Ion drive & Physics question...

[vcf123]

So as I was listening to Astronomy Cast podcast about advanced propulsion, I was thinking what if you combined chemical rocket and ion drive. While we are doing that, I was thinking, why not we add solar sails on it... As I understand the physics, any object in the space will keep it's momentum forever. If that is the case any little acceleration that is added to a probe will gain more speed, right? Then, is it not possible combine other means of propulsion with ion drive? Say, start with two stage chemical rocket from the earth orbit and then deploy solar sail and ion drive. Add a gravitational slingshot on top of that... Would that combination be possible to reach great percentage of the light speed? And, would it be too costly for our generation to launch such a probe to nearby systems for a flyby mission...?

[cjameshuff]

So as I was listening to Astronomy Cast podcast about advanced propulsion, I was thinking what if you combined chemical rocket and ion drive. While we are doing that, I was thinking, why not we add solar sails on it... As I understand the physics, any object in the space will keep it's momentum forever. If that is the case any little acceleration that is added to a probe will gain more speed, right? Then, is it not possible combine other means of propulsion with ion drive? Say, start with two stage chemical rocket from the earth orbit and then deploy solar sail and ion drive. Add a gravitational slingshot on top of that... Would that combination be possible to reach great percentage of the light speed? And, would it be too costly for our generation to launch such a probe to nearby systems for a flyby mission...?

Neither ion drives nor solar sails are free in terms of mass. Depending on your needs, you would likely get more by dedicating the same amount of mass to a bigger ion drive or a bigger solar sail. Sails give better thrust for a given mass in the inner system, but further out from the sun, ion drives give far more...so if you're going to be making a lot of maneuvers far away from the sun (like Cassini, which is constantly adjusting its orbit around Saturn to make flybys of interesting moons), you'll want an ion drive. For probes intended to leave the solar system, solar sails with a fast flyby past the sun can theoretically do better than an ion drive. Large solar sails just haven't been done yet, so there's a lot of unknown problems that could be encountered, and not enough benefit to justify risking an expensive mission on under-developed technology.

[Grey]

And to be clear, neither ion drives nor solar sails nor gravitational slingshots around anything in the Solar system nor any technology with which we are reasonably familiar can get an object the size of a space probe to a "great" percentage of the speed of light. New Horizons, the fastest thing we've launched to date, is moving at about 0.0065% the speed of light.

Probably the fastest vehicle design that we're reasonably sure would really work would be the old Project Orion (not the new shuttle replacement), that would operate by riding the blast waves from nuclear bombs dropped off the back. Top cruise velocity would still only be about 8% to 10% the speed of light, though. It's possible in theory to move a lot faster than that, but we're sadly a very long way from being able to do it in practice.

[cjameshuff]

And to be clear, neither ion drives nor solar sails nor gravitational slingshots around anything in the Solar system nor any technology with which we are reasonably familiar can get an object the size of a space probe to a "great" percentage of the speed of light. New Horizons, the fastest thing we've launched to date, is moving at about 0.0065% the speed of light.

Actually, another use of sails can reach at least small percentages of the speed of light. A very large, very tightly packed, very high power laser array and photon sail combination has the advantage that the craft does not carry fuel, and can get relatively high and nearly constant acceleration. It's possible to brake this way as well, by jettisoning most of the sail and using it as a mirror to reflect and concentrate light onto the remainder. But yeah...nothing that would be described as "a great percentage of the speed of light".

[vcf123]

Maybe, I should my question clearer. I do understand that ion drive or slingshot alone would be enough to reach anywhere near even 1% of the light speed, though sails might be able to some degree. I was wondering if we can combine everything then can an unmanned probe reach greater speed? Maybe greater than 10% of the light speed?

[Grey]

Maybe, I should my question clearer. I do understand that ion drive or slingshot alone would be enough to reach anywhere near even 1% of the light speed, though sails might be able to some degree. I was wondering if we can combine everything then can an unmanned probe reach greater speed? Maybe greater than 10% of the light speed?

