Thanksgiving update, recent ideas

[tevarin]

We're back from Connecticut, after a seven hour car ride featuring stop-and-go traffic and vomiting. Avoid the Nathan's hotdog place at the Molly Pitcher rest stop on the New Jersey Turnpike. Although I'm not entirely sure they're to blame, I wouldn't take the risk

Comments

[tevarin]

Newborn babies plus life support might be pushing the weight limits. How about embryos? They're even freezable for long duration journeys. The problem of recruiting and training embryos for astronaut service is left as an exercise for the student.

I agree, the mass of the cable will have to be taken into account, and it should help add to the total weight of the descending mass.

Hmm. Very roughly, if we assume a cable made from commercial Dyneema or similar material, with a density of 1 and a tensile strength of 3.1 gigapascals = 3.1 billion newtons/sq. meter, a cable with an area of 1 cm^2 will hold 31,000 kg. A cable with an area of 0.1 cm^2 will hold 3,100 kg, which gives us a safety factor of 3, based on a 1000kg (1 ton) weight.

A cable with an area of 0.1 cm^2 and a length of 10 km will have a total volume of 10^-5 x 10^4 = 0.1 cubic meters. Like a coil of eighth-inch rope the size of a semi-truck tire. It will weigh ~100kg, ten percent of the total weight to be dropped. Not too bad, even if we want to double the safety factor, it'd still be only 20%.

So we assume a drop mass of 1000kg, a cable mass of 200kg. If the spool mass is another 200kg, and assorted mounting spars, electronics, etc. are another 200 kg, then the total mass for the balloon to lift is around 1600 kg.

At sea level, air weighs about 1.2 kg per cubic meter. If the lifting balloon were a perfect vacuum, it would have to be ~1300 cubic meters in size, about 11m x 11m x 11m. At 30 km altitude, air density is about 1/100 sea level. So the balloon has to have a hundred times that volume, and would be 130,000 cubic meters.

But thin hydrogen or helium loses ~10% lift compared to a perfect vacuum, and the outer envelope weight cuts lifting efficiency as well. So maybe 200,000 cubic meters is a more realistic requirement.

Late WWIZeppelins had a volume of ~60,000 cubic meters, and carried a payload of 40-50 tons at low altitudes. Our airship would be roughly as big as three or four zeppelins lashed together.

Note that we might be able to save airship size by launching from a lower altitude. Starting high gives us minimal gravitational benefit, but seriously reduces parasitic losses due to air drag on the spinning spool and on the payload rocket after launch.