Cray
02/12/03 08:31 PM
65.32.253.120
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Your Periphery planet just crawling out of its Succession Wars-induced steam age? Was the last fusion engine factory (and its engineers) on your planet nuked by WoBblies? But you still need aerospace support?
Look no further: rules for "internal combustion engine" aerospace fighters are here.
ICE AEROSPACE FIGHTERS (ASFs)
If you stick to reasonable propellants, you're going to have variable thrust ratings and/or fuel consumption. Unless you simplify things to cinematic levels, of course. Let's keep it semi-real.
First, note that 1 thrust point, which gets a ship moving at one 18km AT2 hex per 60-second turn, equates to 300m/s. A thrust point is 0.5G.
Consider that the space shuttle orbiter carries enough fuel for its OMS (orbital maneuvering system, the two medium-sized engines in pods on either side of its tail) to shift its speed by, IIRC, 600m/s, or the equivalent of 2 thrust points. (Remember, an AT2 hex is 18km across, the distance you'll cross in one 60-second AT2 turn after accelerating at 0.5G [i.e., 1 thrust point] for one turn.) Fuel isn't a BIG fraction of the orbiter's mass, though, just several tons. (On the other hand, the shuttle burns 1700 tons of fuel getting into low Earth orbit: 26 thrust points. Actually, a bit more. The total delta-V of the shuttle at launch is around 9000m/s or 30 thrust points vs. the 7800m/s needed to reach orbit. The extra 1200m/s is spent fighting gravity and atmospheric drag.)
So. Pondering an ICE ASF in space...well, let's set some "givens":
1) The vehicle is 100 tons. Nice round number.
2) The fuel used is a super-advanced LOX/LH2 rocket with a specific impulse of 500. Nice round number again, though the rocket scientist in my gaming group would tell me anything over 460 is unreasonable.
Fuel use for the fighter goes something like this:
TP FS VM
0 0 100000
1 5900 94100
2 5600 88500
3 5300 83200
4 4900 78300
5 4600 73700
6 4400 69300
7 4100 65200
8 3900 61300
9 3600 57700
10 3400 54300
11 3200 51100
12 3000 48100
13 2800 45300
14 2700 42600
15 2500 40100
16 2400 37700
17 2200 35500
18 2100 33400
19 2000 31400
20 1900 29500
TP = thrust points spent, FS = fuel spent (kg), VM = vehicle mass. Rounded, obviously.
To get 20 thrust points with this super-efficient chemical rocket, the 100-ton fighter needs to burn 70.5 tons of fuel.
DROP TANKS
If you included a 100-ton drop tank (increasing initial mass to 200 tons) that included 95 tons of fuel (heavier tank construction than the shuttle uses on its ET), you would get an additional 10 thrust points from the 95 tons of fuel.
If you included, say, a 725-ton extenal drop tank (like the shuttle's) with 700 tons of fuel, you would get 31 thrust points from the extra 700 tons of fuel. Those natural logs and exponents are nasty buggers.
Incidentally, large external tanks don't necessarily require big, single tanks like the current shuttle has. There are other options:
Twin tanks on either side of the spacecraft:
www.abo.fi/~mlindroo/SpaceLVs/Slides/sld033.htm
www.abo.fi/~mlindroo/SpaceLVs/Slides/sld019.htm
(The entire slide show at www.abo.fi/~mlindroo/SpaceLVs/Slides/sld001.htm is highly illuminating of the thought processes behind the development of the current US space shuttle. )
However, when you're talking about liquid hydrogen (LH2), you'll be talking about very large fuel tankage (and there isn't really a denser option to give the same level of performance from a rocket...fission, fusion or chemical). The shuttle's external tank carries 6 times as much LOX by mass as LH2, but the LH2 tankage is over 2.5x as large as the LOX tankage. Here's a cutaway of the shuttle ET. The smaller nose tank is the LOX tank, which holds 600,000kg of LOX. The larger rear tank is the LH2 tank, which holds 100,000kg of LH2.
www.lockheedmartin.com/michoud/gallery/lo_res/et_orthographic_lo.jpg
LIGHTER FIGHTERS
While the fuel use isn't linear in relation to velocity, it is fairly linear due to mass. A 10-ton ICE ASF will use 1/10th as much fuel as a 100-ton ICE ASF for the same change in velocity (i.e., for the same number of thrust points). A 90-ton ICE ASF will use 90% as much fuel as a 100-ton ICE.
