Is it necessary for spaceships to include reverse thrusters for deceleration in space?

Question

Set in the not so distant future, space travel becomes commonplace but is limited within the heliosphere. I'm thinking since it is fuel efficient and more economical to use gravity assist to slow down the speed of the spaceship would reverse thrusters become obsolete? I think maybe adding rocket boosters at the front of the spaceship is dangerous especially for nuclear explosion type of propulsion rocket and it is costly to add lots of electromagnetic rings all over the body of the spaceship just to direct the compressed plasma and feed it to the reactors to achieve fusion for thrust. We need to cool and barricade the thrusters and the crew which is expected to add even more mass, so is the reverse thrusters obsolete?

Answer 1

Nope, we don't even use them today. Reverse thrusters are completely unrequired.

Just flip the ship and use the normal thrusters.

All the ship needs is:

• A means to rotate the ship.
• Normal main engines.

Once rotated so that the main engines are pointing in the way the ship is traveling, just ignite those engines. Take a look at a SpaceX booster, the ones they land back on earth.

What we might want reverse thrusters for is docking. Actually a full RCS system with thrusters aimed left, right, up, down, forward, and back that allows the ship to rotate and translate is really handy. These thrusters are not large engines but are very small. The point is to provide a small nudge and wait, rather than putting the pedal to the metal. Which makes them absolutely impractical for accelerating/decelerating from planetary/inter-planetary transit speeds.

If you want to watch a fairly accurate visualisation with a somewhat futuristic ship take a look at The Expanse. They have large ships with a few configurations.

Answer 2

What do you mean by obsolete? They never were a thing to begin with.
AFAIK, nobody even tried to add a propulsion system to the front of a space vehicle.

Sci-fi (or rather space fantasy, as a commenter aptly noted) routinely takes energy for granted, but in reality putting stuff into orbit costs a packet.
An X-wing starfighter apparently carries enough fuel to effortlessly fly away from a planet, but us poor Earthlings actually need 600 to 800 tons of rocket (Proton / Atlas V / Ariane 5) to painstakingly put 10 tons into a 36000 km geostationary orbit.
Using the most powerful rocket available, we barely managed to launch New Horizons and its 30 kg of useful payload toward the outer rim of our solar system, at a snail's pace (9 years to reach Pluto).

(As another commenter aptly pointed out, the probe complete with topped up fuel and engines weighed a whooping 478 kg, but the Atlas V 551 launcher - the most powerful version available to date - weighed about 570 tons, i.e. the actual probe was less than 0.1% of the total mass, and the useful payload roughly 1/20,000th of it. Scaled to an average 75 kg individual, that would not even represent the weight of a quarter.)

So the last thing you want on a spaceship is dead weight, and an extra main engine surely falls into that category.

As for gravity assist, it's not nearly enough to slow down a ship to a landing. Besides, following the necessary trajectories usually costs a lot of time.

What you could possibly use is an atmosphere. With enough air brakes or lifting surfaces you could basically end your trip like a plane.
When no atmosphere is available, you still could use the ground to brake, but that would involve a huge pile of airbags and give your passengers a very rough ride :D
If you want a smooth landing, you'll have to use pretty powerful thrusters at some point, and carry an amount of fuel equivalent to what you need to take off from the planet.

Just to put that in a response: you might want to play around with Kerbal Space Program. You'll soon realize the power of gravity :)

Answer 3

In Star Trek and pretty much every other Sci Fi show you never see them turning the ship around and firing the engines in order to slow down.

They have stupid things like inertial dampers and reverse impulse thrusters to slow the ship down.

In reality, pretty much the only way to slow down in space is to turn around 180 degrees and apply thrust.

Answer 4

Well... yes and no. Mostly no.

If you are thinking spaceship as in Star Destroyer (and definitely if you are thinking rocket ship), then no. Reverse thrusters are a horrible idea. Although it would definitely be possible, having a second main engine would have these effects:

• Extra weight. Think about it: not only do you have one massive drive engine, you now have a second one. This extra weight would FUBAR your ships' maneuvering, as their momentum would be drastically increased. Think about it like this: Which would you rather be driving on a slick road, a sports car or an overloaded 18 wheeler? This extra weight causes an Achilles and the Tortoise - style situation, with the increased weight requiring more engines which means more weight which means more engines which means more weight which means more engines which means... (And so on ad nauseam)

• Huge fuel costs. While having a reverse gear would allow you to decrease your lateral velocity more quickly, the extra weight means that you would need more fuel. To continue the sports car vs. big truck analogy, Which would you rather drive: a car with 20 mpg, or an 18 wheeler which gets 1 mpg.

