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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 1:48 am 
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So I am looking at the Isp numbers again. I look at the ion thrusters, and their ISP of 3,000. That would make for a competitive fighter.

I look at the NERVA numbers, and I made a mistake: 6-8k is the particle velocity. The actual ISP is 6-800, 1200 max. So instead of 188 fuel dots, more like 18.

But let's be fair. The Rafik's ISP couldn't be 3,472. If it's delta V is 146,250m/s, and its weight is 5,300,000 kg

146,250=3472.2*9.8*ln(5300,000/x)

4.298=ln(5,300,000/x)

e^4.298=5,300,000/x
x=72,058 kg= Rafik's dry weight.

That can't be right. The Rafik cannot be 72% fuel tank. It must have a higher Isp rating.

But that is beside the point. I seem stuck. On one end you have an Isp of 1000 for fission rockets as an upper limit of current technology, on the other hand you have an Isp of around "3,000", more like 30,000, and nothing in between. You either get a small, weak rocket, or a massive, efficient rocket. A fighter would need something with an Isp of around 3,000 to have enough dV.

Maybe an ion drive? It has a great ISP, if you have enough power to drive it. If I want 4gs thrust I determined I need rougly 2MN, the size of a space shuttle main engine. 1/2*2MN*55,000kg= 55 Gigawatts. Hmm, better than 3 TW, 1/53rd less powerful than the torch ship, 1/18th as powerful as a LP-13 gunboat. But you can't achieve 55 Gigawatts without fusion power. Can you make a fusion reactor small enough to fit on a fighter, not a torch, but a power supply for the ion drive? I don't know. You say it can't be done...

But if we stick to fusion torches, you are right, there will be no fighters.

I was looking for something like a MIG: small, cheap, short-range, maneuverable. The gunboats are similar, I would like to try them with a high-speed attack against a capital ship. As with fighter strategy, if they come in off-orbit, or are in deep space between planets, they could use high RoC to maximize damage. I certainly look forward to playing with them almost exclusively. But there are several problems compared to a fighter:

One, they probably can't dodge bullets like a fighter could. Although 3 gs is impressive, they are much larger targets. I am not sure how size plays into a ship's vulnerability.

Two, they don't swarm. Instead of 12-16 targets, there are only four. They don't overwhelm defenses and would draw concentrated fire.

Three, they can't go in the atmosphere,

therefore four, they can't hide.

Five, they are produced in space, so they are going to be larger and more expensive than a fighter-sized craft.

Imagine if you could mass-produce fighter sized craft, ship them around the galaxy in shipping containers, assemble them and hide them on any asteroid, forest, desert, freighter. The portability and mass-reproducibility would be a real game-changer, altering the balance of power.

A fighter would need 1) at least 3,000 Isp, 2) a small enough reactor 3) with enough power to create 4gs of thrust. Sure they can't make a small fusion reactor?


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 7:09 am 
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Location: Warsaw, Poland
William Scott wrote:
So I am looking at the Isp numbers again. I look at the ion thrusters, and their ISP of 3,000. That would make for a competitive fighter.

No, it wouldn't. You can't look at Isp alone, you have to also look at thrust. Ion thrusters have no thrust to speak of. :-)

William Scott wrote:
But that is beside the point. I seem stuck. On one end you have an Isp of 1000 for fission rockets as an upper limit of current technology, on the other hand you have an Isp of around "3,000", more like 30,000, and nothing in between. You either get a small, weak rocket, or a massive, efficient rocket.


Yes, exactly. Basically, fusion torches have to be BIG because of physical constraints on fusion chamber size, heat radiation etc. They are extremely handwavy anyway, to enable for interesting games, miniaturising them would require even more handwaving.

It's a bit like complaining that it makes no sense that the US Navy has nuclear-powered aircraft carriers and nuclear-powered subs, but no nuclear-powered speedboats, and that there are no nuclear-powered jets in US Air Force nor nuclear-powered tanks in US Army.

It makes perfect sense when you look at the engineering and physics.

William Scott wrote:
Maybe an ion drive? It has a great ISP, if you have enough power to drive it.

But it has a lousy thrust, due to physical constraints. Sorry.

William Scott wrote:
One, they probably can't dodge bullets like a fighter could. Although 3 gs is impressive, they are much larger targets. I am not sure how size plays into a ship's vulnerability.

Fighters can't "dodge bullets" either, because lasers are pretty much undodgeable. You can't dodge speed of light weapons.

And fragmentation cannisters launched from coilguns should be pretty good at killing fighters, too.

William Scott wrote:
Three, they can't go in the atmosphere,therefore four, they can't hide.