MORE AND LESS FUEL EFFICIENT ROCKETRY
Aside from hideously toxic fuels (usually things involving
fluorine, beryllium, etc.), a hydrogen-oxygen chemical rocket is about as fuel efficient as you're going to get. It has a "specific impulse" (Isp) of about 460 sec^-1 (with simpler LOX/LH2 engines hovering around 420). "Specific impulse" is a measure of fuel efficiency. Specific impulse means "You can get this many pounds of thrust per second per pound of fuel" or "You can get this many kilograms-force of thrust per second per kilogram of fuel." Thus, a good hydrogen-oxygen rocket will use 1 pound of fuel in one second while producing 460 pounds of thrust. Good kerosene-oxygen rockets (like the first stage of the Saturn V) had a specific impulse of about 265.
This number can vary quite a bit. The space shuttle's main engines, the SSMEs, only have a specific impulse of about 350 at sea level. Rocket fuel efficiency is very dependent on internal pressure, external atmospheric pressure, and how the rocket nozzle is shaped to handle both. The SSMEs are designed for high altitude/space work, to supply most of the velocity to get the shuttle into orbit. The shuttle uses the low altitude-optimized solid fuel boosters for most of its sea level thrust, and to clear the atmosphere.
Why do I mention all this?
Well, it's to give you a background to understand this point: the fuel used by a rocket (in relation to changing its velocity) is linearly related to its specific impulse. Double the specific impulse and you double the velocity you can get from a given mass of fuel. Halve the specific impulse and you halve the velocity you can get from a given mass of fuel. (Ignoring gravity - there's times when it's good to burn fuel quickly, like during launch, hence the shuttle uses its inefficient, high-powered SRBs with a specific impulse of ~250). This gives you the ability to look at alternate fuels for your own ICE ASFs - www.astronautix.com takes pains to mention the specific impulses of the engines it lists.
For the most part, any given fuel combination for an ICE ASF is only going to vary between 60% and 90% as effective as the super-engines I used in the above calculations. In other words, the change won't be that significant. You WILL be using a lot of fuel.
However, it might be significant if you decided to look into non-fusion nuclear (fission) rocketry. Hydrogen-fed nuclear thermal rockets like the Timberwind double the specific impulse of a good chemical rocket (to about 1000).
For comparison, the specific impulse of the engine on a 100-ton, fusion-powered ASF is about 125,000. That of a 20-ton fighter (which uses the same mass of fuel per thrust point as a 100-ton fighter) is 25,000. 25,000 is better than many airbreathing jet engines. You might deride canon ASFs as being fuel thirsty compared to dropships and warships, but with rocket engines like a fusion-powered ASF has, you could open the Solar System with quick and efficient space travel.
SCRAMJETS
Generally, the flammable stuff pouring into a rocket engine's combustion chamber(s) is referred to as "fuel." To be nitpicky, there's actually two things going in: fuel and oxidizer. The fuel burns, the oxidizer is the "air supply" for the resulting fire.
Surprisingly, the oxidizer is usually the heavier component. Sure, you only need 1 oxygen atom per two hydrogen atoms to burn, but that 1 oxygen atom is 8 times as massive as the two hydrogen atoms put together. When oxygen burns kerosene, a lot of oxygen atoms are needed to pair off (1 with every 2 hydrogen atoms in the kerosene, 2 with every carbon atom).
In fact, rocket engines almost always run fuel-rich, so there's more fuel in the rocket than there is oxygen to burn it. This is to make sure there's always fuel atoms around to burn with the oxygen, rather than letting the super heated oxygen wander off and burn with, say, the walls of combustion chamber. The shuttle's mainengines run at a 6:1 ratio (mass of oxygen to mass of hydrogen) rather than the ideal 8:1 partly for this reason.
