Boeing’s Low cost Heavy Lift Vehicle Concept VTVL SSTO RLV was part of the company’s NASA-funded Solar Powersat Studies in 1976-77. Known as “The Big Onion” it utilized twenty-four 4.5MN oxygen-hydrogen main engines plus twenty-four 2.25MN lox-kerosene booster engines to generate a thrust-to-weight ratio at liftoff of 1.3. with 227 tonne payload capacity Leo’s base heat shield would be water-cooled and the engine nozzles were to be protected by steam ejection.