• A minimal increase in maneuverability. While having a reverse-thrust engine would be nice, the advantage over just flipping the ship around is minimal.

• Decreased cargo carrying capacity. In exchange for this minimal increase in maneuverability, you are giving up a lot of cargo space in order to make room for the engines. Since a spaceship's prime purpose is usually to move cargo from point A to point B, this alone would be sufficient to keep reverse thrust-capable spaceships off the market.

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That being said, while all of these are valid points, There is one type of spaceship for which this works: Bussard Ramjets. With a Bussard ramjet, reverse thrust is (almost) as simple as reversing the polarity on your magnetic field generator and then running stored hydrogen through it until you get to cruising speed. Since ramjets produce gamma radiation this would be impractical for manuevering close to planets, being mostly reserved for when you want to "get the hell out of Dodge."

Answer 5

Only one sci-fi universe I've seen has had main engines in multiple directions... the "Nightsdawn" series from Peter F Hamilton. The hyperspace rules made for spherical or nearly-spherical ships. Warships needed to dodge, that is, suddenly accelerate in various directions. Thus, each cubic face on the sphere had a full main engine, and a variety of direct and tangential thrusters as well. Going into hyperspace chopped off anything extending outside the sphere, so spaceships were generally urchin shapes that could retract all their spines.

The design has the drawback of enormous waste of space, but also redundancy of engines, fuel tanks, etc. There's no "front" of the ship. There are no specifically weak sides. Tumbling the ship to radiate away laser energy is no problem. There's no dedication of the ship to cinematic aerodynamics (this is outer space, after all, no need for swept wings).

In a realistic setting, you have one or a small set of engines, and use tangential thrusters to spin the ship around in the span of dozens of seconds or minutes. "The Expanse" series shows thrusters spinning a ship around in a small number of seconds, and that's really fast and uses a lot of propellant.

Answer 6

It Depends™

For most cases, no, reverse thrust is not necessary. But there are two cases where it would be beneficial despite the additional cost of mass:

Large military ship

In a space battle, your survivability is directly tied to the ability of your opponent to predict in which area of space you will be by the time their slugs, lasers, or rockets will cross the distances between you two. In ordinary circumstances, a spaceship can always flip and burn, yes. But in combat? You will need to be as quick and as unpredictable as you can. How fast can your ship turn around? Chances are, not fast enough. Having retro thrusters will allow you to abruptly change your trajectory, without telegraphing it to your opponent beforehand, and without throwing off the aim of your own turrets.

Very large or very fragile civilian ship.

To turn, you need to spend fuel or energy. If your ship is too massive, it might be not economical enough to sloooowly turn it around for a retro burn, spending propellant to overcome its inertia, and wasting days not being under a thrust to deliver the cargo in the shortest possible timeframe. Just adding two additional channels to expel your reaction mass might in the end turn out to be more economical and practical than the costs of powerful enough RCS engines and all the additional infrastructure they would require combined with all the time wasted on turning around.

Another consideration is if your ship is just simply too fragile and hyperoptimized to turn around. Something like ISS Venture Star from Avatar: it's very long and very thin. Inertial stresses from RCS thrusts might snap it in half or damage vital parts of it, so it launches from its home system on a light sail powered by lasers, and on arrival, it slows down without turning around via its antimatter engines - both cases create tensile stress around an only single axis of the ship that it's optimized to handle extremely well, since its basically almost just nuclear engines and a crew section tied together by a rope.

Answer 7

Yes it is necessary to have reverse thrusters because spinning in space is a terrible idea- something they strenuously avoid right now. Spinning causes so much difficulties with control of a spacecraft- spinning out of control is a real danger. You can't just spin 180 degrees-chances are a slight miscalculation will cause tumbling. Reverse thrusters and other directional thrusters would be tiny little rockets- mainly for fine control of the spacecraft.