ITSy as a booster
This week space enthusiasts eagerly wait for Elon Musk's presentation in IAC2017, which should bring refinement of ITS architecture for Mars exploration and settlement. Initial scope of the project was seen by many as a bridge to far. Reality check was hinted by Elon Musk as downsizing ITS to maximum 9m width, which would reduce (if linearly scaled in all dimensions) to 42% of the original weight and volume.
Main obstacle that SpaceX faces is financing development and making reduced scale ITS (nicknamed ITSy) pay for itself. Reduced scale would bring it in the similar weight and height class as Saturn V and SLS. With given mass budget and flexible nose/fairing, it could possibly be used as direct SLS replacement, capable of launching Orion and DSG elements. But it is clear that such capability would cause even stronger coalition of old-space companies fighting against SpaceX for US government funding.
ITSy has to provide rationale for its existence on the commercial market, serving as replacement of F9 and Falcon Heavy. It has to provide gradual development path, where pieces can be introduced and put into production before all elements of the stack are completed, in the similar manner how F9 was developed over the years.
Popular option was development of new upper stage based on 5-6m diameter and Raptor engine. It would basically provide logical evolution of Falcon architecture, but it would not enable radical capacity increase like 9m or 12m vehicle. So it is a sensible step forward, but too small step for someone who wants to go to Mars.
Therefore 9m ITSy is the critical part of the launch infrastructure, since it will need to perform atmospheric entry using on its side, turn and land vertically on Earth. It should also be capable of launching from flat surface on Mars and Moon. For its larger cousin, I argued that ITS SSTO should be the first variant to be developed. With calculated 20t LEO payload, it could launch most LEO payloads instead of F9. But for ITSy, 10t LEO payload might be too small, especially for any higher energy orbit. ITSy could also be used with F9 boosters, in the similar manner as Falcon Heavy. Whole stack would be more similar to Ariane V and SLS than to Saturn V and other vertically staged rockets. There lies the main risk of such approach. SpaceX did not anticipate big issues with parallel staging of Falcon Heavy and ended up with complete redesign of the central stage.
How could SpaceX utilize ITSy for launching F9 and FH payloads? Full ITS+BFR is oversized, and can be commercially used only as SLS replacement (which is big political risk). Space tourism is not proven as sizeable market that can bring a billion or more per year. ITS SSTO is too small and applicable to limited LEO payloads only. So there is a clear need for a solution between ITSy SSTO and ITS/BFR capacity that could be used as immediate F9 and Falcon Heavy replacement.
If we drop the idea of ITSy boosters, we could use ITSy itself as a booster. By using smaller engine nozzles applicable to sea level ignition, ITSy could easily boost itself and the payload from the ground in the same way as SSTO variant would. But ITSy will also have huge cargo bay, several times bigger than current fairings (Falcon 9 fairing has less than 180m3 of usable space). It could be estimated to be at least 500m3 of usable volume. This space can be used to pack expendable second stage rocket (lets say 100mt MMH/NTO stage using space optimized Super Dracos with Isp of 300s). In this scenario ITSy would provide ever increasing delta-V (from initial 3km/s up to 8km/s) from initial missions and gradually test high energy reentry under various conditions WHILE launching commercial payloads. First missions would be used for small LEO satellites, while latter missions could expand to SSO/MEO/GTO/BEO orbits. In this scenario ITSy would gradually replace all F9 launches except Dragon flights. Dragon could evolve to "Dragon 3" variant that would offer similar capability as Dragon 2, but would launch on top of ITSy and have much larger delta-V capability.
In such roadmap, Falcon 9 can be phased out or even sold to third party (for example publicly traded spin-off company specialized for NSS and ISS launches), while SpaceX would focus on ITS. Major advantage of this approach is that it would have clear payback period of the development investment. ITSy would start flying suborbital flights very soon after initial development and could be iteratively inspected and enhanced after each (payed) flight. At the same time, it would have clear path to fully reusable architecture by introducing BFR and vacuum optimized Raptors. In such scenarios ITSy can be flown pretty soon (in three-four years), at the same time frame when F9 competition (Ariane 6, New Glenn, Vulcan) will enter the market. It would also rapidly test Raptor, heat shield, precision landing and structures while earning money.
