Space carrier/reaction engines question

[Batman]

So how dangerous would it be for space fighters approaching a reaction driven carrier from behind while the carriers engines are operating? How tightly focused would the engine exhaust be? I realize the most obvious answers are a) depends on the drive type and performance, and b) whatever I want it to, it's my universe afterall.
I'm not asking for a drive that works, I'm asking assuming I have a drive that works, how big would the drive plume be, and how far away from it would landing fighters need to be to be safe.
If the answer is 'insufficient data', as I very much suspect it's going to be, by all means tell me what data you need to attempt to answer the question.

[Whiskey144]

Well..........I've gone through some similar stuff, though it was of a different train of thought. That said-

We'll need tonnage of the carriers, acceleration capability, and the exhaust velocity of the drive. For example, a 500,000 tonne ship with a 150,000 m/s exhaust velocity and 1.0G acceleration has 5 teranewtons of thrust force, mass flow of 3,333.33 kg/sec (approximately), and plume wattage of around 7E13 watts.

IIRC, that's multiple kilotons/second firepower equivalent- in other words, your fighters are probably plasma bursts by now. How far away they start to turn into plasma bursts, no idea.

TBH, magic energy shields or magic armour plating are going to be good ways to get around it- though you now need to up the ante on the offensive systems. That could probably spiral into a circular track.

[Batman]

The power of the engines is moderately irrelevant unless it figures into how tight the engine plume is. I'm not asking how dead the fighters will be once they get into engine exhaust, I'm asking how far away from it they need to stay to be able to land safely.

[Batman]

If it helps any, I'm thinking about a roughly 20 million ton vessel acellerrating at one g.

[Sea Skimmer]

Its not like it would matter, match speed with the carrier off the beam and you can approach sideways for in space. If you want to know the specific answer you need to be able to create a cross section of the exhaust plume intensity at its widest point at which it is still intensive enough to cause damage or discomfort to the fighter. That means you need to 1) know what that point is which is related to the fighter design and the nature and power of the engine exhaust, 2) know the diffusion pattern of the exhaust plume and 3) know how quickly the diffused particles themselves loose energy.

The width of the widest danger point plus a margin for error and safety would then give you how close astern you can approach. But like I was saying, you are in no way obligated to approach from close astern like a fighter landing on a modern carrier. Flying ahead of the carrier and then slowing down to recover into a nose bay would work just as well or many other options.

[Stark]

Why would you even be operating the engines during recovery of fighters?

[Sarevok]

Yeah. Ships with giant glowing engines look cool but it is stupid. A carrier should not be using engines unless performing some nBSG style dramatic "combat landing" maneuver.

[Dwelf]

I assume that the engines would be running since everything went pear shaped and they are trying to salvage as much as possible on the way out of system.

That being said a vacuum is one of the best insulators you can get. As long as you dont linger you should be able to pass pretty close to the drive plume. Exactly how close you could get I can't really say since that starts to become a technology questions.

The image of a fighter comming in tight past the engine plume landing on the flight deck red hot seems pretty cool really.

[Dass.Kapital]

Though wouldn't simply 'moving', if just to make the ship a harder target to hit, be a viable course of action?

Which is where we get into effective ship ranges etc.

[Sarevok]

The image of a fighter comming in tight past the engine plume landing on the flight deck red hot seems pretty cool really.

The added thrill of avoiding your own carriers relativistic exhaust while landing under enemy fire would make a great scene.

[Skgoa]

That being said a vacuum is one of the best insulators you can get. As long as you dont linger you should be able to pass pretty close to the drive plume. Exactly how close you could get I can't really say since that starts to become a technology questions.

This. It really depends on the drive in question and even the ship's size.

So I will try to give you my best guesses:

Classic "rocket" engines
By this I mean anything that accelerates it's reaction mass through pressure. Now, those are pretty straight forward: if it glows, its very hot and should be avoided. But thats only going to be present directly behind the engine and in vacuum the exhaust spreads pretty fast. Thus you have the counter-intuitive effect that your fighters should stay out of a halve-sphere around the aft of the capital ships. It might be an emergancy trick to go as far into the exhaust as possible in order to loose a pursuing enemy or missile. Note that many chemical drives use very toxic fuel, thus it might be SOP to have the "exclusion zone" be several kilometers in radius, so that the ground crew doesn't have to wear hazmat suits. And a little note about Nuclear [insert subgroup of choice] Propulsion: the exhaust will still be radioactive. While it might not be as much of a problem as the whole fighter melting, it could fry sensors and other delicate equipment.

