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Post by walterscientist on Jun 15, 2019 21:31:22 GMT
I am working on designing a realistic small combat spacecraft - I prefer to call it a "strike craft" like in the Homeworld computer game. The general idea is to improve delivery if your projectiles on target by moving your gun closer to the enemy. In order to accomplish that, the strike craft is basically a large gun mated to a nuclear rocket with drop tanks. Intended operation is to launch it from a large carrier-style ship, probably using some kind of catapult so that there isn't an issue of burning an NTR toward a friendly ship. The strike craft then uses fuel in the drop tanks to gain velocity for upcoming intercept. Upon reaching desired intercept velocity or spending all fuel in the drop tanks, the tanks are jettisoned in order to prepare for combat. The strike craft reorients so that gun is facing toward the enemy, this also reduces presented profile and puts internal parts behind the spaced armor. In this orientation the main engine is facing sideways and burning will make the craft dodge with acceleration of several Gs. After combat the strike craft would be left with minimal delta-v - the idea is that a larger "retriever" ship would match trajectory of coasting strike crafts, pick them up and return to base with them. What do you think about the overall concept?
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Post by cipherpunks on Jun 15, 2019 22:17:20 GMT
Upon reaching desired intercept velocity or spending all fuel in the drop tanks, the tanks are jettisoned What about those RCS modules that are shown residing on said droptanks, then? It doesn't needs them anymore?... This brings a few questions, so to say: - how long an "autonomy" you'll expect from the "waiting" craft? - you do realize that "picking up" is only possible in case of either victory, or 'orbit availability' due to 'war situation', right? I mean, picking up might not happen at all... Especially if said craft is left w/o dV AND quickly falling into some gravity well..- why not supplanting the craft with at least some - albeit not powerful - electric propulsion? While on it - is Methane really the best choice here? Care to explain why? - leaving aside gas nature of fixed cannon - why 1st armor layer is plate and is not conical? - I can't really parse heat sink configuration on this one. Elaborate please.
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Post by walterscientist on Jun 15, 2019 22:37:21 GMT
The schematic says "structural elements omitted" - the drop tanks are attached to stub-wings (yeah, it makes the craft look more like an airplane) and RCS modules are on the tips of the stub-wings.
Ingame Children of a Dead Earth delta-v on launch is 6km/s and delta-v without drop tanks is 3.5km/s - that means it should be able to (more or less) kill its momentum after the combat so the launching fleet should be able to more or less scoop them up as they pass by.
If the craft needs to be manned, then I expect the crew should be able to survive at least a week without problem (based on Apollo missions).
Electric propulsion would require a dedicated reactor and large heatsinks. Unless you mean space probe level of ion thrusters, but those would be kinda pointless.
I think I messed up the heatsink configuration when tweaking the design - this wouldn't work well.
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Post by bigbombr on Jun 16, 2019 5:17:52 GMT
The schematic says "structural elements omitted" - the drop tanks are attached to stub-wings (yeah, it makes the craft look more like an airplane) and RCS modules are on the tips of the stub-wings. Ingame Children of a Dead Earth delta-v on launch is 6km/s and delta-v without drop tanks is 3.5km/s - that means it should be able to (more or less) kill its momentum after the combat so the launching fleet should be able to more or less scoop them up as they pass by. If the craft needs to be manned, then I expect the crew should be able to survive at least a week without problem (based on Apollo missions). Electric propulsion would require a dedicated reactor and large heatsinks. Unless you mean space probe level of ion thrusters, but those would be kinda pointless. I think I messed up the heatsink configuration when tweaking the design - this wouldn't work well. What if the enemy alters it's trajectory? Also, combustion light gas guns are pretty heavy. Consider using conventional guns and increasing intercept velocity. IMO, the 155 mm combustion gas gun is likely larger and heavier than what you'd want. A 7.62x51 mm machine gun is likely more cost-effective. Think less of them as combat platforms suited for sustained fighting and more of them as missiles with machine guns strapped to them: the gun is essentially just terminal guidance for the payload, but the velocity of the drone itself towards the target is a significant contributor to the resultant impact velocity.