In Ten Worlds nothing can go into the atmosphere and back up again on its own power and using its own fuel, because this strives to be a realistic SF setting, not a space opera, and spacecraft that start from the planet's surface, then go on a mission, then land back up require pretty magical drives. (Thus the importance of laser launchers)

The same goes for stealth in space.

William Scott wrote:
Imagine if you could mass-produce fighter sized craft, ship them around the galaxy in shipping containers, assemble them and hide them on any asteroid, forest, desert, freighter. The portability and mass-reproducibility would be a real game-changer, altering the balance of power.

And if you could mass-produce invisible space-capable faeries with disintegration wands, that would be a real game-changer, too. But it simply does not make sense in this setting. And neither do space fighters. Space fantasy, space opera - these are all different genres. Ten Words is hard SF.

The same goes for space marines boarding hostile craft. They're a staple of space opera, but in hard SF, a space marine is just a very expensive and not very efficient impactor warhead, because at 3 km/s a 200 kg space marine+suit+weapons assembly "docking" with an enemy warship is just a plasma explosion with a force equivalent to 200 kg of TNT.

From your comments, it looks like the game you're looking for is Squadron Strike, not Attack Vector: Tactical. You're looking for a space opera with a thin veneer of scientific plausibility painted over it, not for a game built ground-up as hard SF. A valid preference, but trying to use AV:T to get such a genre is like trying to use a motorcycle for interstate cargo hauling.


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 7:58 am 
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In terms of dodging bullets, a gunboat is pretty good at juking around incoming kinetics. Anything with a thrust of 5 or greater can do it; thrusts of 10 or higher mean that they can do it preferentially until they run out of gas.

However, the gap between minimum installation size for nuclear propulsion and self powered chemical rockets is why we've all been saying "No, it doesn't work that way..."

This is the engineering background for why there aren't fighters - if you're going to invest in that fusion torch, you might as well invest in making the ship it's on be large enough to be multi-role and capable of at least a month of cruise endurance, more than likely two.

The game balance side of things is that running lots of ships is a pain in the ass.

You should also take a long look at fuel flow rates, exhaust velocity and temperature and total drive output.

In very broad strokes with no math (I don't have time to get dragged into it right now):

As your exhaust moves back faster, it approximates being hotter.
Momentum transfer is largely a function of particle mass in the exhaust
Higher exhaust velocity gives better fuel efficiency
The easiest way to improve exhaust velocity is to reduce the particle mass.

AV:T's combat mode torches use an unspecified and handwavy mechanism to get enough mass flow past a fusion reactor going like gang busters to get their amazingly high thrusts. And they are amazingly high thrusts compared to real rockets, even if, compared to the conventional thrusts of SF, they look like Sopwith camels painstakingly carved out of modeling clay and lead.

Realistic fusion torches might get to 5-12 milligees of thrust, not AV:Ts 750 to 2000 milligees for warships. We do this by magically imparting about 90% of the waste energy of the drive in the exhaust.

In round numbers, anything below 4 milligees isn't useful for interplanetary transits. You spend so much time spiraling out of planetary orbit that you might as well have a secondary chemical rocket shove you out. (Beyond the time value of money, the "spend your time spiraling out in the Van Allen belts" aspect of it is also problematic)

In round numbers, once you hit the 'can break planetary orbit in straight thrust' threshold, which is about 4 milligees for most earthlike planets and 5 for very heavy planets, like Damso in the Ten Worlds setting, the benefit for higher thrust drops considerably for shipping.

What higher thrust lets you do is trade more fuel for flexible departure times. The more your thrust putters at 4-5 milligees, the likelier it is that you have hard launch windows that cannot be altered. In a handful of situations, you can trade roughly 4x the fuel for a 15-30% reduction in travel times if you can get to about 10-12 milligees, but it's usually a poor idea.

I honestly think that if the general problem of putting relative orbital position and their impacts on travel time and delta v requirements can be made 'simple enough to use', that there's a fascinating game of cargo slinging around the solar system to be had. The fundamental game decisions are:

A) Who needs what cargo?
B) When do they need it?
C) What's the least energy launch window to get them that cargo?
D) OK, that's going to take too long.
E) What's the shortest transit time that we can make with the best rocket we have?
F) OK, that's going to cost too much energy.
G) What's the middle case?
H) OK, let's try and get everything ready for launch to make that launch window.

(Other players then play event cards to make you miss your launch window, you try to build up revenue to make more robust rockets with more options, and players compete to sling cargo around the solar system).

As an SF setting, you also get the kind of environment where someone can get stuck at a locale for months on end, because the orbital geometry doesn't work right. Imagine being a captain of a ship trying to balance his needs for paying the idling crew, keeping them out of trouble, and trying to broker a cargo and a reaction mass budget.