Particle accelerators
By this I mean laser, ion, microwave etc. engines. Those are basicly like giant beam weaponry (although not quite as focused) and thus allow your fighters to be anywhere other than in a rather narrow (but long) cone. Depending on what you are accelerating and on your tech level (i.e. do fighters have force shields?) You have be careful about electromagnetic interactions. This might impact sensors just like the noise from the screws of a sea-vessel screws (pun intended ) with sonar. Of course, this also depends on the speed at which the particles exit the ship.
And if you somehow manage to accelerate neutrinos or dark matter, you don't have to worry about anything, because there will be minimal interaction with the fighters.

[Sea Skimmer]

Why would you even be operating the engines during recovery of fighters?

Because maybe you know, you have some place to be? Every second of acceleration lost hurts you a lot in that respect. Also very large engines are highly unlikely to appreciate being frequently turned on and off as is the case with most large machines. Throttling back might occur but I doubt you'd switch off for every single recovery. But approaching from astern is just not a very meaningful consideration.

[Batman]

Can we for the time being assume the engines are operating for whatever reason and fighters do need to approach from the rear because I say so?
What I want to know is how big would the engine exhaust plume be so I know how far away the fighters need to stay to be safe upon landing. So far, the answer seems to be 'unanswerable without knowing a lot more about the drive system', which is hardly unexpected. Unfortunately, all I can tell you about the drive system is that it's vaguely fusion based (think BattleTech drop/warship drives), acceleration is 1.5g tops, and the mass of the carrier is about 20 million tons. If that is still insufficient data to tell (which I very much suspect), so be it.

[Sea Skimmer]

Its totally insufficient and you already have been told what you do need, and if its fiction you can just make up a reasonable answer anyway and nobody will care.

[Batman]

Fair enough.

[GrandMasterTerwynn]

Though wouldn't simply 'moving', if just to make the ship a harder target to hit, be a viable course of action?

Which is where we get into effective ship ranges etc.

Problem. If you're trying to be harder to hit, then, by extension, that means your parasite ships are burning up delta-V trying to keep up with your repeated course changes. The chief "main engines running while the parasites are landing" scenario I can think of is that the carrier is planning to beat some sort of hasty FTL/jump retreat and needs the main engines fired up for this to happen. Or if the carrier is in the process of running hard for the exits and it's cycling parasites to fight a rear-guard action for it . . . i.e. it flushes some vessels to lay down some missile salvos to cover the carrier's retreat, and then those vessels quickly return to the mothership for rearming.

[Batman]

All that is required for the carrier to be engines hot on recovery is for it to need to get somewhere ASAP. That somewhere can easily be within the same system. We're talking one and a half gee of peak acceleration. I fail to see why 'engines hot' equals 'repeated course changes'.
Straight line acceleration absolutely suffices.

[Simon_Jester]

If you're just doing a speed run from A to B, at 1 g you can afford to cut acceleration for a few minutes. It won't make that much difference to how long it takes you to arrive at the target.

[Sea Skimmer]

Yeah it could, the the cumulative time could become very considerable on a long trip. Its not like you'd just have to do recovery just once. Modern carriers land a plane every 1 minute or so ideally, it can be cut to about every 30 seconds or so. A space carrier might do quicker but unless the landing zone is huge you've got a physical time delay to move the landed craft out of the way for the next once. Lets stick with 30 seconds. If you had 100 craft and they each made one mission a day that's 50 minutes lost. In a week that's almost six hours. Rounding to six hours, assuming a constant 1 G burn, and if I did the math right that means you lost out on 211,823.64 m/s of velocity. That's pretty bigbeing about 1/1415th the speed of light assuming the engines allow you to go this fast in the first place. If you had FTL.. who cares, but if this is all slower then light then that's going to be a vast amount of lost travel distance over the life of the ship.

More craft and more sorties per craft per day could increase the figure even if the landing time was a lot lower. With 500 craft and a 10 second interval it'd be about 83 minutes a day for 1 recovery per craft for example, or with 3 missions per day and 250 craft and 5 second gap it'd be 62.5 minutes. Depends on the details, but it seems like a pretty poor option all and all, aside from the fact I've already said, you are just not obligated to approach from astern.

[someone_else]

The exhaust per-se isn't going to be dangerous unless you are eating the nozzle.
Unless you are pulling the fuel out of your arse (or you have a ludicrously low endurance), we are dealing with limited amounts of super-hot stuff being dumped at very high speeds.

Usually the main issue is the waste heat from the reaction chamber. For concepts with a reaction chamber anyway (fusion/antimatter/whatever).

Even if you have Elf Magic keeping the engine in one piece, there will still be a point where the reaction chamber is open to space (the nozzles usually).

acceleration is 1.5g tops, and the mass of the carrier is about 20 million tons

Fp = (F * Ve ) / 2

so, pushing 20 megatons at 1g requires 196'200'000'000 newtons of force.
The exhaust velocity is in the 4000 km/s range since it's fusion-ish and still below half the maximum.

