HN Explorer

New rocket engine concept stuck – help out

2 points by urb ↗ HN
Idea is to lower the dry mass to achieve higher acceleration by shooting fuel pallets from a space based platform to be then collected and ingested by the accelerating spaceship "riding the fuel trail". Problem is kinetic energy of pallet impact on ship seems to equal energy density of monopropellant at relatively low speeds. Thoughts?

7 comments

[ 3.0 ms ] story [ 32.7 ms ] thread
zeristor
I had a similar idea previously for interstellar travel. My idea was to fire fuel pellets ahead of the rocket so it would collect them up as it accelerated, relative velocity being not too much.

However Anything orbiting the sun wouldn’t be able to stay on target to fire the fuel for long. Calculating the trajectory to account for gravity would be problematic. The whole process would be too brittle, if something went wrong it could well be impossible to recover.

Nifty idea was to fire the pellets before and after, so when it flips mid-flight it could collect fuel pellets to decelerate.

neckardt
> Calculating the trajectory to account for gravity would be problematic.

Couldn't you just make every fuel pellet a powered spacecraft just a tiny engine? It could adjust its velocity when it drifts off course. That would add some weight, but a small ion engine might fit the bill. The engine could of course be disposed of to save weight once its captured by the larger ship.

The whole process could be safer if the rocket just keeps some extra fuel in case it doesn't manage to collect say every 10th pellet.

urb
Nice, but the small powered pallets would have to catch up with the ship and quickly won't be able to even if given initial momentum
zeristor
Yes, I had thought of a buffer for that, probably a design trade off.

But if the pellet track is moved off target you have to follow it.

By the way I like to call it the PacMan Rocket.

AnimalMuppet
Exploding fuel pallets. Here's one ton of fuel, a one-ton inert reaction mass, and an explosive charge that will fire at the right time to separate the two. It drives the fuel mass to nearly the velocity of the rocket ship (this also makes catching it easier). It preserves momentum by driving the reaction mass in the opposite direction.

Hint: Make sure the reaction mass misses the rocket ship.

urb
Ok good! But how do you contain the blast without harming the ship or sending it off course?
AnimalMuppet
Well, if you can manage it, the blast can be mostly linear.

I picture it like this: There's, say, a one degree angle between the trajectory of the ship and the trajectory of the fuel pallet. The explosion can happen arbitrarily far before their courses intersect. This means that the blast can be (arbitrarily far) * (sin 1 degree) away from the course of the rocket.

But that [Edit: the fuel pallet] transfers a small amount of sideways momentum to the rocket. To counter that, alternate fuel pallets from the left and the right.