Cray
03/09/04 04:06 PM
147.160.1.5
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Unusually, the spacefaring Nomads use chemical rocket spacecraft for short range work. Nomads are quite capable of building small fusion engines (such as the engines used on their protomechs), making these craft all the more unusual.
You could chalk it up to the logic of their culture. They live on spacecraft and, despite the bounty of their industry, they are leery of wasting elements and compounds used by their life support systems, including water, hydrogen, and oxygen. They produce quite a bit of waste oxygen during hydrogen production that is used for very little. The kilogram or two of oxygen used by an adult human each day is almost entirely recovered by the life support recycling systems. Naturally, Nomads decided to burn the oxygen with some fuel (hydrogen was handy) and blast it out a rocket nozzle. That was less wasteful.
Plus, something about the chemical rockets appeals to Nomad pilots. With 1 ton of hydrogen, a typical small fusion rocket can accelerate at 1G for 40 minutes (or 2Gs for 20 minutes, or whatever). That's a delta-V of 23.5km/s, more than twice what was contained in the Saturn V's three stages. It was no challenge to pilot such a vehicle. But to change orbits with sips of fuel, boost off a moon or asteroid while using so rapidly that your rate of acceleration changes noticeably, to have just seconds of hovering time when landing - that tested a pilot.
And, truth be told, the flights between jumpships in a Nomad Squadron consumed so little fuel even with chemical rockets, what was the harm? Chemical rockets were certainly much easier to manufacture than a fusion rocket. The material, engineering, and manufacturing challenges were trivial compared to the exercise in applied esoteric physics that was a small fusion engine. A two-walled, milled copper reaction chamber and nozzle, a pressure feed system to avoid the trouble with fuel turbopumps, and some aluminum tankage that could handle 20 atmospheres of internal pressure, and some valves. Simple.
The typical Nomad Utility Bus is an exercise in simplicity. The pressurized crew section is typically an aluminum sphere 5 meters in diameter. It is swathed in insulation (fiberglass or alumina fibers are preferred; the Nomads are up to the armpits in waste silicates and oxides from their mining operations) and then a debris shield of ferro-aluminum armor (re-processed scrap from dropship hull production). On the dorsal surface (if one side of the sphere can be termed dorsal) is the "glass wart" or the "glass nipple," a 1.5m radius hemisphere of layered, fused quartz, alumina and a tough polymer. The pilot seat is mostly or entirely within this bulge. Some pilot positions will have extensive analog dials as backup vehicle health monitors (just going with the whole "primitive chemical rocket" aesthetic), while others are satisfied use data piped to the canopy and/or spacesuit helmet HUDs. Controls are built into the seat: control sticks, pedals, etc., leaving a very "clean" cockpit. The seat can usually pivot through several axes to give the pilot a good view for docking, landing, and maneuvers in many directions. The rest of the body is open volume to be configured into passenger seats, cargo space, or whatever as needed. An airlock is typically mounted on the nose, though this may be replaced by large clamshell doors dor releasing large payloads into space (i.e., satellites). Since the cockpit is rarely isolated from the interior, the pilots of purely cargo Buses must wear spacesuits. But since they face ridicule for leaving a large vessel (jumpship or dropship) for not wearing spacesuits, they would wear the suits anyway.
Behind the spherical crew section is the engine module. This is a 6m diameter assembly of tubular aluminum framework encompassing a large, spherical hydrogen tank (4m in diameter). Six spherical oxygen tanks (1.5m diameter each) cluster around the base of the hydrogen tank. The large hydrogen tank is designed to hold two tons of hydrogen while the six small oxygen tanks hold a total of 12 tons of oxygen. (Yes: the smaller tanks hold a lot more than the big one. Liquid oxygen is 16 times denser than liquid hydrogen.) Finally, sticking out of the tail is the exceedingly large nozzle of the engine. It is a vacuum-optimized nozzle that suffers a lot of inefficiency in an atmosphere, but that's not a problem for Nomads. The Bus isn't meant to land on planets and they have plenty of fusion-powered shuttles to handle the odd moon with a thick atmosphere. Nomads tend to swath the engine module in a cylinder more ferro-aluminum, giving the Bus the appearance of a giant pistol cartridge.