The Saturn V's kerosene-LOX first stage was in a similar situation. It burned 2.27 pounds of LOX per pound of kerosene burned.
WTF does this have to do with scramjets?
Think about it: since oxygen is the heavier component in rocket engine fuel, as much as 6/7 of the mass, if you could get that oxygen from the air rather than an onboard fuel tank, the engine could increase its fuel efficiency (as far as carried fuel is concerned) about 7-fold. A hydrogen scramjet engine would have a specific impulse of about 3500 versus a hydrogen-oxygen rocket with specific impulse of 500. That 70 tons of fuel used by an ICE ASF to get 20 thrust points suddenly becomes 140 points of fuel in the upper atmosphere.
ICE AEROSPACE FIGHTERS, IN REVIEW
So, you've got a load of information for primitive, semi-realistic ICE aerospace fighters. What are you going to do with it?
Well, if you're going to build the fighters, I recommend looking at the thrust point chart I put up and figuring how many (many, many) tons of fuel your ICE ASF will need. Something on the order of 10-11 points sounds reasonable; that "only" uses half the fighter's mass as fuel. Use external tanks for the initial boost to orbit (~25 points) or to close with "hostiles" if already in orbit. Remember, the fuel use is linear with fighter mass: a 50-ton ICE ASF will use half the fuel mass of a 100-ton fighter for the same number of thrust points.
I'd also recommend ignoring turning fuel costs, at least for vehicle facing. If you want to actually change the ICE fighters' movement, spend fuel. If you're using the advanced movement rules to just "spin in place," ignore fuel costs. There's no sense in burning 15 tons of fuel to spin 180 degrees - the 100-ton shuttle uses a few kilograms of fuel to do that (as do fusion-powered ASFs).
I also recommend picking a single thrust rating for your fighter and sticking to it, even as the fighter gets lighter. You're free to recalculate thrust, and the fuel point usage chart I put up above doesn't care how fast the thrust points are used, just how many are used. However, realistically, chemical rockets try to keep below a certain peak G-force level with a combination of throttling and staging (dropping lower stages) to use smaller, weaker engines.
Don't think in terms of fusion ASF dog fights. An ICE ASF ain't chasing down any fusion-powered space craft. It can pop up to orbit, or maneuver a bit, fire a volley of ammo, and head home. Consider them for Periphery plants with pirate problems.
Chemical rockets are very mass efficient - 40:1 and 100:1 thrust-to-weight ratios (for the engine's weight vs. it's own thrust) are not unknown. On the other hand, a 300-rated fusion engine has about a 15:1 to 18:1 thrust-to-weight ratio. I'd recommend treating an "ICE ASF engine" like an XL engine for mass. Besides, any mass savings on the engine are going to be dumped into heat sinks (ICEs don't have them...), power amplifiers, and fuel (a 9.5-ton 300 ICE rocket engine frees up room for, what, 2 to 4 thrust points on a
100-ton fighter?)
Fission rockets are somewhat heavier, though still impressive (in thrust-to-weight terms). Treat fission rockets as heavy as normal fusion engines. However, fission rockets are still "ICE": they have no heat sinks, and energy weapons on a fission rocket ASF still need power amplifiers. Why? Because fission rockets are just hot masses of radioactive fuel that do nothing but heat hydrogen squirted through them. They do not have the integral power generation systems of fusion engines, nor do they need heat sinks - the reaction mass squirted through them keeps them cool. Why use fission rockets since they're heavier and don't have heat sinks? Because they're about twice as fuel efficient as chemical rockets - you'll save many tons with them.
Scramjets are not so weight efficient. If you want your ICE ASF to have scramjets rather than plain rockets, I'd use normal ICE engine weights. Assume an ICE ASF with scramjets can function as a plain rocket, too - if you can get a scramjet to work, converting the scramjet to rocket operation is easy. And at low altitudes where conventional fighters can operate, your wonder scramjet can presumably function as conventional fighter's ICE, with all the
associated fuel savings. Hey, your ICE ASF will probably be able to stay in the air much longer than an ICE conventional fighter - the ASF probably has much more fuel tankage.
