of the solar wind decreases according to the inverse-square law. Thus, the rate at which you are accellerating will decrease (dramatically) as you move farther away from the sun; thus probably precluding Pluto's orbit in 5 years. I'll confess to not actually having done the math, though... I'd probably have to dig out a reference or two to make sure it was done right, and I haven't got the time just now.


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Post    Posted on: Tue Jun 21, 2005 11:07 pm
Yes, that assumes constant acceleration, 100 mph per day x 1,000 days = 100,000 mph. The distance covered during that 1000 days, assuming 0 starting velocity, would be 1/2 * 100 * 1,000^2 = 50,000,000 miles, a little past Mars. According to: http://www.planetary.org/solarsail/faqs.html which also quotes the above numbers,
Quote:
However, once you get much beyond the orbit of Jupiter, energy from sunlight is too weak. When far from the Sun, lasers can be directed at the sails. Lasers stay in a tight beam so that most of their energy can be imparted to the sail and not diffused into space. Very large lasers in Earth orbit or in the inner Solar System could be used to help us travel to other stars. In the future, people may travel to distant stars using laser powered solar sails.
So maybe a little past Mars is not too far.

Cosmos 1 was launched today. It is not yet clear if it is operating or not as the expected signals have not been detected. However, even if it is successful it is not expected to get anything like 100 mph per day acceleration because it is too heavy for the size sails it carries. To get 100 mph per day acceleration you need a VERY light vehicle and VERY large sails.


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Post    Posted on: Wed Jun 22, 2005 4:04 am
Sawss1jun21 said:
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However, the particle density of the solar wind decreases according to the inverse-square law.


Not to be a little prick going around correcting everyone, but I believe it's light pressure we're concerned with here. The solar wind would be employed by a mag-sail.

I wonder if in the future there will be a poster here named sawcosmos1jun21? It's getting to be an interesting day of the year.

Peter quoted:
Quote:
When far from the Sun, lasers can be directed at the sails.


What I want to know is, how difficult is that? It seems to me it would be extremely difficult. I mean sure, you can predict with orbital mechanics how things are going to move, but with the laser itself orbiting one object, and the target moving, and the distances involved, it seems like it would get really hard just to physically aim the laser.

EDIT: I just read the news right after I posted. That sucks dude.


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Post    Posted on: Wed Jun 22, 2005 9:17 am
May be aiming the laser in't that problem yet today - given the increasing speed or frequency of computers, their increasing precision and the highly increased and still increasing precision of telscopes, radars, interferometers and communication techniques and technologies. Think for example of the laser communication experiment by one of the next Mars probes.

The failure of the russian rocket may be a good argument for manned delivery of probes etc. to space and their installation in orbits. Or - more eseentially - a very good argument to invest significantly more thoughts, research and development into human or manual control of engine shut down, engine ignition and much more including steering a rocket or a vehicle. And it is an argument against exclusive control by computers and automation which alread is under discussion by at least one other thread.



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Post    Posted on: Wed Jun 22, 2005 2:56 pm
campbelp2002 wrote:
So maybe a little past Mars is not too far.


Yeah, but you used constant accelleration, and by the time you get to Mars' orbit the light pressure (which is ALSO governed by the inverse-square rule, but thanks for the clarification, Marshall) is less than 25% of what it is in Earth's orbit (orbit of mars > 2 A.U.)

You have to do the calculus where you integrate the changing accelleration over the distance covered (that's the math I was avoiding)


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Post    Posted on: Wed Jun 22, 2005 3:14 pm
SawSS1Jun21 wrote:
(orbit of mars > 2 A.U.)
Not quite. Orbit of Mars varies from 1.37 to 1.66 A.U. But 50,000,000 miles is only 0.53 A.U. Light at 1.53 A.U. would have an intensity of about 43% of Earth normal. Still, you are correct, acceleration is not really constant.


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Post    Posted on: Wed Jun 22, 2005 3:22 pm
campbelp2002 wrote:
Not quite. Orbit of Mars varies from 1.37 to 1.66 A.U.


DOH!!! I did the km/mi thing! (saw orbit of mars as > 200M and figured "that's more than twice 93M") Perhaps I can get a gig at JPL now.

So, you don't want to do the integral either, huh?

P.S. Ekke: Here is the answer to the question asked by this thread:
http://science.howstuffworks.com/solar-sail1.htm
Quote:
Eventually, however, the continuous force of the sunlight on a solar sail could propel a spacecraft to speeds five times faster than traditional rockets.

... we just need one of our chemical impulse gurus to tell us what the speed limit is (or look it up ourselves), and multiply by five.


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Post    Posted on: Wed Jun 22, 2005 3:35 pm
Of course the maximum speed of a chemical rocket depends on numerous assumptions like mass fraction and exhaust velocity. The maximum sail velocity depends on other assumptions like total vehicle mass, sail area and local light intensity. A 99.9999% mass fraction chemical rocket could go WAY faster than a 1,000 kg vehicle with a 1 square meter sail starting from Saturn. Ridiculous assumptions, I agree, but my point is that statements like "a sail could go 5 times faster than a rocket" are just wild guesses at best. At least right now.


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Post    Posted on: Wed Jun 22, 2005 3:38 pm
Is there not a real, hard speed limit to be obtained by chemical impulse propulsion, regardless of mass fraction and assuming a vaccuum for the entire burn? I thought there was. Perhaps I am mistaken.


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Post    Posted on: Wed Jun 22, 2005 3:53 pm
SawSS1Jun21 wrote:
So, you don't want to do the integral either, huh?
OK, I calculated it would take 1717 days and 107,000,000 miles in reality instead of 1000 days and 50,000,000 miles assuming constant acceleration. I just used Excel. The first day the vehicle is 93,000,000 miles from the sun at 0 speed and accelerating at 100 mph per day. After 1 day it is 93,000,100 miles from the sun at 100 mph speed and accelerating at 100 * (93,000,000 / 93,000,100)^2 and repeated until the speed was 100,000.


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Post    Posted on: Wed Jun 22, 2005 3:56 pm
SawSS1Jun21 wrote:
Is there not a real, hard speed limit to be obtained by chemical impulse propulsion, regardless of mass fraction and assuming a vaccuum for the entire burn? I thought there was. Perhaps I am mistaken.
No. In theory a mass fraction infinitely close to 1 would get you an infinite final velocity. In reality there are many engineering limits that prevent really large mass fractions.


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Post    Posted on: Wed Jun 22, 2005 4:02 pm
Ah, integation by parts! Not as accurate, of course, but for >1700 parts it should be close enough.

So it takes 70% longer than advertised, I guess that's not terrible. If we assume some nominal accelleration continues throughout the remainder of the trip, I guess we can make pluto in less than five more years (we've already used about five years getting to just past Mars, right?)

OK if you're not in a hurry. I would still favor NTP or even NEP if I was making a serious interplanetary jaunt, however.


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Post    Posted on: Wed Jun 22, 2005 4:09 pm
Hello, SawSS1June21,

I don't have available other data currently and had doubts if the data provided by the german article are correct - they seemd to be a little bit high.



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