Assorted other subsystems dot the engine module and crew section. Inside the debris shield are batteries (with enough stored power to run lifesupport for 1 person for 6 months - this IS a society with batteries fit for personal laser weapons), sometimes hydrogen-oxygen fuel cells or internal combustion APUs, radiators, life support, autopilots, navigation equipment, RCS motors, etc.
The typical Nomad Bus is not a speed demon compared to aerospace fighters, let alone any fusion-powered vessel capable of strategic (heat-expansion) fuel use. With a typical 7-ton fuel load, it has a total of 1480m/s of delta-V (5 thrust points), while its full fuel capacity (14 tons) amounts to 12 thrust points and spare change (3700m/s). However, it is a "pilot's spacecraft." The reaction control system enables translational maneuvering in all directions (up-down, side-to-side, backwards-forwards), in addition to pitch, yaw, and roll. Thrusters are crisp and precise. While autopilots are standard, most Buses also carry manual navigational tools: sextants, mechanical timers, slide rules, etc. Nomad pilots take pride in being able to calculate orbital maneuvers manually.
Not all Buses have landing legs; for purposes of landing, Nomad Buses with landing gear are treated as spheroid dropships. Since most Buses only "land" on small asteroids, the operation is more like zero-G docking than landing.
The ferro-aluminum armor is one high tech wonder on the Bus that opens new vistas for such a "primitive" spacecraft. The material is quite capable of enduring the rigors of atmospheric entry; Nomad pilots LOVE to save fuel with aerobraking. They do, however, tend to letting the autopilot handle such maneuvers. It doesn't take much to turn an aerobraking maneuver into a lithobraking maneuver, and no Nomad pilot can stand the idea of being razzed in his eulogy for having ended it all in "an auger job."
NOMAD BUS
25 tons
0.25 tons 25-rated chemical rocket*
....Thrust: 3**
....Overthrust: 5
....Structural Integrity: 3
0 tons 0 heat sinks
3 tons cockpit
7 tons fuel (tankage for 14 tons)***
3.75 tons FF armor
....Front: 22
....Sides: 15
....Rear: 15
11 tons cargo (4 tons cargo with 14 tons fuel)
*Engine specific impulse: 460
**Note: "ICE" spacecraft need to expend thrust points to re-orient themselves (change facing), but do not use fuel to do so. Of course, they do need to spend fuel to change their heading.
***7 tons of chemical fuel for *this* engine gives 5 thrust points *if* the Bus's starting mass is 25 tons. 14 tons of chemical fuel is good for 12 thrust points, again for *this* engine and *if* the bus starts at 25 tons. A lighter ship will get more delta-V.
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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Toontje
03/09/04 04:56 PM
217.123.31.80
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I propose the use of oxygen+lithium or sodium engines, the hydrogen is used for the fusion engines after all.
What reacts good with oxygen, and results in the formation of a large number of gaseous molecules and has little or no hydrogen attached?!? Carbon is a possibilty. I like the idea btw, if I ever have anything to say about an RPG, I'll include it.
Rather to blow up, then.
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Cray
03/09/04 05:32 PM
68.200.110.103
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Quote:
I propose the use of oxygen+lithium or sodium engines, the hydrogen is used for the fusion engines after all.
A dribble of hydrogen is used by fusion engines - the 2 tons of hydrogen this bus can use in 2.4 minutes (plus 12 tons of oxygen) gives it 3700m/s delta-V, while the same 2 tons of hydrogen (with no oxygen) will propel a 2500000-ton warship for 42850m/s. The needs of the large, fusion-powered engines of Nomad fleets are not significant dents in Nomad hydrogen production, so (like the fluff says) something needs to be done with all the waste hydrogen production.
Also, lithium & sodium have always proved troublesome in rocket engines - hard to store, hard to pump, poor gas behavior, etc.
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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