Is it useful to use a reverse thrust system on a huge interstellar spaceship (1.7km) to slow down in space?

I’m working on a personal 3d animation project: A big, BIG interstellar spaceship (about 1.7 km long), propelled by two antimatter engines (Like the Venture Star, in the introduction of Avatar). https://kimbody1535.files.wordpress.com/2013/04/isvdiagram18.jpg

In the film, the spacecraft makes a 180° turn which propels the ship in the other direction and slowing down. But is this maneuver dangerous with a ship of this size? If so, why not use a reverse thrust system, as on aircraft engines?

And would it be enough to simply open nozzles in order to propel matter in the opposite direction? Like in this exemple: https://engineering.purdue.edu/~propulsi/propulsion/images/jets/basics/Frevers1.jpg

Thank you for taking the time to read.

• Thank you for your answer. Yes, I have just watched a documentary on rocket engines and I understand why a reverse thrust system wouldn’t work. But this is a chemical rocket, which operates in "short distances".Can we imagine an interstellar ship engine using the reverse thrust system? Would this system be useful and possible to operate in space? And what will be the structural changes for the engine (compared to a chemical rocket nozzle)? Since would a simple flow-back transfer (like on an airplane) work in space? – Space1113 Dec 15 '20 at 0:45
• As usual, do not answer in comments – L.Dutch - Reinstate Monica Dec 15 '20 at 4:10
• I would suggest building a scale model of your ship in Kerbal Space Program and seeing how it would react. I would expect it to shear in half. – Aaron Lavers Dec 15 '20 at 6:22

There is a risk in a 'flip' maneuver for a High C vessel This is dependent on three factors.

Firstly the final 'cruising' velocity of the ship. At a significantly high % of the speed of light even impacts with small particles, smaller even than a grain of sand can cause significant, perhaps catastrophic damage to a ship due to the kinetic energy accumulated by the vessel as it accelerates. At high enough speeds even gas molecules become problematic.

For this reason ships traveling at a high % of the speed of light would probably tend to try and minimize their horizontal cross-section (along their line of travel) i.e. be as long and 'thin' as possible rather than wide. This is a passive design measure as opposed to whatever 'active' (e.g. lasers?) countermeasures the designers decide to include.

So during a 'flip' maneuver the ship is potentially exposing its flank to incoming projectiles.

Second is the amount of time the ship needs to complete the flip (how powerful its attitude thrusters/gyroscopes etc are in proportion to its mass). Obviously the quicker the transition the less the danger.

Lastly its the density of potentially dangerous particles along your flight path. Which presumably can be calculated based on research and experience.

You end up with a risk calculation, your final velocity multiplied by the time it takes to flip and the probability of encountering dangerous particles during this process (minus a risk adjustment for any active countermeasures you can build in.)

So depending on how dangerous you decide to make the likelihood of impacts the designers might consider the extra mass and complication of a 'forward' facing drive nozzle to be worth the trouble. Its up to you.

(It might even be useful as defense measure e.g. the ship detects an incoming particle on a collision course and briefly fires up the front drive at low power to destroy/deflect the inbound object, assuming maneuvering or other options aren't available.)

• Thank you for your comment. I already assume that the ship is powered by two antimatter engines. So, it is not impossible to imagine a reverse thrust system... In my opinion, the most important factor is that passengers will receive particles from the interstellar medium. – Space1113 Dec 15 '20 at 17:16

The maneuver is not dangerous for the ship. The worst that could happen is flexing of the structure as the rotational force is applied, but the answer to that is simply knowing how rigid your vessel and how much sideways thrust you can safely apply. The turn doesn't need to be rapid, and the thrust for rotation doesn't have to be applied continuously. The RCS (Reaction Control System) which would be used for the purpose of turning the ship would likely be incapable of generating such large forces.

If you're in vacuum and you have a planned trajectory, the process can be very gentle. If you know you need to point your ship retrograde (backwards, in relation to your trajectory) in x hours from now, give it a gentle nudge from RCS, so that ships starts rotating. It's in vacuum and zero-G, which means it will keep rotating untill you counteract the turn with equal force to the one that caused it. There's no acceleration acting on the ship as it rotates, hence there are no forces that could cause strain on the structure. It would not even be noticeable to anyone on board. Once the ship has rotated 180 degrees, you fire the RCS again, in the opposite direction to the first time, stopping the ship's rotation. The ship is now pointed retrograde and ready to burn for deceleration.

Of course if you need to do very rapid maneuvers things can get more messy, as the ship is unlikely to be be 100% rigid, and the more sideways force you apply the more it will flex.

Putting more engines than is absolutely needed is not a practice in spacecraft, as more engines mean more weight and more weight is bad. It means less acceleration and less delta-v, the latter being usually more important. I am unsure how the antimatter engine in this case is supposed to work, but chemical rocket engines are not as simple as closing one nozzle and opening another. The solution would be to rotate the entire nozzle of the rocket (gimballing, which is done to a certain extent in order to use the rocket's thrust for steering), but from an engineering standpoint why would you do that if it's simpler to turn the entire ship?

It's really easy to turn in vacuum in zero-G. There are no forces acting against you.

• additionally, designing a ship for reverse thrust would necessitate to make everything capable of withstanding accelleration in both directions. – ths Dec 15 '20 at 1:03
• Thank you for the details of the manoeuvre. For my project, I will very slowly animate the rotation of the ship. Also, I will slightly thicken the cable that tracks the back of the ship. But, the manoeuvre must be fairly rapid, as the crew is exposed to interstellar particles during the manoeuvre. – Space1113 Dec 15 '20 at 17:24

To avoid a flip, your “reverse” thrust would need to equal your “forward” thrust. Since mass is your enemy on any type of spacecraft, having mutually exclusive sets of engines makes no sense. This is why everyone with remotely realistic physics uses the flip maneuver.

If you have some handwavey exotic propulsion field, though, just reverse its polarity to slow down (or to exit hyperspace) and call it a day.

• "thrust would need to equal your “forward” thrust." I understand. In fact, I ask this question because while the ship (1.7km long) is turning 180°, it is exposed to the dangers of space, at a very high speed; maybe 2/3 of the speed of the light... In addition, passengers will receive particles from the interstellar medium. This can influence the health of passengers. They need to be protected and the operation must be quick. So I was thinking about planes that keeps the same orientation, while braking quickly... with reverse thrust – Space1113 Dec 15 '20 at 1:20
• @Space1113 Space is big. You’re looking at days, weeks or even months on the burn to get anywhere useful. A few minutes for the flip is nothing. – StephenS Dec 15 '20 at 1:46
• Ok, thanks fot the help. – Space1113 Dec 15 '20 at 17:03