Cray
02/17/03 03:36 PM
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ICE DROPSHIP ENGINE TYPES: CHEMICAL, NUCLEAR, OR SCRAMJET
"Internal combustion" dropships are basically built like normal dropships. The major difference is in the engine type and size of the fuel tankage:
As with chemical rockets on fighters, chemical rockets on dropships remain very weight-efficient critters. They deliver 1 thrust point for about half the tonnage of a fusion dropship engine (1 thrust point per 3% of the dropship's tonnage). On the other hand, Big Rocket Engines guzzle fuel at an incredible rate. (That picture isn't the Saturn V's first stage engine, the F-1, by the way. That pic is of the somewhat less powerful but much larger M-1, a planned successor to the 5 J-1 hydrogen-oxygen engines used by the Saturn V's second stage. One M-1 would replace the 5 J-1s.)
Nuclear fission rockets are available in two varieties to dropships: solid core and gas core. A solid core rocket is the NERVA/Timberwind we all know and love: very hot, but solid lumps of uranium ceramics superheating hydrogen gas blown over them. Emphasis is on the solid state of the fuel. Gas core rockets actually heat the uranium to a plasma, not a "real" gas, but close enough. With a much hotter fuel, the reaction mass can get a lot hotter, too. There are variants of the gas core reactor: the "nuclear light bulb" the keeps the uranium in a transparent reactor vessel while hydrogen flows along the exterior of the bulb, and the direct injection method that squirts hydrogen through the gaseous fuel. The "light bulb" keeps fuel from escaping (i.e., no spewing of
uranium in the rocket exhaust), but it can't heat the hydrogen as much, so it's less efficient. Since you asked, yes, there's "liquid core" nuclear rockets, but they're an annoying middle ground between solid and gas core, with few of the benefits of either.
Gas Core nuclear rockets are not available for fighters. Their minimum size is too large.
Solid core rocket fuel efficiencies are unchanged from the IC ASF document (900-1000), while gas core can range from 2000 (light bulb) to 3000-5000 (gas core). Solid core rockets are 6% of the mass of the dropship per thrust points, while gas cores are 12% per thrust point.
Scramjets...well, they're like those of fighters, heavy and with limited benefit. 12% of the dropship's mass per thrust point, specific impulse of 3500 in the upper atmosphere, like an ICE conventional fighter in the lower atmosphere, or like a chemical rocket in space. Scramjets are also limited to dropships of 1000 tons or less.
LAUNCHING
Remember, 1 thrust point equals 0.5G. If you have 2 or fewer thrust points available from your engine even in overthrust, a dropship or fighter cannot lift its bulk off Earth. At 3 thrust points (such as from 2/3), the fighter or dropship actually has performance comparable to a modern rocket on the launch pad. (The shuttle and other big rockets often launch at 1.25 to 1.5Gs. After subtracting the 1G pull of Earth, they accelerate upwards at 0.25 to 0.5Gs. As their fuel is burned, they can reach 3Gs or more.) Planets with lighter or heavier gravities may affect the minimum thrust needed to launch. A Scout jumpship (with its unusually powerful stationkeeping drive: 0.2G thrust, 0.3G overthrust) could easily land and takeoff from Luna (0.17G), and any jumpship could easily land and launch from Ceres (0.03G).
TURNING
All non-fusion dropships are assumed to be able to either pivot (not change movement heading) by 3 hex facings per turn, or as many hex facings per turn as they have thrust points (without actually using thrust points). This assumes you're using the advanced movement rules.
LESS THAN 1 THRUST POINT
Dropships restricted to space can very easily get away with less than 1 thrust point (0.5G), unless they're combatants. Or even if they are. 0.2 thrust points (0.1G) will conquer a solar system if your engine is fuel efficient enough and in space, how much fuel you have matters more than how fast you can burn it (unless you have a really weak engine, like an ion
engine).
Dropships with less than 1 thrust point on their main engine should use the advanced turning rules, above. To change their velocity by 1 hex per turn, they have to accumulate enough fractional thrust points to equal a full one. For example, a dropship with a 0.1G engine (0.2 thrust points) would spend 5 turns speeding up or slowing by 1 hex per turn. Until the full thrust point is accumulated, treat them as moving at their previous velocity.
Why would you want engines so weak? Well, if you have a 10000-ton spaceship with a gas core nuclear engine that needs to cross interplanetary distances (say, from Earth to Mars at closest approach) in a reasonable time (about 2, 2.5 weeks), and stop when you arrive but have no fuel for the return flight, you need 9000 tons of fuel. This leaves 1000 tons for the cargo, crew, structure, and engine. A 1 thrust point gas core engine is 1200 tons. So maybe a 0.2 thrust point gas engine looks fairly reasonable.
ADVANCED THRUST CALCULATIONS
In the ICE ASF rule post, I suggested simply recalculating fighter thrust when they carried an external tank in the usual AT2 fashion. Thus a 100-ton ASF with a 200 "ICE" engine would have a 4/6 performance, while adding a 100-ton external tank would slow it to 3/5.
However, that's not very realistic, and it leaves open the question of fighters with HUGE external tanks, like 725-ton drop tanks (like the 100-ton shuttle rides). So, instead, here's a more realistic method:
1) Find the thrust of your fighter without an external tank. Select the engine and whatever normally.