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Post by airc777 on Jun 16, 2019 11:30:59 GMT
Even with just todays tech I doubt it would be necessary to crew a craft with a single weapon system meant to operate from a carrier with a standard operational run time of 1 week. Not having a crew makes the craft far more expendable, not being mission critical gives you more freedom to put pressure on the enemy without risk of counter pressure. What is the projectile mass and velocity? 30 tons fighting weight with a 155mm cannon means this thing could have a fairly strong recoil impulse, if the gun has a degree or two of gimbal for fine aiming that may cause it to burn a lot of rcs to maintain orientation, having said that it looks like the base of the gimbal might be near enough or directly on top of the center of mass depending on the mass of the barrel so that might not be a problem at all. What is the target that you intend to engage that you've determined it's necessary to use a 155mm projectile? What does the target's ciws look like that you've decided to use a 155mm cannon on a 30 ton strike craft to deliver the payload instead of a 155mm missile launched directly from the carrier craft? Gun mantel armor should probably be at least a little conical, unless you're being really through about storage efficiency and workspace concerns on the carrier craft? Why does the target presumably have enough defensive weapon systems to warrant a frontal gun mantel but no escort at all that could provide flanking fire into the side of the craft? How long are you intending to fire the main gun without being flanked that you've determined you need active cooling on your weapon system? I don't think this is inherently bad design, but it does make me wonder what are the circumstances of the fight it was intended for.
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Post by Rocket Witch on Jun 16, 2019 20:11:47 GMT
What is the projectile mass and velocity? 30 tons fighting weight with a 155mm cannon means this thing could have a fairly strong recoil impulse, if the gun has a degree or two of gimbal for fine aiming that may cause it to burn a lot of rcs to maintain orientation, having said that it looks like the base of the gimbal might be near enough or directly on top of the center of mass depending on the mass of the barrel so that might not be a problem at all. A recoilless gun could be used, but needing both ends clear does impose significant design limits on a system with this ratio of gun to ship.
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Post by AtomHeartDragon on Jun 17, 2019 20:21:22 GMT
My 2 cents: - I like the sideways NTR (for jinking) - note that it will need very good roll capability to be effective. Alternatively you could go for 4 rapidly gimballing engines in place of your RCS pods and NTR.
- For this kind of small, agile craft you will want to put your crew module just behind the center of gravity. It seems too far back for me.
- In most cases you will want to keep your centre of mass fixed - "twisted" rotational symmetry might be a big thing for a spacecraft, so for example instead of droptanks under your stub wings, you get one under and one over (possibly with a gun pod/missile/recoilless MLRS on the other side).
- The gun seems oversized. Huge cannons don't seem to work that well in orbital context losing to a lot of DAKKa on one end, and missiles on the other, even when we stay in the low-power regime. Also without PD of any sort it's a flying coffin. Just two mini-turrets would be a lot of improvement, same with some varied armaments (comparable to a single seat fighter or attack craft). Note that in Homeworld strikecraft were almost exclusively of flying dakka variety.
- In orbital combat around massive body the often cited 4x delta-v disadvantage for a fighter as opposed to a missile disappears, however...
- ...You still need a decent reason to pack a precorpse (or a dashing fighter jock if you manage to improve survivability) or two in there, as opposed to making it a drone.
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Post by walterscientist on Jun 17, 2019 22:43:31 GMT
Almost all mass is close to longitudal axis, rolling would need little effort. I probably should put the crew module between the gun and the main tank. I realize that crew is kinda undesirable in hard sci-fi setting. Unless there is a story reason to have the craft manned, I suppose it could just as well be without onboard crew. It wouldn't make much difference to the operation. Center of mass of fuel tanks is on same plane as center of mass of the craft. I see little issue with shifting center of mass. Having droptanks elsewhere than under wings would make jettisoning them while NTR is burning not possible. Point-defenses are under consideration. Flying dakka similar to Hellfire drones might be more effective. I find it hard to come a conclusion. In part because ships seem to have atrocious aim in CoaDE - missing shots even at distances where the enemy ship cannot possible have time to dodge out of the way.
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Post by AtomHeartDragon on Jun 18, 2019 18:20:41 GMT
Center of mass of fuel tanks is on same plane as center of mass of the craft. Which it wouldn't be if you put the CM in there. Maybe put the tankage around the CM to double as some extra armour and radiation protection? Ammo. Especially with a gun this huge. Good point. OTOH dakka really seems to work well outside of the atmosphere, and is really helpful when the target buzzes by at several km/s. It's also harder to PD against. Huge shell can be dodged (and miss completely, unless it's a nuke or something), or shot down and neutralized completely (and nukes are fragile devices). A rain of dakka less so. Huge shell also seems to be a worse way to deliver a whole lot of blam at once than missile or a whole bunch.
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Post by walterscientist on Jun 19, 2019 22:53:28 GMT
So in order to get more dakka, I mounted 7 time-proven Bofors guns on my drone chassis instead of 1 CLGG. I tried to point the engine sideways, but because of how AI steers the ship, it seems the engine has to point backward. I tried to improve muzzle velocity, but with how CoaDE guns work, any attempt at improvement mostly increased weight and cost, with little change in performance. In the end, I just copied real-world Bofors L/70 gun as closely as possible, in part for the fun of it. The resulting drone costs about same as a Stinger drone, but if it can execute its strafing run one drone can destroy most stock ships. I suppose it is in part because stock ships suck, but I am pretty happy about it nonetheless
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Post by airc777 on Jun 20, 2019 11:04:53 GMT
looking at your 40mm drone I feel like the armor could be just a bit leaner, which would give you more delta V to toy around with. With 8G dry acceleration and only 19t mass can the enemy kenetic weapons track this thing at all? If they can't you could focus on armoring against lasers specifically. If the yard stick for combat success id killing stock cutters then you could armor against that with just some nitrile rubber, or amorphous carbon if you have the ablation cap off. If the kinetics can hit you then maybe a whipple shield on top of a spalling liner? But yeah, the drone seems an agreeable shape and decent performance numbers.