Engine rating is determined like normal for ASFs.
Drop tanks. External fuel tanks are 3% of the mass (rounded to the nearest ton) of the fuel they carry. Extra armor can be added, though I'm too lazy to integrate them into fighter hit locations. Above 1000 tons, armor is applied as with dropships. Recalculate thrust if a drop tank is carried.
I'd treat ICE ASFs as ICE mechs for heat: DHS are acceptable, heat generation is like a normal ASF (including any from thrust), the engine comes with no heat sinks. Of course, if you're making an ICE ASF, then DHS are a bit unlikely.
Oh, hey, bonus: chemical rocket engines frequently cool themselves with their own fuel. Tons of cryogenic oxygen and hydrogen make a great heat sink (in the usual "absorber and holder of heat" meaning of the term, not like BT heat sinks). As a consolation prize for being stuck with so much fuel, an ICE ASF can use its fuel to absorb extra heat, a lot of it. Each heat sink they have can operate at double efficiency while dumping heat into the fuel. The limit to this bonus: each ton of fuel can soak 3 points of heat. More than 3 points of heat per ton and the fuel tank explodes from internal pressure.
For whatever's left (like atmospheric maneuvering, armor, etc.), treat the ICE ASF as a normal ASF.
CALCULATE YOUR OWN FUEL USE
Want to know how much fuel your own design will use without looking at my chart?
Okay, to find out how heavy a fighter will be after using its engine for a while, you start with this equation:
Final Mass = Initial Mass / [e^(Change in velocity / exhaust velocity)]
"Final mass" is how massive the fighter is after running its engine. "Initial mass" is how heavy it was at the beginning of the engine "burn." (I recommend working with kilograms; BT uses metric tons, and kilograms will give a fairly precise answer.)
"Change in velocity" is how many meters per second you want to add or remove from your fighter. Remember, 1 thrust point = 300 meters per second.
"Exhaust velocity" is how fast gas is farting out of the rocket engine. It's equal to the engine's "specific impulse" (which has the units of 1/sec) multiplied by G (9.8m/s/s).
For example, a 100-ton (100000kg) fighter that expends 1 thrust point (300m/s) with the realistic space shuttle main engines (specific impulse of 455):
Final Mass = 100000 / [e^(300 / 455 * 9.8)]
Final Mass = 93493kg
Fuel used: 100000-93493 = 6506kg
Got more gas than you know what to do with? (What no matches and no friends to entertain?)
You can turn the above equation inside out:
Change in velocity = exhaust velocity x natural log of (initial mass / final mass)
For example, you've decided to give your 20-ton fighter a 20-ton drop tank (with 19 tons of fuel) to go with its 455 Isp engine and want to know how many thrust points that drop tank gives you:
Change in velocity = 455*9.8 x ln (40000 / 21000)
Change in velocity = 2873m/s
Thrust points = 2873 / 300 = 9.5
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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masdog5
02/13/03 09:37 AM
205.213.145.33
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Wow...that left my brain spinning...
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Vapor
02/13/03 11:01 AM
202.123.139.14
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I like your post. It's very well thought out and very well done. I do have a few things to point out, however.
1) An ICE will not function in space. Some people consider rockets to be a form of ICE, but I don't. ICE typically refers to either a reciprocating engine, or a turbine. A rocket engine is another beast entirely, since it operates on slightly different principles. An ICE requires atmosphere in order to produce thrust. A rocket engine is not hampered by that limitation.
2) A scramjet would work at high altitudes and airspeeds, but again, wouldn't function in space or at low airspeeds. Basically, a scramjet is a highly specialised turbine engine. The scramjet's main limitations come from it's design, however. It doesn't have a compressor section. Instead, it relies on the forward movement of the engine to force air into the intake. So, unless the scramjet is already moving at a high velocity when it is started, it won't produce any thrust. However, once it is started, it is extremely efficient.
3) An aircraft mounting an ICE would be limited to ballistic and missile weapons only. This is due to the fact that no ICE could produce enough thrust to keep the aircraft flying, and at the same time produce the energy required to power energy weapons. Mounting another ICE (preferrably a turbine) to power energy weapons might solve that problem, but again, that's extra weight to carry around.