I think the answer would be "probably not". Even if you assume that the propulsion techniques were additive, stacking three or four methods that each alone can achieve less than 1% of the speed of light certainly isn't going to somehow get you to above 10%. As cjameshuff points out, adding additional propulsion systems also adds to the mass of the craft. So if anything, two propulsion systems together woould be less than twice as effective as either one alone, not more than twice as effective. If you put a second motor on your boat, you'll be able to go faster than you could with just one, but not twice as fast.

[Grey]

Actually, another use of sails can reach at least small percentages of the speed of light. A very large, very tightly packed, very high power laser array and photon sail combination has the advantage that the craft does not carry fuel, and can get relatively high and nearly constant acceleration. It's possible to brake this way as well, by jettisoning most of the sail and using it as a mirror to reflect and concentrate light onto the remainder. But yeah...nothing that would be described as "a great percentage of the speed of light".

Yup. Didn't mean to suggest that we couldn't achieve a small fraction of the speed of light using variants of those techniques. I figure that the point at which we'll actually try something like an interstellar probe is when we think there's a reasonable chance of successfully getting it there within the lifetime of the people planning the mission. Sadly, we've got a ways to go.

[cjameshuff]

Yup. Didn't mean to suggest that we couldn't achieve a small fraction of the speed of light using variants of those techniques. I figure that the point at which we'll actually try something like an interstellar probe is when we think there's a reasonable chance of successfully getting it there within the lifetime of the people planning the mission. Sadly, we've got a ways to go.

I'm a little more optimistic...I think it'll occur when the cost of a probe with a flight time of a century or so gets down to around the cost of a project like the LHC. People can and do tackle projects that they won't see the completion of, interstellar probes are simply too expensive at present.

I think the answer would be "probably not". Even if you assume that the propulsion techniques were additive, stacking three or four methods that each alone can achieve less than 1% of the speed of light certainly isn't going to somehow get you to above 10%. As cjameshuff points out, adding additional propulsion systems also adds to the mass of the craft. So if anything, two propulsion systems together woould be less than twice as effective as either one alone, not more than twice as effective. If you put a second motor on your boat, you'll be able to go faster than you could with just one, but not twice as fast.

Yeah. A solar sail will give an early boost when you do a slingshot of the sun, (look up "fast solar sailing"...essentially, use the sail to cancel out gravitational pull on the trip out of the gravity well, allowing the craft to keep velocity gained on the fall inward...gravity and photon pressure both fall off with the square of distance, so a sail that just barely supports the craft in a stationary position against gravity will allow it to keep all the speed it achieves at perihelion), but that boost will be greatly diminished when it has to carry the mass of an ion drive's power source and fuel, and will only occur close to the sun. And then your ion drive has to move the mass of the sail...once you get out of the inner system, a kg of ion drive fuel is worth much more than a kg of solar sail.

The fact that each is better than the other at a different part of the journey means that the optimal solution might be a staged launch, a solar sail at the beginning that gets jettisoned when the ion drive is started up. However, for an interstellar trip, an ion engine propelled vehicle will require a monstrous amount of fuel and a huge power source, and the solar sail required to carry that will be monstrous even by the most fantastic solar sail standards. And a solar sail's not going to contribute much to the final velocity...the amount of benefit it ultimately provides could easily turn out to be just too small to be worth the trouble.

If you're doing a laser sail launch, the opposite may be true...you want vehicle mass as low as it can be, to maximize the acceleration, and you want as much sail area as you can get, to extend the range at which you get useful thrust from the launch laser and to increase the thrust you can achieve without damaging the sail. A kg of sail might very well end up giving the vehicle far more speed than a kg of ion drive propellant ever could.

[eburacum45]

Sails are good for fly-bys, but not so good if you want to stop in your destination system.

