One curious omission in the speech concerned SpaceX’s prior interest in an all-in-one island facility, which had been assumed in the run-up to the speech.
This plan called for the ITS to be built in a factory within the island facility. Once a rocket was built it would be sent to a nearby test stand, where it would have been fired. After testing finished it would be moved a short distance to the launch pad, and then launched and landed there. Nearby hangars would allow SpaceX personnel to maintain and fix the vehicles as needed.
Such a site would have allowed SpaceX to build, test, launch and land the ITS away from population centers and maintain a rapid launch rate without drawing as much complaint from locals.
Experts at NSF and elsewhere raised questions about the sheer size of the vehicle as proposed at the time, ranging from the event of an anomaly during launch – which may have required a much larger Blast Danger Area (BDA), through to the issue that the design produced so much thrust that it could not be handled by either the 39A or 39B launch pads at Cape Canaveral.
Built in the early 1960s for the Apollo program, the pads were designed to handle up to 53,379 kN (12,000,000 lb) of thrust. While they could be upgraded at a substantial cost in time and money, this would also lose SpaceX, NASA and others all use of the pads during the construction work.
BFR launching from 39A – notional – from SpaceX
Another concern with the vehicle’s thrust was the sheer acoustic vibration would complicate its engineering and draw more opposition to launches. In addition, the core stage was still large enough that had it had an accident while attempting to land on the launch mounts, an explosion large enough to substantially damage the pad would result.
Other concerns centered around the ITS Spaceship. The primary concern was its complete lack of a Launch Abort System (LAS), a feature NASA was notably required for SpaceX’s Dragon capsule. Should there have been an explosive failure, it was not clear that the crew could be gotten to safety. With up to 100 people on board, the potential for loss of life multiple times worse than any Shuttle disaster was dangerously high.
The use of three landing legs entailed lessened stability versus four or more leg designs and unlike five or six-leg designs, had no redundancy if a leg failed. The speech also did not heavily cover the design’s radiation protection. Given the radiation levels astronauts and colonists would encounter on the way to Mars, the relative dearth of details on the Spaceship’s radiation protection was surprising.
The most pressing issue with the design was the lack of details on how to make money with it apart from Mars. Development would likely take years and cost in the order of 10 billion dollars, but its uneconomical size and lack of satellite launch capability drastically hurt its business case. The issue was even referenced in the speech by Musk.
Following the speech NSF learned in February 2017 that the design was evolving in a smaller direction. One design of the ITS under consideration featured a diameter of 10 meters (32.81 ft) and as few as 16 Raptor engines on its booster. Within months, however, SpaceX decided to go with an even smaller 9 meter (29.53 ft) diameter design in order to avoid building an all-new factory. This would ease production at a cost of more complicated and expensive logistics for stage transportation and testing.
In the run-up to the next speech in September 2017, Raptor engine testing continued at a vigorous pace. The sub-scale test engine possessed a chamber pressure of 200 Bar and a thrust of 1 MN (224,808 lbf). According to Musk, it also used a new alloy to help its oxygen-rich turbopump resist oxidization. By the time of Musk’s September International Astronautical Congress speech in Adelaide, Australia, the sub-scale prototype had completed 1200 seconds of firings across 42 tests.
Musk’s IAC 2017 speech revealed the Raptor engine’s designed sea level thrust had been shrunk by 44.3% from 3,050 kN (685,700 lbf) to 1,700 kN (382,200 lbf). Its initial version would have a lower chamber pressure of 250 Bar, which was 50 Bar, or 16.7% less than the final planned version. With a nozzle diameter of 1.3 meters (4 ft 3 in) this allowed it to develop an Isp of 330 seconds at sea level. The vacuum version was to have 1,900 kN (427,100 lbf) of thrust produced from a 2.4 m (7 ft 10.5 in) nozzle and had an Isp of 375 seconds.
Only 31 Raptor engines would be attached to the updated BFR’s 9 m diameter, 58 m long booster stage, which would allow it to produce a still impressive 52.7 MN (11,847,400 lbf) of thrust at sea level.
BFR flow at 39A as envisioned by Jdeshetler for NSF/L2
This was a 58.9% drop compared with the prior design and coincidentally was just low enough for SpaceX’s 39A launch pad to handle with minimal modifications.
The rocket would still produce 55.7% more thrust than the Saturn V moon rocket, the current world record holder for most capable and successful carrier rocket. Other benefits of the size change included a much smaller exclusion zone, lower development costs and a lower risk of engine failure during launch.
Attached to the booster stage could be any of three different upper stages, all sharing the same outer mold line and powered by a mixture of four Raptor vacuum engines and two Raptor booster engines. The six engines would produce 11,268 kN (2,533,100 lbf) of thrust after stage separation.
BFS cross section – via SpaceX
Once the stage’s thrust to weight ratio had improved, the landing engines would shut down. Each stage would boast a modest delta wing and four landing legs, allowing the stage to de-orbit and land vertically.