As the title says, can any form of known (or even theorized) form of rocket leave a contrail in the vacuum of space?
Question comes after watching that new Battlestar Galactica, very neat battle sequence, but it got me to wondering whether it was even possible to have contrails, since I personally thought that any emissions would dissipate almost immediatly.
Short answer: yes.
longer answer: Leaving aside sci-fi engines, a rocket works by ejecting mass out the back at a high rate of speed. If that mass is visible, then your rocket will leave a visible trail - not a “contrail” but a “smoke” trail.
Larry Niven once postulated that really efficient fusion rocket engines would also be horrifically efficient weapons - being hit by a beam of superheated plasma travelling at relativistic speeds would hurt like heck whether it was a weapon of war or just the wake of a passing freighter. He used the idea to good effect in some of his novels, most notably in the diplomatic vessel “Lying Bastard” that didn’t have a single device on it that was officially a “weapon” but which was still the most lethal thing ever to fly out of a Puppeteer factory.
Normally, the smoke from a rocket is moving a little sideways as well as backwards, making the trail cone-shaped. This would mean a smoke trail would likely drift apart fairly quickly, although if the trail is visible it may be pretty long in proportion to the spaceship. To leave a more persistant trail, you would have to get the mass ejected to be almost perfectly parallel (which would also make the rocket more efficient unless it required energy to constrict the blast). The smoke trail would also fan out pretty quickly if the ship was turning, much like when you swing a water hose.
Since you can in theory build a rocket that uses any sort of propellant imaginable, provided you have some way to get it moving, you could build a rocket that fires two liquids that react and polymerize when they mix. This could leave behind a solid “smoke trail” that would stay behind until forcibly damaged.
If you really wanted to, you could take this to an absurd extreme and have a spaceship that carried a large coil of rope and propelled itself by continuously ejecting the rope. This isn’t likely to be a very efficient propulsion system, but it would leave a visible and persistant trail, and it can actually accelerate a ship in the vacuum of space.
MS,
You seem to understand the principle, but I don’t think you quite got what I was suggesting when I mentioned the weapon part.
Leave aside the limits of chemical rockets for a second and consider the rocket principle in general: in order to get the best result from your engine you want to maximize the amount of thrust you get while minimizing the amount of mass you eject. that means you want the ejecta to have as little lateral spread as possible (as you noted) and as high a velocity as you can manage. At the extreme, this means shooting a stream of photons or plasma at as close to light speed as you can get and in as tightly focused a beam as you can manage.
Now consider the legendary “death ray” of science fiction: a canon that emits a beam of deadly energy. But what is that beam of deadly energy? It is, in fact, a stream of particles or photons in a tightly focused beam.
In other words, in space travel there is no difference between a powerful, efficient motor and a powerful, efficient cannon.
Classically, a contrail is formed when humid jet exhaust mixes with cold air in the upper atmosphere, thus causing the moisture to condense and freeze into an ice crystal. Many rocket engines which operate in space have humid exhausts (in fact, the most widely used space engine, the RL10, burns hydrogen, so its exhaust is all water), and space is of course extremely cold, so the two prime ingredients are present. An engine burning a hydrocarbon fuel would probably generate both moisture and particulates around which droplets could nucleate.
However, a rocket engine in space exhausts to a vacuum. The plume you are used to seeing when a rocket or jet engine operates in the atmosphere becomes greatly expanded in space, because there is no atmospheric pressure causing it to hold its shape. Therefore, even if the ingredients were present for a visible exhaust plume or contrail to form, it would almost immediately disperse. I don’t think you would see a trail.
I can say with confidence that the detectability of rocket exhausts in space has been extensively studied. I can also say that such studies are almost certainly classified, because they are used to detect and track rocket engine firings in space by people who aren’t supposed to be observing them.
Porkchop_d_clown, I understood your point about using rockets as weapons. I just wasn’t following that particular tangent.
Hyperelastic, I hadn’t considered the possibility that the pressure in a gaseous rocket exhaust might cause the smoke to spread out even if it was initially parallel. The cold of space might counteract that a bit. That’s probably quite a thermodynamics and heat transfer problem there, since (1) space is only slightly warmer than absolute zero, (2) the pressure of a gas at that temperature is going to fall considerably as it cools, and (3) with nothing to conduct heat, it may not cool all that fast. I don’t want to try calculating how fast that would expand, but it’s probably not trivial.
My Alma Mater did some telescope work during the Apollo program. They’d film Apollo rockets doing LIO burns and such from their observatory. (They were even able to catch the command module in lunar orbit when it was lit up against the dark of the moon. E.g, near the crossing of the terminator.) Anyway, so there was a lot of video about it at the time.
They and NASA were quite surprised. The vapor trail dissipated rapidly. But once you think about it: Gases in a vacuum dissapate. The hotter the gas, the faster it spreads.
As to one big aspect of the OP: Contrail mean “condensation trail”. There ain’t no condensation gonna happen in that environment.
So then, the vacuum of space WOULD dissipate anything that was ejected? (this is what I thought by the way, but I read an interview about this series that they were trying to actually stay as much as possible within “real” science. So I started to wonder if it was at all possible to get that sort of trail in space.)
For anyone who has not watched it, the bad guys shoot rockets/missles at the good guys/civilians. The rockets/missles always leave trails.