[PeteG]

And to be clear, neither ion drives nor solar sails nor gravitational slingshots around anything in the Solar system nor any technology with which we are reasonably familiar can get an object the size of a space probe to a "great" percentage of the speed of light. New Horizons, the fastest thing we've launched to date, is moving at about 0.0065% the speed of light.

Probably the fastest vehicle design that we're reasonably sure would really work would be the old Project Orion (not the new shuttle replacement), that would operate by riding the blast waves from nuclear bombs dropped off the back. Top cruise velocity would still only be about 8% to 10% the speed of light, though. It's possible in theory to move a lot faster than that, but we're sadly a very long way from being able to do it in practice.

Project Daedalus is another reasonably feasible interstellar probe concept I've seen that can reach a somewhat considerable percent the speed of light.
However the cost of creating (or mining from Jupiter, as the project suggests) 50,000 tons of helium-3 and deuterium and the launching of another 50,000 or so tons of materials into orbit to be constructed there, would be a little over most space agency's budgets for the time being

[PeteG]

Sails are good for fly-bys, but not so good if you want to stop in your destination system.

Just a thought, but wouldn't sails be the best choice to stop, or atleast slow down once you reach the destination system?

The sail could be used as a kind of 'solar parachute' using the wind from the destination star to decrease the probes velocity. Whilst means such as conventional rockets or ion drives would require the craft to carry their fuel all the way.

[cjameshuff]

Sails are good for fly-bys, but not so good if you want to stop in your destination system.

Sails *can* be used to stop. If the sail is designed in concentric parts, and the outer ring designed to focus light, you could jettison the outer ring, allowing it to accelerate on ahead while you use the light reflected from it to brake. This would probably mainly be useful with beam propulsion within a system, though. Robert Forward used the concept fairly believably with an interstellar mission in Rocheworld, but that's a long, long way to shine a laser...

If you used the sail to launch, with no laser assist, you can of course use it to stop at the target system, as long as the star is sufficiently bright. There's a bit of leeway there, since you don't have to decelerate to the velocity you initially had in your source system, you only have to drop below escape velocity for the target system, after which you can just spiral in. It'll be a long trip without a laser launch, though. Likely thousands or tens of thousands of years.

Sails *could* help decelerate in a target system, but they won't do much good until you get right into the inner system. And then they still won't get rid of very much of the velocity required to make an interstellar trip in a few centuries, so you have to carry something that'll take care of the remainder during the time you have left before you exit the system. If you've got engines that can do that, you're still probably best off jettisoning the sail and decelerating hard on your way into the gravity well...the further out you get, the more velocity you have to lose to get below escape velocity.

[eburacum45]

An image of the reflection/deceleration concept for a sail here
http://www.sff.net/people/Geoffrey.L...sail/sail1.gif
but to decelerate at the destination using a beam sent from the home system is just a little optimistic, as lasers spread out a great deal by diffraction over such vast distances.

[cjameshuff]

An image of the reflection/deceleration concept for a sail here
http://www.sff.net/people/Geoffrey.L...sail/sail1.gif
but to decelerate at the destination using a beam sent from the home system is just a little optimistic, as lasers spread out a great deal by diffraction over such vast distances.

That can be resolved by using a large enough aperture, or by adding on lasers over the decades. A bigger problem may be tracking the thing over a century or so with years of lag. If you lose tracking, good luck regaining it...

The problem may be solvable by sending off subsequent sailcraft carrying thin film holographic optics (which could consist of the same material with patterned gaps in the aluminization) to keep the beam collimated and adjust its direction to track the ship. With a spacing of 1 light month, they could then notice loss of tracking within a month instead of the corrected beam will arrive within 2 months of the occurrence of the error, rather than 8 years or so. But then you've got to worry about one of them failing, and about imperfections scattering the beam...

Well, like I said, it might be more suitable for travel to the outer system. The gas giant moons, and the icy rocks in the Kuiper belt...