Using NTO/MMH engined derived from SuperDracos also enables keeping 2nd stages in orbit as long term space tugs. They could be refueled and reused for additional missions in the future (such as distribution of LEO constellation satellites or capturing and de-orbiting larger pieces of space junk). Due to Raptor efficiency and high wet to dry mass ratio of ITSy, the upper stage would not be optimized for high Isp, but for versatility and durability.
Main obstacle that SpaceX faces is financing development and making reduced scale ITS (nicknamed ITSy) pay for itself. Reduced scale would bring it in the similar weight and height class as Saturn V and SLS. With given mass budget and flexible nose/fairing, it could possibly be used as direct SLS replacement, capable of launching Orion and DSG elements. But it is clear that such capability would cause even stronger coalition of old-space companies fighting against SpaceX for US government funding.
ITSy has to provide rationale for its existence on the commercial market, serving as replacement of F9 and Falcon Heavy. It has to provide gradual development path, where pieces can be introduced and put into production before all elements of the stack are completed, in the similar manner how F9 was developed over the years.
Popular option was development of new upper stage based on 5-6m diameter and Raptor engine. It would basically provide logical evolution of Falcon architecture, but it would not enable radical capacity increase like 9m or 12m vehicle. So it is a sensible step forward, but too small step for someone who wants to go to Mars.
Therefore 9m ITSy is the critical part of the launch infrastructure, since it will need to perform atmospheric entry using on its side, turn and land vertically on Earth. It should also be capable of launching from flat surface on Mars and Moon. For its larger cousin, I argued that ITS SSTO should be the first variant to be developed. With calculated 20t LEO payload, it could launch most LEO payloads instead of F9. But for ITSy, 10t LEO payload might be too small, especially for any higher energy orbit. ITSy could also be used with F9 boosters, in the similar manner as Falcon Heavy. Whole stack would be more similar to Ariane V and SLS than to Saturn V and other vertically staged rockets. There lies the main risk of such approach. SpaceX did not anticipate big issues with parallel staging of Falcon Heavy and ended up with complete redesign of the central stage.
How could SpaceX utilize ITSy for launching F9 and FH payloads? Full ITS+BFR is oversized, and can be commercially used only as SLS replacement (which is big political risk). Space tourism is not proven as sizeable market that can bring a billion or more per year. ITS SSTO is too small and applicable to limited LEO payloads only. So there is a clear need for a solution between ITSy SSTO and ITS/BFR capacity that could be used as immediate F9 and Falcon Heavy replacement.
If we drop the idea of ITSy boosters, we could use ITSy itself as a booster. By using smaller engine nozzles applicable to sea level ignition, ITSy could easily boost itself and the payload from the ground in the same way as SSTO variant would. But ITSy will also have huge cargo bay, several times bigger than current fairings (Falcon 9 fairing has less than 180m3 of usable space). It could be estimated to be at least 500m3 of usable volume. This space can be used to pack expendable second stage rocket (lets say 100mt MMH/NTO stage using space optimized Super Dracos with Isp of 300s). In this scenario ITSy would provide ever increasing delta-V (from initial 3km/s up to 8km/s) from initial missions and gradually test high energy reentry under various conditions WHILE launching commercial payloads. First missions would be used for small LEO satellites, while latter missions could expand to SSO/MEO/GTO/BEO orbits. In this scenario ITSy would gradually replace all F9 launches except Dragon flights. Dragon could evolve to "Dragon 3" variant that would offer similar capability as Dragon 2, but would launch on top of ITSy and have much larger delta-V capability.
In such roadmap, Falcon 9 can be phased out or even sold to third party (for example publicly traded spin-off company specialized for NSS and ISS launches), while SpaceX would focus on ITS. Major advantage of this approach is that it would have clear payback period of the development investment. ITSy would start flying suborbital flights very soon after initial development and could be iteratively inspected and enhanced after each (payed) flight. At the same time, it would have clear path to fully reusable architecture by introducing BFR and vacuum optimized Raptors. In such scenarios ITSy can be flown pretty soon (in three-four years), at the same time frame when F9 competition (Ariane 6, New Glenn, Vulcan) will enter the market. It would also rapidly test Raptor, heat shield, precision landing and structures while earning money.
Using NTO/MMH engined derived from SuperDracos also enables keeping 2nd stages in orbit as long term space tugs. They could be refueled and reused for additional missions in the future (such as distribution of LEO constellation satellites or capturing and de-orbiting larger pieces of space junk). Due to Raptor efficiency and high wet to dry mass ratio of ITSy, the upper stage would not be optimized for high Isp, but for versatility and durability.
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