"For those about to rock, we salute you." - AC DC
"The evil that can come, from the heart of a man, must be answered in kind 'till it disappears, and we're safe." - Kansas
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GiovanniBlasini
02/13/03 04:09 PM
204.210.29.44
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In reply to:
1) An ICE will not function in space. Some people consider rockets to be a form of ICE, but I don't. ICE typically refers to either a reciprocating engine, or a turbine. A rocket engine is another beast entirely, since it operates on slightly different principles. An ICE requires atmosphere in order to produce thrust. A rocket engine is not hampered by that limitation.
Yes, but if you read the volumes Cray was kind enough to post here, he's referring to rockets. In Battletech, ICE is usually used to differentiate from nuclear power, or simply running off batteries.
In reply to:
2) A scramjet would work at high altitudes and airspeeds, but again, wouldn't function in space or at low airspeeds. Basically, a scramjet is a highly specialised turbine engine. The scramjet's main limitations come from it's design, however. It doesn't have a compressor section. Instead, it relies on the forward movement of the engine to force air into the intake. So, unless the scramjet is already moving at a high velocity when it is started, it won't produce any thrust. However, once it is started, it is extremely efficient.
Um, most of our more recent scramjet projects have involved converting the scramjet into, essentially, a rocket, for use out of the atmosphere.
The main problem with scramjets working at low altitude is the main problem with scramjets working at all - supersonic speeds for air passing through them. I'm guessing that problem will eventually be solved, allowing for practical scramjets. That'll probably mean the same engine will have to act like a ramjet at low speed & altitude, scramjet at higher altitude (and speed), and a rocket outside of an atmosphere.
In reply to:
3) An aircraft mounting an ICE would be limited to ballistic and missile weapons only. This is due to the fact that no ICE could produce enough thrust to keep the aircraft flying, and at the same time produce the energy required to power energy weapons. Mounting another ICE (preferrably a turbine) to power energy weapons might solve that problem, but again, that's extra weight to carry around.
Dude, I have issues with ICE engines on conventional vehicles being able to generate enough power, but the generators tied to them in the Battletech universe appear to be up to the job.
Besides, you might recall we have several airliner-type aircraft in real life that've been fitted with experimental laser weapons. Remarkably, they do not fall out of the sky. 
Member of the Pundit Caste
"Which side are we on? We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go. I'm surprised you didn't know that." -- Col. Saul Tigh, BSG2003
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Cray
02/13/03 05:28 PM
65.32.253.120
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1) I used ICE mostly to tie into BT ground vehicle terminology. However, "ICE" refers to an "internal combustion engine." It doesn't matter what kind of engine it is (turbine, piston, Wankel rotary, rocket, etc.), so long as the fuel directly acts on the engine instead of via an intermediary fluid (like steam, which is an external combustion engine). And, of course, may rocket engines include large and powerful gas turbines to power their fuel pumps.
2) A scramjet is not a turbine engine. Pointedly, it lacks a turbine (in addition to lacking a compressor). I'm aware that scramjets don't work at low speeds, or in a vacuum. Which is why I said, "you can get a scramjet to work, converting the scramjet to rocket operation is easy." You close off the air intake and start pumping oxidizer into the scramjet as well as fuel: a rocket.
For low speed operations (and vacuum), there's the "ram augmented rocket" or "rocket augmented ramjet." By placing a rocket in a (sc)ramjet, the ramjet basically gets a lot of air sucked through it - the rocket exhaust entrains a lot of external air, fast enough to get the ramjet operational even on the ground. Low speed operation fuel efficiency is poor, only slightly better than a rocket, but at mach 1 and faster you get the usual fuel efficiency gains of ramjet, then a scramjet if you can make a scramjet work at all, and finally you just use the rockets for the final kick to orbit.
In reply to:
3) An aircraft mounting an ICE would be limited to ballistic and missile weapons only.
First, this is incorrect according to the AT2 errata. Conventional ICE fighters can now carry energy weapons when given the usual power amplifiers and heat sinks. Reality based concerns are secondary. However...
