To Be or Not to Be
Anyone with an interest in SF spaceships has probably come across the website of a man going by the moniker of BTE Dan; the website is Build The Enterprise, in which he sets out a proposal of how the American government might, if it so desired, build a real life version of Star Trek's famed Enterprise - minus the warp drive - the feasibility of which is hotly contested. My personal opinion is that it is not feasible: technical considerations only, politics and economics are beyond my pay grade. This blogpost will look at several elements of the BTE design, pointing out any possible flaws, and ways to remedy them, providing a case study in the design of a hard SF spaceship.
Along with budget the biggest factor affecting the design of a spacecraft is the mission it will fill. Military and civilian technologies follow different approaches to the same problems, and within the civil field there are a multitude of roles with different technical requirements. As we already have a design - the Enterprise - it remains to find a mission by matching it to the Enterprise's characteristics as set out on the BTE website.
- large personell capacity - 1000 people for a few years
- fast travel - 90 days to Mars
- spin ring - simulated gravity in living quarters so that long term missions are not medically hazardous.
- safety - tripple redundancy in major systems
these give the Enterprise a clear role as a transport for humans throughout the solar system. The spin ring and enclosed cargo areas make it unsuitable for a dedicated cargo ship, but as research and support facility it is right on target. It is unlikely to be used as an exploratory vessel; the reason being it is simply too capable, that kind of overkill is reserved for unlimited budgets. A small proof of concept ship might well take the first humans into Mars orbit, however.
On the we site is is stated that one of the goals of the design is to retain the recognisable exterior of the Original Enterprise. Connected with this are the two biggest problems of the design, the size and the shape. A thousand person capacity is vast - the ISS has only - representing significant overkill for any possible mission. Realistically a hundred crew would be the max, and for any spacecraft built in the near future 10-20 is far more likely. This also reduces the tech requirements, helping to make it more plausible. Shape; it has often been remarked that the Enterprise has pretty much the worst design ever from an engineering standpoint. Those thin trusses connecting the Main to Auxiliary Hulls are a nightmare. To give adequate margin of safety thy would be ugly expensive and grossly overweight for the job they do. However, while the design can be scaled down, it cannot be changed it the goals of BTE Dan are to be met, making this the single most likely factor to making it unfeasible.
|Yes, I know, this is a HALL thruster, |
Engines & Power
One thing that BTE Dan does get very right is the choice of power and propulsion systems, i.e. a nuclear-electric drive. For fast interplanetary travel this is pretty much the only choice with current technology. There is confusion over the naming of the drive with the selected VASIMR being called a 'ion drive', which it is not, but the choice is sound. VASIMR is a plasma drive using magnetic containment and radio frequency coils to energise the plasma; from a design standpoint it is similar to a fusion drive with energy provided by external power instead of internal fusion. Among the advantages are proven design, hydrogen remass, long life and reliability, good electrical efficiency, high Isp, and the ability to trade Isp for thrust. Although BTE Dan decided on three drive units, it has been proposed that the VASIMR drives be installed in pairs, helping to cancel out their magnetic fields external to the drive itself.
For the BTE design liquid hydrogen-oxygen engines are proposed for manoeuvres that require high thrust. This seems like a good idea apart from the difficulty of storing LH2 for long periods of time, but given the amount of electric power available large electro-thermal rockets might be better. Although giving slightly less thrust these have a higher Isp if using LH2 remass, and are very simple mechanically, where liquid-fueld engines are notoriously complexed.
Although ideally provided by a fusion device the power is most likely to be fission-thermal. Even if a project like the ITER tokamak succeeds, it would need years before being perfected to the point where it is space-worthy. With the greater experience in fission reactors these are a better be in the forceable future despite concerned about radiation. Advanced concepts could be used, however, like MHD generators, sub-critical fission, vapour of liquid cores, etc. One thing that the BTE design seems to gloss over is that of wast heat. The Enterprise BTE Dan proposes needs 2.5 GW of electric power. Assuming a fission to electricity efficiency of 40%(modern water cooled power reactors are around 30%) that is still 6.25 Gigawatts of was heat to dissipate. BTE solution is to use the aluminium outer hull as a radiator, but it seems unlikely that this is an adequate solution. Nor does the shape of the 'classic Enterprise' allow for external radiators without changing vastly the visual appearance. Obviously a smaller version of the Enterprise with lower power needs becomes more realistic.
Alternative, the Kelvin
Given the flaws of waste heat, overall structural design, and the ridiculously large size proposed the BTE Enterprise seems to be a dead end for a spacecraft that could someday be built. But are any of the Star Trek ships viable? I propose the Kelvin, seen in the beginning of the 2009 Star Trek movie under the command of Kirk's father. Over the next few weeks I will be designing a spacecraft with the aesthetic of the Kelvin, capable of supporting a crew of 10-15, and able to make a rapid transit to Mars, or a destination in the Asteroid Belt. I will include calculations, diagrams, specs, and possibly a 3D Blender model, so it will not be finished soon, but keep a lookout for a Build the Kelvin post.