196'200'000'000*4'000'000/2= 392'400'000'000'000'000 watts in the exhaust. Or 392'400 Terawatts.

Now, with these parameters the mass flow (= F / Ve) is around 49'050 kg/s, so you have around fifty tons at 4000 km/s per second. Deadly or not depending on how big is the area they are spread on.
If half of your craft is fuel, you'll empty the tanks in 204080 seconds, or 3400 minutes, or 56 hours.

But anyway, since your engine isn't going to be 100% efficient, you are generationg more than 400'000 terawatts and a significant part of it will leak out as IR from the plasma or neutrons for less-clean fusions.
If you have a 50% efficient engine, that's 400'000 terawatts of radiations.

At one meter from the center of the reaction chamber you have 400'000 terawatts spread on 12.6 m2 (a sphere of 1 meter radius) or 31'746 Terawatts per square meter, at 1 km from it the flux is 2.5 gigawatts per m2.

If those are IR you may be able to survive (kinda), if a significant fraction of that are neutrons... heh.

If that is still insufficient data to tell

Would be interesting to know how much nedurance you plan them to have, since that will tell a lot about engine performance.

If you had 100 craft and they each made one mission a day that's 50 minutes lost.

What kind of missions can they do when in open space anyway? The mothership can easily mount sensors that can see well farther than where a fighter can go, and this cuts a lot the recon missions.
And there must be a very significant limit on the targets they can reach if you don't want to make obsolete the carrier (or transforming it into an uninspiring FTL-platform that serves a whole solar system).

I see fighters deployed only when relatively close to a target, and loaded fast for escaping a lost battle. And I don't see why you cannot use missiles instead, but that's another issue alltogether.

[Simon_Jester]

Yeah it could, the the cumulative time could become very considerable on a long trip. Its not like you'd just have to do recovery just once. Modern carriers land a plane every 1 minute or so ideally, it can be cut to about every 30 seconds or so. A space carrier might do quicker but unless the landing zone is huge you've got a physical time delay to move the landed craft out of the way for the next once. Lets stick with 30 seconds. If you had 100 craft and they each made one mission a day that's 50 minutes lost. In a week that's almost six hours. Rounding to six hours, assuming a constant 1 G burn, and if I did the math right that means you lost out on 211,823.64 m/s of velocity. That's pretty bigbeing about 1/1415th the speed of light assuming the engines allow you to go this fast in the first place. If you had FTL.. who cares, but if this is all slower then light then that's going to be a vast amount of lost travel distance over the life of the ship.

The other half of the question is- how far do you travel in that week of 1 G burn? About 1.8 billion kilometers. You lose something less than 10% off of that by turning off the engines while landing small craft. Which raises the obvious question- where are you going, anyway? Anything less remote than Pluto means that you've already hit and passed turnover.

At continuous 1 G burns, a solar system is not all that large a place. You sacrifice hours off your trip time by cutting the engines to recover small craft, it makes the difference between getting where you want to go in 80 hours or 90 hours, sure, but except in the most desperate of situations this is unlikely to matter very much.

As to landing small craft while the carrier is accelerating, this requires the small craft to keep accelerating themselves while in close proximity to the ship. You're handling their exhaust plumes too, and the "fighters" off a twenty million ton "carrier" may have pretty impressive exhaust in their own right.

[Dwelf]

How common would the requirement to land fighters while under power be? If it is a common use case then the carrier can be designed to provide the capability without too much effort. The most obvious solution is to kick the landing strip off to the sides ala battlestar galactica. I suspect that you would still want to approach from a few degrees off course regardless just as a safety margin though.

[Batman]

As to landing small craft while the carrier is accelerating, this requires the small craft to keep accelerating themselves while in close proximity to the ship. You're handling their exhaust plumes too, and the "fighters" off a twenty million ton "carrier" may have pretty impressive exhaust in their own right.

We're talking out-accelerating a 1.5g vessel, that shouldn't require all that impressive engine performance. Also, shouldn't it be possible for the fighters to build up speed away from the carrier and then coast in so by the time they arrive at the landing deck, they're moving at the same speed upon touchdown?

[Simon_Jester]

Tricky- you are, in effect, dropping the ship into the landing bay at 1 or 1.5g acceleration, even if the fighter's engine exhaust is not an issue.

How big are these fighters, anyway? Remember what the exhaust from a hundred-ton mass accelerating at 1g looks like...

[hermitbob]

I would like the question answered of how durable the fighters are? If they can survive re-entry of a planet's atmosphere than they should be durable enough to last one to several seconds in the exhaust heat wise. Actual particle strikes might be another story based on efficiency, lower efficiency engines having higher density exhaust.