In reply to:
This is due to the fact that no ICE could produce enough thrust to keep the aircraft flying, and at the same time produce the energy required to power energy weapons
Second, this is incorrect, too. Electrically powered lasers (diode lasers) are being eyeballed for near-future fighter applications. 10kW demonstrators have been used on the ground; a flight version is a matter of harnessing more laser heads into a common emitter. Stumbling blocks are cooling (a 100kW laser will produce about 900kW of waste heat), probably to be solved by using the fighter's fuel supply as a heat sink, and getting enough diode lasers to work together. Power is trivial: 100kW, or 1000kW, can be delivered by a very small turbogenerator (on the order of a few hundred pounds).
Third, chemical lasers can deliver megawatts of lasing power without any more electricity used than what their targeting computers and fuel pumps need.
Fourth, for AT2, "power amplifiers" may well be that secondary power plant. After all, a vehicle with a 10-rated ICE and a 3-ton power amplifier can power 4 ER PPCs. That 3-ton power amplifier may be a high-strung, fuel-gobbling turbo-generator set.
Fifth, jet engines (and chemical rocket turbopumps) have more shaft horsepower than you can shake a stick at (100,000shp and more is not unusual). Those horsies are harnessed for secondary power applications through a gear box, such as for propellers (turboprop aircraft and US Navy surface warships...US gas turbine-powered warships use the DC-10's jet engine to drive their props) and electrical generators (USN ships again, which get megawatts for their Aegis radars from those former jet engines).
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Cray
02/13/03 05:31 PM
65.32.253.120
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In reply to:
Besides, you might recall we have several airliner-type aircraft in real life that've been fitted with experimental laser weapons. Remarkably, they do not fall out of the sky.
www.airbornelaser.com
The Airborne Laser uses a chemical laser - basically, it burns a nasty rocket fuel, which produces a lot of laser light while burning. Electricity demands are minimal.
However, as I noted in my other post, jet engines are quite capable of delivering megawatts of electrical power even while operating as propulsion power plants.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Vapor
02/14/03 03:56 AM
202.128.69.122
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Ok, I will admit that I was looking at this from a real-world viewpoint rather than from a BT viewpoint. I tend to do that a lot when I think about BT technology, simply because a lot of BT tech is based on real-world physics, and it doesn't take a whole lot of imagination to "bridge the gap," so to speak.
In reply to:
1) I used ICE mostly to tie into BT ground vehicle terminology. However, "ICE" refers to an "internal combustion engine." It doesn't matter what kind of engine it is (turbine, piston, Wankel rotary, rocket, etc.), so long as the fuel directly acts on the engine instead of via an intermediary fluid (like steam, which is an external combustion engine). And, of course, may rocket engines include large and powerful gas turbines to power their fuel pumps.
I know what ICE refers to.  lol I still don't consider a rocket engine to be an ICE, however. I realise that is my opinion, and most people would disagree with me, but I will explain my reasoning (or try to. Sometimes my thought patterns get a little complex and when I try to explain myself I end up confusing everyone else, as well as myself lol).
My main distinction comes in the fact that a rocket engine utilizes a completely self-contained system. That means it carries not only the fuel for the engine, but the oxygen to burn the fuel, with it. A reciprocating engine or turbine engine only carries the fuel on the aircraft, relying on the atmosphere to provide the oxygen.
Secondly, the point of ignition in a rocket engine is exposed to the outside of the engine. I'm not an expert on rockets, so I don't know all the terminology, but the point of ignition occurs when the fuel and oxygen are mixed, and then the resulting flame (for lack of a better term) is immediately ejected from the engine. The force of the flame and resulting energies leaving the engine is what creates the thrust. In reciprocating or turbine engines, the point of ignition is wholly inside the engine itself. If you see flames coming out of either a reciprocating or turbine engine, than something is seriously wrong with the engine (the exception, of course, being a turbojet engine mounting an "afterburner" kit, where flames exiting the exhaust cone are normal when the afterburner is engaged).
