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I've seen many sci-fi scenes that feature a ship underway on a deep space journey. As the ship passes the vantage point we can see engines burning during sub-FTL speeds (The Expanse most recently).

Why would engines need to burn once target velocity is reached?

Is there resistance in the vacuum of space that would slow velocity, or is the ship implied to be under constant acceleration? Wouldn't the trajectory continue at the same speed and direction without needing engine assistance?

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  • $\begingroup$ While people here can answer, perhaps you should ask this in Physics.SE $\endgroup$ – nzaman Apr 19 '18 at 15:53
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    $\begingroup$ Do you want to know reasons to explain why this might be the case (i can think of a couple) or just assurance that you are right that this doesn't have to be this way ? The thing is you can easily come up with a reasons for burning engines, and you are asking about fiction in general - you can't say this is wrong without a concrete example. $\endgroup$ – Raditz_35 Apr 19 '18 at 16:07
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    $\begingroup$ @RonJohn This would be better suited for movie or scififa se then if it's specifically about one show. But currently I don't understand it as him asking exclusively about the expanse $\endgroup$ – Raditz_35 Apr 19 '18 at 16:18
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    $\begingroup$ Well, I asked a question on constant-burn transfer orbits to maintain artificial gravity. It may be of interest. $\endgroup$ – Joe Bloggs Apr 19 '18 at 16:46
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    $\begingroup$ Remember Newton's first law: an object in motion stays in motion until acted upon by another force. Yes, there's dust in the universe that would cause drag, but it's miniscule and irrelevant (if dark matter exists then it may be something more than miniscule, but still pretty much irrelevant). Of course, you have everything from nebula to asteroid fields that can cause drag. But in space, pretty much nothing. Juice it up and coast (almost) forever. $\endgroup$ – JBH Apr 19 '18 at 23:27
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Until you get up into relativistic velocities, how much you burn is only constrained by fuel. The Expanse provides a good example. They have handwaved super-high Isp engines. Since there's no compelling reason to conserve fuel, the fastest way to get from one place to another is to accelerate up to the halfway point (more or less), then flip for a deceleration burn all the way to the destination. If there's a need to conserve fuel, then break it up into distinct burns and coast in between. In The Expanse, it's handy because they can cruise at 1g acceleration and use the engines for artificial gravity. They only have to deal with zero g during the flip maneuver or when docked.

If you care about relativistic speeds, you probably don't want to keep burning past about 0.7 or 0.8c. See this cool chart. At around this point, more acceleration is starting to increase the time dilation effect and you are losing out on actually getting anywhere faster. You can keep burning if speed is critical, but your losses in terms of fuel consumption will start to eat you alive. So a long burn and coasting at low relativistic velocities would be the way to go if you are doing far future sci-fi.

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    $\begingroup$ In the Expanse book series maintaining artificial gravity is one of the main reasons the ships stay under constant burn: It's not just about getting places fast. In fact there's more than one instance of them flying their ships around in a big 'ol circle just to maintain gravity. $\endgroup$ – Joe Bloggs Apr 19 '18 at 16:43
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    $\begingroup$ Fuel consumption would eat you alive only if you cared about actually increasing speed in the galactic reference frame. If the reason you're accelerating is to maintain 1g in the local reference frame, it might make sense to keep the engines on, even if has negligible impact on your galactic reference frame speed. $\endgroup$ – R.M. Apr 19 '18 at 19:23
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    $\begingroup$ Note that relativistic time dilation only applies to outside observers. To the people on the ship, it will appear that they do get there faster. $\endgroup$ – jamesqf Apr 19 '18 at 19:49
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    $\begingroup$ "how much you burn is only constrained by fuel." Actually, for rockets, reaction mass is more of a constraint than fuel. $\endgroup$ – Acccumulation Apr 19 '18 at 21:59
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    $\begingroup$ "At around this point, more acceleration is starting to increase the time dilation effect and you are losing out on actually getting anywhere faster." I'm not sure that's true: for one thing, it's speed that matters; for another, even if you experience time dilation, that just means that it appears to you that the journey takes less time. It doesn't actually slow you down. $\endgroup$ – HDE 226868♦ Apr 20 '18 at 1:35
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Continuous high ISP small engines are a great way to travel.

