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Comment by thomasmeeks | Hacker News Reader

Turn the craft around halfway to your destination and burn to decelerate at the same rate you accelerated. Keeps a nice 1G on the body the entire time.

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Propulsion is provided from the earth /LEO/GEO (imagine blowing on a kite with giant fan).

turning the kite around will not make it slow down/return to you while you keep blowing on it.

Sail boats are able to sail into the wind. This, however, requires the drag of the keel to work. It might be possible to use a traditional rocket engine to act like a keel to allow using the laser energy to slow down the craft.

You use 2 kites on board the spacecraft. One gathers the transmitted beam and reflects it to kite #2, which then reflects forward again. Careful design is important to ensure the spacecraft in the middle isn't struck by the photons. And you will have significant losses by the double-reflection, so you have to turn earlier than 50% of the way there.

Wouldn't your idea, at best, null-out the forward propulsive force, but not actually decelerate the ship?

Not my idea - it's been around for decades.

It works because you can treat the two kites as one system, because they're both connected to the ship.

> Not my idea - it's been around for decades.

> It works because you can treat the two kites as one system, because they're both connected to the ship.

Care to supply some citations?

Physicist Robert L. Forward mentioned it in his book Rocheworld (later republished as The Flight of the Dragonfly)in the early 1980's, with the concept work being done in the 1970s.

https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight

That's a science-fiction book, though. I'm still not getting how the ship could basically sail straight into the laser that's pushing it (e.g. decelerate). The main sail is still getting pushed forward by the laser, even while the second sail is pushing backward (with the reflected light from the main sail).

His credentials in the field are pretty good, and his books are always hard-science (no elves or wizards, and very little hand-wavy stuff).

https://en.wikipedia.org/wiki/Robert_L._Forward

If I recall the design correctly (forgive me - it's been over 15 years since I read it), there's a significant size difference in the sails. The larger sail collects photons from the distant laser and focuses them on the smaller "backwards facing" deceleration sail.

Yeah, for this idea to work as transportation, you really need lasers at both ends: one to accelerate the ship, and other to decelerate.

So it doesn't really solve any problems of initial colonization (beyond fly-by surveys), but it could be used for commerce once a colony has already been established.

You'd also probably want the lasers based on the moon: no atmosphere but large enough not to fly off in the opposite direction.

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