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Hull temperature: 318 Centigrade | 400 comments | Create New Account
Comments belong to whoever posts them. Please notify us of inappropriate comments.
Hull temperature: 318 Centigrade
Authored by: ailuromancy on Sunday, May 13 2012 @ 06:12 AM EDT

Things cool down because of conduction, convection and radiation. Space ships are not in contact with anything, so conduction does not work. There is no atmosphere or ocean outside a space ship, so convection is not available. The only option is radiation. Radiation is proportional to surface area. From the picture, the area is:

  • 250m radius disk (both sides): 2xπx250² = 400000m²
  • 100m diameter cylinder 500m long: 500xπx100 = 160000m²
  • two 50m diameter cylinders 500m long: 2x500xπx50 = 160000m²
  • total: about 720000m²

Now lets take a look at how much heat is generated: There are one 1.5GWe and two 0.5GWe nuclear power plants for a total of 2.5GW of electricity. To get that 2.5GW of electricity, the nuclear power plants create 5GW of waste heat. Lets pretend all the electricity is used for propulsion and that all the waste heat from propulsion goes into heating the propellent. (Apart from the 100MW laser, all the other proposed uses for the electricity end up heating the space ship.)

The magic formula we need is: T = (P/σA)¼

  • T: temperature in Kelvin.
  • P: power in Watts.
  • σ: Stefan–Boltzmann constant = 5.67x10-8Wm-2K-4
  • A: area in m²

Plug the numbers in, and the hull temperature is: 591K. Subtract 273 to get 318°C, or 605°F. Oops, we fried the crew. Try that again without using the saucer section for cooling: 451°C. Still not good. That rather generous 33% efficiency for nuclear thermal to electricity conversion assumed a massive source of cold water (big power stations are coastal for a reason). Without the cold source, there will be much more than 5GW of waste heat when generating 2.5GW of electricity.

Quick! Turn on the air-con. Part of electricity generation is involves letting heat flow from a hot place to a cold place. Air conditioning is the reverse process - using electricity to pump heat from a cold place to a hot place. If air conditioning and power generation are as efficient as possible, they cancel each other out. In the real world, the electricity used by the air conditioning is more than the extra generating capacity provided by the lower temperature.

I would love a return trip to Mars, but not on this Enterprise.

[ Reply to This | Parent | # ]

We can build the first generation of the USS Enterprise – so let’s do it !
Authored by: Anonymous on Sunday, May 13 2012 @ 01:32 PM EDT
One major flaw in the described plans is that it uses both British Imperial
units and Metric units.

The second flaw is that it doesn't consider why a permanent station at Earth
parking orbit is required, for this project to have a hope of succeeding.

[ Reply to This | Parent | # ]

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