If your engines have more power than you need to get to your destination in the time you want, and the ISP response to power rate is flat, a short strong burn followed by coasting will be slighly more fuel efficient than a long weak burn.

On the other hand, if engine efficiency goes down as its output goes up by any decent margine, a long slow burn will remain more efficient than a fast short burn.

Current space rockets do fast short burns because our technology doesn't really give us good slow efficient burn options. This is beginning to change with our ion engine technology.

A craft with high efficiency low-power engines and low efficiency high-power engines would want to continuously burn the low-power engines to get somewhere fast.

A craft where their engines exceed 1 g of thrust will want to continuously burn rather than burn stronger just for passenger comfort.

The distance you travel is the integral of your speed. Long burns are slopes, fast burns are steeper slopes, and coasting is a line. Based on the burn-speed to efficiency curve, you can work out how fast or slow a burn you'd want for a given distance profile.

In theory orbits and the movement of your target also factor into it; with orbits, often you want to put all of your thrust into a short window based on orbital geometry (apogee, perigee, or the nodes of a Hohmann transfer orbit).

So there are lots of reasons why you'd have short burns, and lots of reasons you'd have long burns.

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  • $\begingroup$ +1 for mentioning real life ion engines. $\endgroup$ – Schwern Apr 20 '18 at 17:25
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    $\begingroup$ "Current space rockets do fast short burns because our technology doesn't really give us good slow efficient burn options." That's not really true - stronger rockets are much more efficient when you're worrying about getting into orbit, so there's just a lot of incentive to develop rockets like that first. $\endgroup$ – Rob Watts Apr 20 '18 at 19:43
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    $\begingroup$ "a short strong burn followed by coasting will be slighly more fuel efficient than a long weak burn." A lot more efficient if there is a massive planet nearby. See Oberth effect. $\endgroup$ – Phil Frost Apr 20 '18 at 20:39
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It's not accurate, as you already surmise.

Those big burners at the back strongly suggest the ships fly by pushing out reaction mass backwards to get thrust forwards. Even with future tech in mind, you're stuck with the tyranny of the rocket equation. Basically, as you go for higher speeds, you need more and more reaction mass to accelerate and decelerate, which makes your ship heavier, so you need bigger engines and more fuel, until your ship is one giant fuel tank with and engine and cabin strapped to it.

Given that, it simply doesn't make sense to go faster and burn longer than you absolutely need to. A military ship on a short range intercept mission would likely do this, but if it burned non-stop for 30 days in deep space, it would also take 30 days to undo that burn if the situation changed, so the faster it went the more it would be locked into one course.

Flying backwards

Assuming future tech somehow discovered a way for ships to have/get unlimited fuel/reaction mass (maybe siphoning it from an alterante dimension in-flight?), they would still do things quite differently from the way depicted in the movies.

Any ship approaching a planet or other non-accelerating object would fly (and burn) normally at first but then halfway it would turn around, point the engines forward and slow down. For an accelerating target like another ship, the goal would be to match velocities or in battle to at least slow down enough to be in range of the target for more than 1-2 seconds. That means burning retro, left, right, up, down, basically any way except forward until you're behind them.

Mounting multiple sets of engines in the various directions would just make the ship heavier and slower, so there will always be main engines in one direction, with small extras for quick and unpredictable moves.

Movies would suck

Of course, orbital maneuvers and the specifics of deep space navigation are so alien to the average movie viewer that entertainment value overrules realism even if the makers know how it should look. The feeling movies aim for is often WW2 fighter planes. bombers and battleships. Those are familiar, there's lots of action and its within easy visual range, so that's what movie spaceships look and move like.

