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A Russian billionaire wants to send postage stamp-sized probes off to Alpha Centauri using meter-square light sails. Regardless of the practicality of such an attempt, is there a lot to be gained by using a small chemical rocket to boost a deployment mechanism out of LEO up to some speed before deploying the sails, or is the gain so small that it's not worth the effort? Would we gain years, or only days compared to just launching them from an orbital platform* by unfurling the sails and shooting a laser at them?

*By "orbital platform" I mean one in Earth orbit.

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  • $\begingroup$ What do you mean by "boost a deployment mechanism up before deploying the sails"? The way they plan to do it is to use a conventional rocket to get to GTO, then unfurl the sail and push on them with lasers. The sail has to be out of the atmosphere otherwise it probably won't be able to overcome Earth's gravity and the atmospheric drag. $\endgroup$ – ventsyv Apr 13 '16 at 22:21
  • $\begingroup$ Sorry to be unclear (I'll edit), but I meant using a chemical rocket with LEO as a starting point. $\endgroup$ – J.D. Ray Apr 13 '16 at 22:34
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Chemical rockets would be to get out of the region where the sail is not designed for and wouldn't work; e.g. drag in the thermosphere is greater than the thrust, or the delicate sail can't handle the 2,000 °C atoms no matter that there's not many of them, or whatever.

Designing the sail to handle conditions closer to Earth instead is presumably a net loss compared to optimizing them for the cruise phase and supplimenting them at an early step.

No matter what, you'll need conventional rockets to get off the ground and into some orbit, so planning that step to be higher than the minimum possible for a more conventional platform makes sense.

It's not going to add to your total speed. That kind of comparison just doesn't make sense. In a cross-country drive, would you arrive sooner if you parked your car in the livingroom so you didn't waste time walking to the garrage?

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Using a chemical booster makes sense with a "classical" lightsail, that is one with a diameter measured in hundreds of metres to kilometres in diameter and powered by sunlight.

The sail in LEO will suffer significant atmospheric drag (due to its vast size and low density), and the operator needs to do some pretty dramatic orbital maneuverers to accelerate out of Earth orbit (orienting the sail edge on to the sun when the orbital path is against the desired direction of travel, then tacking the sail to maximize the sun's energy when it provides a positive deltaV, which is also difficult with a very large and flimsy sail.

A large sail which is boosted to HEO, geosynchronous orbit or even given a kick into interplanetary orbit will simply take advantage of the higher orbit and velocity and run with it. The main advantage of any sort of sail is the ability to continuously accelerate or decelerate without using any reaction mass, so starting with a higher initial velocity and away from any drag will amplify the advantages of using the sail in the first place.

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  • $\begingroup$ Fair enough, but what I was driving at is "how much time can we shave off by using initial-boost chemical propulsion and letting light sails take over after that?" $\endgroup$ – J.D. Ray Apr 13 '16 at 22:54
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    $\begingroup$ @J.D.Ray I don't know how much we can answer that from the limited information the article provides. For example, they suggest using 100GW lasers to propel the craft forward. If you were to put all that power onto 1 m^2 of sail that's a few hundred atoms thick, even if its reflectivity was 0.999999, you'd still have a crispy piece of tin foil. The alternative sources of energy like lasers really factor into the question of whether its worth it. $\endgroup$ – Cort Ammon Apr 14 '16 at 4:46
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No. Those rockets are punny. Let's assume it can accelerate you to 9.3 km/s (what's needed to reach LEO). That's 33,480 km/h, then you deploy the sail. According to the article the mission will reach top speed of 160,000,000 km/h. (100 million miles)

That's an increase of 0.02%, 99.98% of your speed still comes from the sail.

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  • $\begingroup$ And that's not even taking into account the additional costs that would be incurred during the launch. Adding a bunch of weight (many, many small chemical rockets for the many, many sail-based miniature satellites you're sending up) to your large rocket, necessitating additional fuel, which adds weight, necessitating additional fuel (repeat until you converge between weight and fuel) for a 0.02% gain probably isn't a great return on investment. $\endgroup$ – Ellesedil Apr 14 '16 at 0:34

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