OTTA and HTTA dewars Following Apollo, in 1969 and 1971, under NASA contract, Beech built two ground test dewars to demonstrate techniques for very efficient storage of cryogens. They remain among the most thermally efficient dewars ever built. The oxygen thermal test article (OTTA) was 7..0 ft. diameter near-sphere with a 6,456 liter tank volume. It had fiberglass tank supports, a total of 46 silverized Mylar and silk layers in the MLI blankets and two vapor cooled shields. It was tested with liquid nitrogen and liquid hydrogen and demonstrated boil-off rate of 0.022%/day and 0.056% per day
respectively.

The hydrogen thermal test article (HTTA) was 21.8 ft long x 9.2 ft. diameter. It used all known technologies for maximizing thermal performance in a flight like dewar including fiberglass strap supports, 68 layer, aluminized Mylar MLI blankets and two vapor cooled shields. It was made from spun and welded 2219 aluminum. It was tested with liquid hydrogen and demonstrated a boil-off rate of 0.022%/day or 7.72%/year

http://iopscience.iop.org/article/10...1/1/012086/pdf

https://ntrs.nasa.gov/archive/nasa/c...0090037584.pdf

Structure fractions of 10.5% (propellant fraction of 89.5%) are achieved with this 50 year old technology. A vehicle the same shape and size as the External Tank of the Space Shuttle, with its inter-tank section removed, using the best available 1960s era technology, carry 815 metric tons of propellant and 91.6 metric tons of structure.

With an average exhaust speed of 4.3 km/sec during ascent, a SSTO system can put 13.1 metric tons into LEO.

Two elements operating in parallel puts 117 metric tons of payload.