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    $\begingroup$ You don't need unlimited fuel, the extra fuel must only be cheaper than the benefit from burning engines is worth to whoever pays for it. Might sound like a nitpick, but this changes the character from "very unreasonable to "situationally accurate" $\endgroup$ – Raditz_35 Apr 19 '18 at 16:30
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    $\begingroup$ It's not so much the cost of the fuel (actually reaction mass), it's all the extra mass you need to accelerate and decelerate, which costs more fuel again, etc. Unless of course reactionless thrust is a thing, then you don't need those big burners at the back anyway. $\endgroup$ – Cyrus Apr 19 '18 at 19:26
  • $\begingroup$ Your edit basically describes how this works in The Expanse. $\endgroup$ – TemporalWolf Apr 19 '18 at 20:07
  • $\begingroup$ Yeah, love that series, and they get a lot closer to realistic there, though the fights get conveniently short range every time thanks to stealth and surprise. The first big fight is basically considered over after they fire a bunch of torpedoes at long range, but then the spoilers happen... $\endgroup$ – Cyrus Apr 19 '18 at 20:12
  • $\begingroup$ You don't need to suck reaction mass from an alternate dimension: there's hydrogen floating around. See Bussard ramjet. $\endgroup$ – Phil Frost Apr 20 '18 at 20:46
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While traveling long distances all energy given to the engines effects apparent travel time equally.

A little push over a long time rather than a big shove and coast for a long time is a reasonable engineering trade-off, in fact our current most fuel efficient engines are good at providing a little thrust over a long time. If you have the (unreasonable) energy required to accelerate a ship to fractions of c keeping the humans comfortable with a steady acceleration is probably a better choice than smashing them on launch and then letting them float. It also requires less power (smaller engines) to do a long (relatively) light burn than a short heavy one.

If the ship has power though the whole flight (temperature above 3 K, light) you probably can afford to give some to the engines.

There is interstellar mass that would slow you down eventually; something like 10$^{-7}$g per m$^2$ of cross section per light-year, but that's not really enough to worry about.

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Why would engines need to burn once target velocity is reached?

Because screenwriters:

  1. aren't very educated in science, and
  2. assume that moviegoers are stupid.

Wouldn't the trajectory continue at the same speed and direction without needing engine assistance?

Correct. But movies don't deal in reality.

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    $\begingroup$ Not true in case of Expanse, where constant burn provides 1g and makes travel shorter than short burn, passive flight mode. $\endgroup$ – Mołot Apr 19 '18 at 16:41
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    $\begingroup$ @Mołot In that case, "target velocity is not reached", since you're under constant acceleration. (ISTM a lot of your mass would be fuel. Doesn't sound very efficient.) $\endgroup$ – RonJohn Apr 19 '18 at 16:51
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    $\begingroup$ this particular show is explicitly mentioned in the question and in the show (season two) it is explicitly explained that they solved issue of efficiency. Handwavium, yes, but they pointed at it and pretty much said "this is what we are handwaving". $\endgroup$ – Mołot Apr 19 '18 at 17:57
  • $\begingroup$ Yes it requires some handwaving as to how to make an engine efficient enough that it's feasible for that level of acceleration, but for sci-fi that's pretty realistic. The principle is not only possible, but is in use in modern spacecraft in the form of ion engines such as the one used on the Dawn mission. Dawn is much smaller than any ship in the Expanse, and accelerated at 90 mN instead of ~1g, but by keeping it's engines running for hundreds of days it was able to visit Ceres and Vesta. $\endgroup$ – Nick Apr 19 '18 at 20:34
  • $\begingroup$ @Nick those low-thrust ion engines are what I was thinking of too, which is why I didn't mention them. Also, they don't "burn". $\endgroup$ – RonJohn Apr 19 '18 at 21:06

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