to the relatively low re-entry speed anticipated, it was believed that complex heat-resistant tiling could be dispensed with in favour of simpler and cheaper nickel-alloy panelling across the vehicle's underside. Mustard was to be crewed by between three and six astronauts. Operationally, there were two primary vehicle configurations, the orbiter and booster stages, respectively. The orbiter vehicle, which carried the desired payload, featured ducting to receive fuel from the boosters, while the booster units incorporated systems for transferring fuel across to the orbiter vehicle or between one another. In this fashion, the orbiter could remain fully topped-up for its long orbital injection flight, while all the vehicles could still share a standardised fuel tank design. According to Smith, the orbiting vehicle would have been capable of performing between 30 and 50 launches before needing to be replaced, while the booster engines, which wouldn't have been subject to as much heat and stress, would have remained usable for up to 200 times. Clustering and stacking Various clustering and stacking arrangements were explored. Where the Astro would have launched as a two-stage step-rocket, for which the booster would have been much larger than the orbiter, Mustard comprised from three to five near-identically sized modules. Early studies focused on a vehicle with a shallow 120° "vee" underside to both body and wings so that three could be clustered in a triangle. Some included a fourth, orbital vehicle mounted on top of three boosters. The most efficient regime was to empty one booster at a time, keeping the others topped up for as long as possible, so that the first-stage booster could be dropped as soon as possible. The three boosters would be emptied in turn. But this led to an asymmetric mass loading which BAC believed to be a significant problem, so later designs used a sideways stacking system in which flatter modules were stacked more like sheets of paper. At , at around 30 nautical miles, the last of the booster units would separate; once clear, these units would glide downwards and land upon a runway similar to conventional aircraft, allowing for their reuse. The spacecraft would place its payload into orbit at around 1,000 nautical miles, which would be achieved roughly 10 minutes after launch, and then return to Earth via a controlled gliding descent before conducting a landing in a similar fashion to the booster units. Originally, it was envisioned