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Post by walterscientist on Jun 21, 2019 20:55:13 GMT
Shedding some weight, mostly by halving armor thickness, reduces the fueled weight to 15tons with these results: Weight: 19.3t -> 14.9t Delta-v: 2.9km/s -> 4.2km/s Full acc: 5.3m/s2 -> 6.9m/s2 Dry acc: 8.7m/s2 -> 13.9m/s2 That means over 10g possible acceleration through most of the engagement. That could do some hardcore dodges, especially with the engine perpendicular to the flight path as in the original design.
Total delta-v with drop tanks goes up to 8.3km/s - staying inline with the plan to have half delta-v in drop tanks and half in internal tanks. This rather nice amount of delta-v means that a flight of strike crafts could almost always hunt down stock ships if they don't have a tanker in their fleet.
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Post by walterscientist on Jul 7, 2019 18:59:12 GMT
Did anyone try to estimate how effective would be small rockets in comparison to auto cannons?
Hydra 70 with 10-pounder warhead has 70 mm, 4 kg payload, 6 kg motor and ~2kg launch tube. Velocity after burn (lasts ~1 s) in vacuum should be around 1 km/s similar to a cannon shell. After the motor finished burning, the rocket is effectively no different from a cannon shell.
A 75mm Bordkanone 7,5 - granted, that is WW2, but nobody tried to make such kind of cannon since - weighed 1,200 kg and fired 3.2 kg shell at 990 m/s - total weight of round 9.55 kg (Pzgr 40) - or 6.8 kg @ 792 m/s with total weight of 12 kg (Pzgr 39).
That means rockets are pretty much always more effective per weight as they remove the need to have a heavy gun.
100 rockets with their tubes weigh same as a cannon to fire the same shells. That means that if you expect to fire less than 100 rounds through one gun, rockets are definitely the way to go. Skysweeper 75mm AA gun fired at 45 rpm, that means firing those 100 rounds would take over 2 minutes. That is a lot of time again favoring rockets.
So the revised design for the strike craft might be not so much a flying turret, but a flying rocket pod - a very big one. Rockets would also avoid issues with achieving extreme rates of fire per barrel, as each tube is single shot and you can fire them all almost simultaneously. This really reminds me of some Nazi ideas for wonder and last-ditch weapons - mostly the Natter semi-disposable rocket-powered and rocket-armed airplane (https://airandspace.si.edu/collection-objects/bachem-ba-349-b-1-natter-viper).
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Post by airc777 on Jul 7, 2019 21:14:36 GMT
That's probably why most of today's strike aircraft are missile based instead of auto cannon based. If you only need to be able to fight effectively for about a minute but you really, really need to be able to dodge what you end up designing doesn't really look like a battleship. CDE skews this because of very power dense minimally shielded reactors and extremely high powered lasers and extremely high velocity electromagnetic guns. If you're going this route it may or may not also be worth having a secondary weapon consisting of a much smaller machine gunset to only target shots instead of ships. It's definitely also worth considering adding flares.
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Post by dragon on Jul 7, 2019 21:56:01 GMT
So the revised design for the strike craft might be not so much a flying turret, but a flying rocket pod - a very big one. Rockets would also avoid issues with achieving extreme rates of fire per barrel, as each tube is single shot and you can fire them all almost simultaneously. This really reminds me of some Nazi ideas for wonder and last-ditch weapons - mostly the Natter semi-disposable rocket-powered and rocket-armed airplane (https://airandspace.si.edu/collection-objects/bachem-ba-349-b-1-natter-viper). Nazis weren't the only ones with that idea. The US F-94C Starfire interceptor also carried a bunch of rockets in folding launchers in the nose. This proven troublesome, so they were replaced by conventional pods, but the general idea was the same. In general, however, unguided rockets have a major problem: low accuracy. That makes them, in practice, vastly inferior to guns in terms of effective range. This is why, aside from early Cold War/late WWII designs (where firepower was the problem, heavy bombers of the era were though) they are mostly relegated to air to ground role. There's nothing better than a rocket pod when you just need to cover an area with explosions, but for attacking single, hardened targets their performance is less than stellar. Rockets only get better than cannons when you have reliable guidance for them. Which is harder than it might seem.
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