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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 2:55 pm 
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I like the idea of space trading and cargo. It could be used to set up the initial vectors for intercepts for AV:T. If Isp is so precious, limited under 1000, launch windows would be necessary, although with the dVs of warships launch windows become less an issue. And with launch windows, everyone in the galaxy would know when you would be launching, and would be waiting for you. It would be a gauntlet. You would have to travel in convoys. Sounds like fun!

To simulate interplanetary journeys you could easily reduce it to 2 dimensions. In orbiter we seldom if ever leave the planetary plane. I am sure there are relatively simple relationships between distance and travel time that could be calculated on a play aid. It is done all the time in Orbiter, but it can also be done manually. Triplanetary I hear was good at this I hear, although I haven't played it.

There is just one problem.

If there is no way to get off the planet without chemical rockets or fission rockets, with ISPs under 1000...you are essentially stuck with 20th Century technology, and no one is going anywhere. No space cargo, no space trade, no ship building. How do you get a 75 ton reactor into orbit? How do you assemble a 5,000 ton warship when a space shuttle can only cary 25 tones? Space transit would be a one-way affair: from asteroids and moons down to the planets, but not back up again. And if you can't trade your goods, well, why be in space at all? If you can't get a fighter into space, you can't get anything into space, not economically.

But a reactor can be miniaturized. They put fission reactors in in bombers. Look at the NERVA experiments of the 60s. Even today they are designing fission reactors the size of bathtubs. They are not as efficient as a large reactor, but the goal isn't efficiency, the goal is size.

Here is a good paper on some alternatives to the fusion torch.
http://ntrs.nasa.gov/archive/nasa/casi. ... 001506.pdf
http://fusionenergy.lanl.gov/Physics/Ma ... Fusion.htm

My favorite is magnetized target fusion. It isn't so much a contained reactor as a series of small explosions. You feed pellets of deuterium fuel into a chamber, zap it with lasers, it explodes, magnetic fields focus the plasma out the nozzle, recharging the laser. Or, you feed a stream of deuterium into the chamber, lasers zap the stream.

It wouldn't be as efficient as a 3 TW reactor. But it would be cheaper to build, and smaller. You could get your cargo, and fighters, into space. You would also get an Isp of 70,000 seconds. The NASA design specs say it would be heavy 116 metric tons). But that is not a theoretical limit. If you quartered the size, you should be good to go.

As a side note,

Space marines are doable. You can make a landing craft with chemical rockets that can pull enough translation to land on a moving, rotating craft. The marines go directly into the target ship, so no "splat". The landing craft wouldn't need much dV, just enough thrust to get from ship A to ship B. Unless the rules change, zone defense wouldn't the armored landing ship.


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 3:24 pm 
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William Scott wrote:
Space marines are doable. You can make a landing craft with chemical rockets that can pull enough translation to land on a moving, rotating craft. The marines go directly into the target ship, so no "splat". The landing craft wouldn't need much dV, just enough thrust to get from ship A to ship B.


To paraphrase an old movie: "Show me the math!"

Mike Z


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 4:40 pm 
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Location: Warsaw, Poland
William Scott wrote:
If there is no way to get off the planet without chemical rockets or fission rockets, with ISPs under 1000...you are essentially stuck with 20th Century technology, and no one is going anywhere.


That is simply not true. External power enables orbital lift without magical fusion drives capable of operating in an atmosphere. And if you have lasers useful for space combat at ranges of hundreds of kilometers, you can also have lasers useful for launching cargo into orbit.

Which, incidentally, is the case in Ten Worlds. Concord had only one spaceport, it broke down and it lost orbital facilities and contact with the rest of the words.

William Scott wrote:
If you can't get a fighter into space, you can't get anything into space, not economically.


One does not follow from the other. Really, I suggest you read Atomic Rockets, it covers the various types of drives quite comprehensively. If you assume the sole two options are: either realistic internal drives and no space presence at all, or magical drives enabling fighters capable of SSTO/mission/SSTE, then you've clearly got some catching up to do.

William Scott wrote:
Space marines are doable. You can make a landing craft with chemical rockets that can pull enough translation to land on a moving, rotating craft. The marines go directly into the target ship, so no "splat". The landing craft wouldn't need much dV, just enough thrust to get from ship A to ship B. Unless the rules change, zone defense wouldn't the armored landing ship.


Show me the math, because now, you're just making bold statements without anything to back them up. Basically, you need something like kinetic impactors, but besides enough veer to overcome target jinking, you also need a single impulse chemical engine powerful enough to overcome pretty instantly the RoC (rate of closure) of the "Space Marine Mk 1 Splat Projectile", which means that you can only shoot at low RoCs and you have to make sure the target does not maneouver to increase the RoC to a lethal range where your engines won't be able to prevent the splat, but that means that the target can instead reduce the CV to a negative number, which means that the marine will never reach the target and will die in space.