As far as how thrust is created, rocket engines are very similar to turbojet engines. They both rely on the energy created by the ignition of the fuel to push the engine through the air. Reciprocating engines, turboprop engines, and turbofan engines instead rely on the engine to drive another object which pulls the engine through the air (a propellor, in the case of a reciprocating or turboprop engine, or a fan, in the case of a turbofan engine).
A quick question about your statement about the fuel pumps, though. Do you know exactly how those pumps function? If they do not burn fuel in order to create motion, then they would be considered motors, not engines.
In reply to:
2) A scramjet is not a turbine engine. Pointedly, it lacks a turbine (in addition to lacking a compressor).
Ok, I wasn't clear enough when I made my statement. I appologize. I meant to say that the scramjet is modeled on a turbojet engine (ie. air enters the front of the engine, is compressed, fuel is added and ignited, and the resulting hot gases are expelled out the back of the engine, creating thrust). It doesn't really have any moving parts inside it, hence it's inability to operate at low speeds.
In reply to:
First, this is incorrect according to the AT2 errata. Conventional ICE fighters can now carry energy weapons when given the usual power amplifiers and heat sinks.
Ok, I appologize for making this point. I'm not too familiar with AT2 rules, yet (I'm still trying to make my way through the book for the first time ). I was also (again) thinking along a real-world viewpoint (I really should stop doing that).
I was also unaware of the advances in laser technology. Thank you for bringing me up to speed on that matter. 
"For those about to rock, we salute you." - AC DC
"The evil that can come, from the heart of a man, must be answered in kind 'till it disappears, and we're safe." - Kansas
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Cray
02/14/03 06:45 AM
65.32.253.120
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In reply to:
Secondly, the point of ignition in a rocket engine is exposed to the outside of the engine. I'm not an expert on rockets, so I don't know all the terminology, but the point of ignition occurs when the fuel and oxygen are mixed, and then the resulting flame (for lack of a better term) is immediately ejected from the engine.
This is like a gas turbine/jet engine, too. The combustion chamber is open to air from the compressor and for exhaust to go to the turbine. The combustion chamber of a rocket is at least as "internal" as that of a jet engine - the throat between the combustion chamber and rocket nozzle can be very narrow.
In reply to:
A quick question about your statement about the fuel pumps, though. Do you know exactly how those pumps function? If they do not burn fuel in order to create motion, then they would be considered motors, not engines.
Turbopumps on rocket engines are called turbopumps because they include very powerful gas turbines, over 100000shp on large engines like the F-1 and M-1. They burn quite a bit of fuel, often in a low temperature, fuel-rich reaction, the exhaust of which is vented into the rocket nozzle to act as an insulator between the rocket nozzle and the exhaust from the combustion chamber proper. This is why the exhaust of the Saturn V's F-1 engines has a black appearance for several feet below the nozzle: sooty exhaust from the turbopumps is being spewed out, only to complete combustion with atmospheric oxygen.
This link actually gives an excellent view of the F-1 engines on the bottom of the Saturn V stack, their turbopumps (the upright cylinders beside each rocket nozzle), and the exhaust duct that wraps around the middle of the rocket nozzle to vent turbopump exhaust into the nozzle:
www.astronomers.net/saturn5.htm
The above link on the M-1 also has a similar turbopump venting system partway down the nozzle. Above that point, it used circulated, unburnt fuel (hydrogen) in a grid network of tubes (which you can see) to keep the nozzle cool.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Vapor
02/14/03 07:22 AM
202.128.69.122
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I suppose it could be argued either way endlessly, as to whether or not a rocket engine is actually an ICE. Nothing I can find actually states that a rocket engine is an ICE, though I can find several places that call it a "reaction engine," which isn't what we are discussing in the first place, so it doesn't help. lol
In reply to:
Turbopumps on rocket engines are called turbopumps because they include very powerful gas turbines, over 100000shp on large engines like the F-1 and M-1. They burn quite a bit of fuel, often in a low temperature, fuel-rich reaction, the exhaust of which is vented into the rocket nozzle to act as an insulator between the rocket nozzle and the exhaust from the combustion chamber proper.
