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Comment by ISL | Hacker News Reader

By rotating the craft to point opposite the direction of travel and actuating the engine again.

And then just fly into your still-fissioning exhaust? Wouldn’t that be bad?

Well, presumably you wouldn't be accelerating just before you want to slow down, so the atomic bombs wouldn't be recently detonated and their debris wouldn't be nearby.

If you flip around and fire the engines to decelerate, you would be flying into the exhaust.

Edit: Actually this hurts my brain. If you're going 1,000,000MPH and fire propellant at 500MPH in your direction of travel, it would travel at approximately 1,000,500MPH and you would decelerate below 1,000,000MPH, so you actually wouldn't run into it. There's no drag on your exhaust in space like there is in an atmosphere...

Oh wow nice explanation. Thanks.

According to A. Einstein you shouldn't be able to tell the difference between two inertial frames of reference. Physics would behave exactly the same. So if you don't hit your exhaust in the first instance you won't in the other either.

Yeah, I'm just so used to watching Falcon 9 landings where it's falling through the exhaust because of atmospheric drag.

You don’t fly into your exhaust until you accelerate enough to start going in the opposite direction. If you’re driving down the road and throw an apple out the window behind you, you don’t run over it if you put on the brakes, you’d have to accelerate to cross zero velocity and then start going backwards to catch up. An apple will just sit on the highway, rocket exhaust will be moving with quite a bit of speed as well.

Your analogy isn’t really valid, because in the rocket situation you’re flipping the rocket and throwing exhaust forward to slow down. That would be analogous to throwing apples forwards, not backwards.

Aren’t the plume generally supposed to be faster and in opposite direction compared to the vehicle?

There’s no real concept of “faster” exhaust compared to yourself. The speed of the exhaust is relative to the frame of reference of your rocket which is an accelerating frame, that speed is always the same when measured against the stationary rocket in that frame. When measured from a fixed inertial frame the speed changes as your rocket accelerates.

For a more concrete example, for a moon rocket the speed of the rocket exhaust is around 3 or 4 km/s. On the ground the speed of the rocket is obviously 0, in low earth orbit the speed of the rocket is 7 or 8 km/s, and to initiate the transfer orbit to the moon you have to accelerate to about 10 km/s. (these would all be in earth centered, nonrotating frame speed measurements)

The rocket exhaust doesn’t have to get faster to get you to those higher speeds because you’re taking it with you.

The more you can increase the rocket exhaust relative to yourself though, the more efficient your rocket is.

Say a ship in 1D is flying at 50km/s, exhaust velocity is -25km/s, relative to a static frame.

Last bit of departure burn or correction burns will fly at 25km/s towards the ship, so if the ship decelerated to below that, the plume could catch up. Meanwhile, plume from deceleration continues at 75km/s away from the ship.

I was thinking that high Isp engines generally have insane exhaust velocity, like hundreds of km/s or more, that problems like this is not an issue even for interplanetary transfers. But interstellar is a bit different, depending on other factors such as dispersion, I guess?

A one dimensional rocket traveling from A to B: all of the exhaust emmitted after the rocket speed exceeds the exhaust nozzle speed will end up hitting B.

In three dimensions though in a hard vacuum, particles coming from a fluid with a bulk velocity of kilometers per second are clearly not going to be nearby the path of the rocket for very long at all

In an atmosphere, yes, with the engine pointing forwards to slow you down, the head wind would blow nuclear dust all over you as you were braking.

In space, the exhaust just flies off into the distance no matter which direction you are facing.

A different but related problem: if you arrive at my space house for space dinner in your nuclear space car, it would be quite rude for you to shower me with nuclear space dust (from when you hit the nuclear space rocket brakes to stop at my house) moments before you arrive.

This isn’t an issue if you go into orbit around a planet, where the braking maneuver is at right angles to the direction to the planet surface.

The good news is that If it's space winter those rude space guests won't need to bring a space heater.

I imagine spaceships would need specially designated lanes and directions where they can accelerate and brake, so everybody knows which places to avoid if they don't want to get a blast of radioactive material in their face.

That's what the red and green buoys in the shipping lanes are for. It's a near universal agreed upon standard. It's just those guys from NGC7835 that refuse to accept as they are unable to see that particular shade of green, and are waiting for the intergalactic disabilities act to be ratified and used.

> This isn’t an issue if you go into orbit around a planet, where the braking maneuver is at right angles to the direction to the planet surface.

Unsure. Sounds like your exhaust could enter the planet's orbit. It seems prudent to have a secondary engine for use near planets, to avoid filling their upper atmosphere with radioactive garbage.

There is one case where it would be bad: if the velocity of the exhaust was lower than the escape velocity required to leave the gravitational field of your (massive) spaceship.

> By rotating the craft to point opposite the direction of travel and actuating the engine again.

IIRC, one of Heinlein's juvenile novels referred to this as a skew-flip maneuver; I was quite impressed reading about it at about age 12, several years after its publication date.

[Just remembered the title: Have Space Suit - Will Travel, a play on the title of the TV show Have Gun - Will Travel]

https://en.wikipedia.org/wiki/Have_Space_Suit%E2%80%94Will_T...

In the Expanse books/show they use this all the time and call it "flip and burn". Pretty cool to see what nearly-plausible everyday spaceflight might look like.

(same way as all space craft)

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