And really, that assumes magically supertough space marines of the WH40K fame, because in reality, you need to make sure the RoC and side veers are almost absolutely zero, whereas normally the rounding system assumes rounding in favour of the attacker. But here, having a rate of closure of 0.1 will kill the marine as dead as a rate of closure of 10 or 100, only with less bang.

Space speeds are high. Its not boarding of cargo ships by speedboats, it's worse than trying to board fighter jets using parachutes.


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sat Sep 25, 2010 11:37 pm 
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The lowest Rate of Closure (RoC) used in game for kinetics is 0.5 hexes per segment.

This is 4 hexes per turn. One hex per turn is 156.25 m/sec; 4 hexes per turn is 625 m/sec.

In round numbers, Mach 1 at sea level is 2 hexes/turn (312.5 m/sec). 1 hex per turn is slow airline speed. 4 hexes per turn is a bit shy of Mach 2.

We've already established that shuttlecraft (which is what you're basically describing) are damaged by zone defense fire and obey the normal rules for zone defense fire and small targets. Your fighter is basically a shuttlecraft, and is unarmored, and is vulnerable to zone defense fire.

Your mission, and it will make you fantastically wealthy if you succeed, is to find a way to board an F-22 in flight, disable the pilot, and fly it to North Korea.

When you've done this - it's about comparable to the energies and difficulties of ram-boarding a ship - I'll believe in your space fighters ramming and boarding a ship.

I'm figuring that wind shear is roughly comparable to the "pulse the engine a few times to fry the crew" in terms of the boarding action.

You're still going to have to find a way to get the boarders on board without them getting shot at or the plane going a different direction.

The F-22 has less armor to overcome.

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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Sun Sep 26, 2010 5:48 pm 
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William Scott wrote:
Space marines are doable. You can make a landing craft with chemical rockets that can pull enough translation to land on a moving, rotating craft. The marines go directly into the target ship, so no "splat". The landing craft wouldn't need much dV, just enough thrust to get from ship A to ship B.


Lets assume that Ship A and B are at rest relative to each other and they are only 5KM apart. In AV:T terms, this means that they are in the same hex, at the same altitude, and with the same vectors.

Let us also assume that the landing craft wants to reach the enemy ship in one segment. After all, each segment in flight gives the opposing ship more time to change their vector or shoot down the landing craft.

So, the landing craft has to go 5000m in 16 seconds starting and ending with velocity of 0. We'll also assume that the landing craft can make the turn-over in 0 seconds.

As both halves of our flight are symmetric, we only have to figure out how much acceleration we need to cover 2500m in 8 seconds with a starting v of 0;

Our old friend d = vt + 0.5att will do nicely.

2500 = 0*8 + 0.5*a*8*8
2500 = 0.5*64*a
2500 = 32*a
2500/32 = a
78.125m/ss = a

So we need to pull at least 7.9g's for 16 seconds in order to make it. With the proper training and equipment, that is doable by our theoretical marines.

Now, lets say that the two ships are just 1 hex apart. To cross 20km in 16 seconds, the landing craft would have to pull 31.8g's. Whoops, everyone just blacked out and if they live will likely be blind or have sever eye damage for the rest of their lives.

If we keep the acceleration at 9g's for the duration of the flight, it would take 30 seconds, or 2 segments to travel 1 hex between ships.

Now, this is all assuming that the ships are at rest with each other, something that is not going to happen unless both sides are cooperating.

Hopefully, it is clear that once in a situation where the sides are not trying to match vectors that the use of landing craft are highly unlikely. The amount of g's that the marines would have to withstand is at the limit of human conditioning and the amount of time to reach the opposing ships is long enough that it could either alter the vector enough to avoid the landing craft or destroy the landing craft with one of the smaller ship weapons.

Mike Z


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Mon Sep 27, 2010 7:28 am 
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I don't think the problem is the docking. The problem is doing without getting annihilated by zone defense. It would be like trying to land on an aircraft carrier with a helicopter when they don't want you there.


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 Post subject: Re: Orbinaut's perspective: The need for non-gunboats
PostPosted: Mon Sep 27, 2010 2:12 pm 
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William Scott wrote:
I don't think the problem is the docking. The problem is doing without getting annihilated by zone defense. It would be like trying to land on an aircraft carrier with a helicopter when they don't want you there.


Well, more like it's another problem. If something has let you match vectors at that close a range, either it's captained by someone with a quadruple lobotomy, it's surrendered, or it's some scary kludge design with a few size 8 SLRS mounts under a cargo hatch.

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