Ok, that would be considered an engine, though it would be simply a utility engine (similar to an APU), and isn't responsible for creating any thrust.
"For those about to rock, we salute you." - AC DC
"The evil that can come, from the heart of a man, must be answered in kind 'till it disappears, and we're safe." - Kansas
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Cray
02/14/03 08:55 AM
147.160.125.185
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Well, a rocket ain't an external combustion engine, and its exhaust works directly on the engine to produce thrust (pushing directly against the rocket bell or aerospike, just like it would against a piston or turbine in another ICE). Close enough to an internal combustion engine for my tastes. YMMV.
As for the turbopump, I wasn't trying to indicate that made the rocket engine an ICE, I was pointing out that a chemical rocket potentially had a great deal of power available for an electrical generator for energy weapons.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Edited by Cray (02/14/03 08:57 AM)
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Karagin
02/15/07 07:22 PM
70.123.166.36
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So Cray would these make better low tech ASF/high tech Conventional Fighters then what we have currently?
Karagin
Given time and plenty of paper, a philosopher can prove anything.
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Cray
02/15/07 11:21 PM
68.200.109.191
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Quote:
So Cray would these make better low tech ASF/high tech Conventional Fighters then what we have currently?
Conventional fighters would be superior in the atmosphere due to superior fuel endurance.
There are no low tech ASFs currently available except for those with the "archaic equipment" penalty of Merc Supplement 2. Those MS2 "low tech ASFs" would dance circles around these "ICE aerospace fighters."
The short answer: no.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Karagin
02/15/07 11:25 PM
70.123.166.36
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Okay I was wondering about the SV rules as I was re-reading this.
Karagin
Given time and plenty of paper, a philosopher can prove anything.
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Cray
02/15/07 11:51 PM
68.200.109.191
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Quote:
Okay I was wondering about the SV rules as I was re-reading this.
Speaking of the SV rule fixed wing aircraft, have you found an SV fixed vehicle that is ton for ton better than a conventional fighter? So far, you've only pointed out a 200-ton vehicle that is better in one area (bomb capacity) than conventional fighters, and haven't demonstrated any SV fixed wing designs that beat out conventional fighters on all counts at the same tonnage.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Karagin
02/16/07 12:38 AM
70.123.166.36
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I am working on it...between myself and a couple of buddies I am sure we will have something soon.
Karagin
Given time and plenty of paper, a philosopher can prove anything.
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Karagin
08/24/09 01:15 PM
72.178.75.99
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Old topic, new question...
Why didn't this make it into AT2 or one of the other books?
Karagin
Given time and plenty of paper, a philosopher can prove anything.
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Cray
08/24/09 02:17 PM
147.160.136.10
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Quote:
Why didn't this make it into AT2 or one of the other books?
AT2R, which I helped review, was meant to fix AT2's issues, not introduce niche equipment that would consume several thousand words (minimum) for a proper rules treatment, which AT2R couldn't spare. And chemical rocketry is a niche in BT: BT makes very high performance fusion rockets a 2020's invention (see Magellan probes), so even primitive Deep Periphery hicks with any pretensions toward spaceflight will probably harness fusion rocketry in short order.
Tactical Operations and Strategic Operations were more of the same - they had a lot to address applicable to existing units, equipment and factions. Spending several thousand words for ultra-niche chemical rocketry in grossly-over-word count books (like TO) was a non-starter.
The same sort of problem resulted in sails being deleted from support vehicle rules. There just wasn't enough page space to address sail-powered movement.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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Karagin
08/24/09 03:35 PM
72.178.75.99
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Okay, fair enough...but would not stuff like this be prime material for follow up books and supplement items?
Karagin
Given time and plenty of paper, a philosopher can prove anything.
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Cray
08/24/09 03:41 PM
147.160.136.10
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Quote:
Okay, fair enough...but would not stuff like this be prime material for follow up books and supplement items?
Yes, when there's an appropriate product and room. The topic was touched on when addressing satellites in Strategic Operations (note the miserable fuel capacity of non-fusion satellites), so there's a toehold.
Mike Miller, Materials Engineer
Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
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