2) Now, convert the thrust (and overthrust) ratings into a measurement of tons of thrust. Do this by:
2a) Divide the thrust and overthrust ratings by 2, keeping fractions. This is how many G's your fighter can pull at thrust and overthrust.
2b) Multiply the results of 2a by the tonnage of your fighter. These are how many tons of thrust your fighter generates at thrust and overthrust ratings.
3) To figure out how many G's your fighter pulls with the full external tank, divide the tons of thrust by the (tonnage of the fighter + the tonnage of the external tank).
4) Multiply this result by 2 to convert back to thrust points.
For example: a 100-ton fighter with a 200 rated engines decides to mount a 100-ton external tank.
The fighter has 4/6 movement. This means it can pull up to 2Gs under thrust and 3Gs in full overthrust. To get 2Gs, it must have 200 tons of thrust from its engine, or 300 tons in full overthrust. When you load up the 100-ton external tank, the fighter now masses 200 tons, and its engines have more to push. At full thrust, the 200 tons of thrust can push the 200 tons of ET+fighter at 1 G (200/200), or 2 thrust points. At full overthrust, the 300 tons of thrust can push the assembly at 1.5Gs (300/200), or 3 thrust points.
Another example: the same 100-ton fighter is strapped onto a 700-ton external tank.
The fighter has, as noted, 200 and 300 tons of thrust from its engines. At full thrust, that 200 tons of thrust labors to move (100+700) 800 tons of mass. 200/800 = 0.25Gs, or 0.5 thrust points. At full overthrust, that 300 tons of thrust manages to move the 800 tons at 300/800 = 0.375Gs, or 0.75 thrust points.
SOLID ROCKET BOOSTERS (SRBs) AND LIQUID ROCKET BOOSTERS (LRBs)
Solid rockets have abysmal fuel efficiencies. Their specific impulses range from 250 to 300. They can only be used once. However, they can be very powerful engines, and the military likes them for their near-indefinite storage potential. They're also simple and have few concerns about ignition
(other than blowing up or burning through a segmented joint), unlike many liquid fuel engines.
Liquid fueled boosters are popular with the Rooskies, but have seen negligible use in the West.
Boosters, in AT2 terms, amount to external fuel tanks, but ones with their own engines. To keep it simple, boosters allow fighters (and dropships) to use their normal thrust without recalculating their thrust due to the mass
of the booster they're carrying. SRBs only have a thrust rating (that of the fighter or dropship they're attached to), no overthrust rating, only operate at their full thrust rating (nothing less), and will burn until exhausted - they can be jettisoned prematurely. LRBs can throttle like a fighter normally.
How to use boosters of any sort with AT2: Pick solid or liquid. Pick a specific impulse (recommended: 250 for the solids, 450 or 500 for the liquids). Pick an overall tonnage for the booster, up to 10 times that of the fighter or up to the mass of the dropship. For a SRB, 15% of the mass is structure, with the remainder split between fuel and (if any) armor. For a LRB, 5% of the mass is structure and engine, with the remainder split between fuel and (if any) armor.
ORBITAL VELOCITY & GRAVITY LOSSES
For Earth, low Earth orbit is at about 7800m/s, 26 AT2 hexes/turn.
But getting a rocket into orbit means more than getting the rocket moving parallel to the ground at 7800m/s. It also means climbing up above the atmosphere (c200km) to avoid being slowed by atmospheric drag to the point you're no longer moving fast enough parallel to the ground to avoid the horizon (which gravity is pulling you toward...remember, in orbit, you're not really in "zero-G", you're in "free fall" toward the planet, being pulled toward the planet constantly by gravity.)
In the process of climbing above the atmosphere, you have to fight gravity (rather than letting it pull you down, as you do in orbit) and you have to fight atmospheric drag.
I could probably give you an equation for gravity losses (something like: time to orbital velocity in seconds multiplied by the local gravity in m/s/s gives the velocity loss in m/s) and probably approximate the hideousness of atmospheric drag, but it boils down to about this: 1200m/s, or 4 thrust points of fuel, for a basically Earth-sized, Earth-dense planet.
Thus, to get into orbit, your IC aerospace fighter will need 30 thrust points of fuel, or about 9000m/s of "delta-V" (change in Velocity). A 100-ton aerospace fighter with a realistic hydrogen-oxygen chemical rocket (Isp 460, miraculously even at sea level) would need 87 tons of fuel at launch to get into orbit by itself.
Note that small changes in a planet's size and makeup can result in surprising differences to orbital velocity and thus required fuel mass. Getting into lunar orbit from the lunar surface takes about 2000m/s (7 thrust points) would only required 38 tons of fuel by the preceding aerospace fighter.
ESCAPE VELOCITY
Escape velocity (for a spherical body) is always equal to the orbital velocity multiplied by the square root of 2 (1.414). For an Earth-sized planet, that's 43 thrust points.
The preceding aerospace fighter would need to have 95 tons of fuel at launch to get its (originally) 100-ton butt to escape velocity. A 1000Isp nuclear rocket version of the fighter could reach escape velocity with about 75 tons of fuel.
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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