# Thread: How to propel one's spacecraft

1. In stock car racing the brake discs can glow red hot. The kinetic energy has to show up somewhere when you decelerate, car or spaceship.

Its those pesky inconvenient laws of conservation.

If you wanted to destroy a solar system accelerate a few kg to light speed and quickly decelerate it.

2. Originally Posted by steve_bank
In stock car racing the brake discs can glow red hot. The kinetic energy has to show up somewhere when you decelerate, car or spaceship.

Its those pesky inconvenient laws of conservation.

If you wanted to destroy a solar system accelerate a few kg to light speed and quickly decelerate it.
Check my arithmetic please.

I assume my spaceship has mass of 2 gigatonnes and is traveling 30,000 km/sec. i.e. almost 1 trillion km/year.
Assume the star we're approaching is about the size of our Sun. i.e. 2 ronnatonnes, or 10^18 times our spaceship mass.
To transfer the (2 gigatonne X 1 trillion km/year) momentum to the sun we will have to impart it with 10^-18 of our spaceship velocity, or 1 millimeter per year if my arithmetic is correct.
I doubt whether this 1 millimeter-per-year velocity impulse will be enough to "destroy [the] solar system."

3. Originally Posted by Swammerdami
Deceleration, at least naively, might be as big a problem as acceleration. Getting to Sirius in 300 years will do little good if we're racing at 30,000 km/sec when we get there and can only wave as we sprint past.

By accurately aiming the spacecraft toward the Star (but not on an exact collision course) can't we do a hair-pin turn, and repeat the manoeuvre a few times to get a less eccentric orbit?
Just skimming close by a star won't change your velocity relative to that star - you will be on your way out with the same velocity (in the star's reference frame) as you came in, and won't enter an orbit, not even an eccentric one. The velocity gained during the approach exactly matches the velocity shed while you move away. You will change your direction and may well have slowed down relative to Earth/Sun, but that's no good for staying in the system. You'll need a third body (planet will do, companion star is better) to change your velocity relative to the star. It's for this reason that long-period comets, even if they get close to the sun on their closest approach, generally need their orbit to be perturbed by Jupiter in order to become short period comets.

What could work is if you have a light sail that you can extend while approaching the star and fold up while retreating. That way, the velocity gained during the approach remains less than the velocity shed on the way out. If your initial velocity was not much above the system's escape velocity, this might just be enough to get you into an eccentric comet-like orbit.

I imagine the motive force for the steering to be provided by some sort of magnetic sail. And with such a sail near a star, can the need for ejecting matter for impulse be avoided? Can't momentum be exchanged with the star via magnetic interaction?
Not sure how that would work. Maybe someone else can elaborate more.

4. Originally Posted by Swammerdami
Originally Posted by steve_bank
If you wanted to destroy a solar system accelerate a few kg to light speed and quickly decelerate it.
Check my arithmetic please.

I assume my spaceship has mass of 2 gigatonnes and is traveling 30,000 km/sec. i.e. almost 1 trillion km/year.
You only accelerated to 0.1 c. That's not Steve's scenario. Of course what he said is literally impossible, but allowing for some artistic license, presumably he meant accelerate a few kg to near light speed. Depending on how near, that could be an awful lot more energy than your piddling 10^29 J. Back of the envelope, Steve's method would destroy a star if he stopped a few kg in the star from a projectile going at one light-year minus a micron per year.

(Of course, it's not obvious the few kg would actually stop. Maybe his projectile would just punch through and come out the other side of the star without slowing down much.)

5. Originally Posted by Swammerdami
Deceleration, at least naively, might be as big a problem as acceleration. Getting to Sirius in 300 years will do little good if we're racing at 30,000 km/sec when we get there and can only wave as we sprint past.

By accurately aiming the spacecraft toward the Star (but not on an exact collision course) can't we do a hair-pin turn, and repeat the manoeuvre a few times to get a less eccentric orbit?

I imagine the motive force for the steering to be provided by some sort of magnetic sail. And with such a sail near a star, can the need for ejecting matter for impulse be avoided? Can't momentum be exchanged with the star via magnetic interaction?
A slingshot maneuver doesn't change your velocity relative to the object used for the slingshot. NASA plays cosmic billiards because the measurement that's important is relative to the sun, not to the planet they used.

If you want to use a star to slow down the only way you're going to do it is an aerocapture maneuver--plow through the edge of the star. (And, yes, it doesn't really matter that it's a star--if the star can damage your spacecraft the proverbial snowball in hell will be in a far better position than you will be against the energy of the aerobrake.)

Besides, I've worked it out in the past and the acceleration is extreme. Makes a bullet at the moment of firing look positively placid.

6. How about a big reconciles gun in space, or a magnetic rail gun?

I think in Heinlein's Moon Is A Harsh Mistress rail guns were used to send people and material to Earth from the moon.

There was an idea to use a rotating catapult n space.

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