BFR upper stage as a booster
This is a follow-up post related to the previous speculation of using ITSy (BFR upper stage) as a booster. After the presented numbers during Elon Musk's speech on IAC2017, I can provide a refined estimate.
While new design is more in line with super-heavy lift vehicles of SLS, Saturn V and New Glenn, it is intended to be fully reusable architecture from the start. With full re-usability and 150mt LEO capacity and huge cargo space, it obviously targets SLS as a direct replacement. Moon is (finally) represented as a worthwhile destination of equal value as Mars. It obviously targets any cis-lunar infrastructure such as Deep Space Gateway and Moon Village concepts.
But the most important part of the presentation - how to fund and pay for ITS was not really explained well. While Elon Musk made it obvious that BFR is intended to completely replace Falcon 9 and Falcon Heavy and Dragon 2, it did not explain how do they plan to launch satellites to different orbits.
Any BFR mission should not be priced more than existing Falcon based mission, which really puts an upper price range between 50 million USD (for LEO/SSO/GTO satellite missions) and 140 million USD (for BEO/GEO/ISS missions).
Upper stage is on the critical path for the development and it will need to perform a full envelope testing of the re-entry and precision landing. Doing it purely as a Grasshopper style, will require a lot of time and financial resources.
If SpaceX would replace four vacuum Raptors with sea-level engines, it would have a lift-off force of 1040t. That is more than the mass of fully fueled 2nd stage without any cargo (which is projected to 1185t). But with tanks not full, it could provide up to 860t of fuel and delta-V of roughly 7.9km/s. The payload should include additional kick stage motor to provide additional 2km/s to reach LEO or 6km/s to reach GEO.
Here the issue is vast volume of the BFR cargo space of 825 m3 which is not used for much. It seems strange to even consider putting even the largest geostationary satellites into such a large space, that is five times larger then current fairings on EELV class vehicles.
But once vacuum Raptor engines are replaced with their sea level equivalents, up to 4 additional Raptors can fit inside the the same engine area. Which brings lift-off thrust up to 17MN, so the total stack can weight up to 1600mT. Enough for full propellant load of 1100mT, and 400mT of payload delivered to 3.4 km/s delta-V. That is not much, but that 400mt could easily fit four Falcon 9 upper stages. Or 17 Atlas V second stages (Centaurs). Or three iCPS (SLS upper stages). Ok, maybe length would be an issue there.
But why bother with packing a second stage inside the cargo space? Why not dedicate most of this space to additional propellant tanks that will transfer the propellant in-flight into the main tanks? If we add 400mT of additional propellant and 15mT of additional mass for tanks and engines, the same stack can reach LEO with effective delta-V of 10km/s. Fully reusable SSTO with couple of tonnes capacity. More than Vega, or even Soyuz. Maybe even Atlas V in smallest (401) configuration. And it will still have more available space in cargo bay that the current Falcon 9 fairing.
That fairing volume is significant problem for any large constellation deployment, since satellites (especially for LEO) will usually be quite constrained with available volume. So larger volume can mean larger solar arrays. And larger solar arrays mean either more power or use of cheaper, but less efficient silicon based cells.
And if upper stage can perform as SSTO, it can also do a suborbital flights between the cities, just like the whole BFR. Of course, with at least 15 times less capacity. But there is no staging. And there is no mating of upper and lower stage. Which leads to rapid turnaround. There might not be a market for hundreds of passengers wanting to pay tens of thousands of dollars for 30-minute travel between Shanghai and New York. But what about tens of passengers per day? Maybe doing a hop from Hawthorne to Adelaide and back in the same day? Private jet costs 200.000USD per day. But it takes at least 24 hours sitting and living in an airplane to do a round trip. Paying ten times as much for one hour flight? Sounds good if you are wealthy. Because time is a scarce resource for anybody.
Once the second stage is operational and has flown a full envelope to LEO and back multiple times (using it for low cost commercial launches, possibly in the cost range of the small launch vehicles such as Rocket Lab's Electron), it can be extended with BFR first stages of varying length, starting from the half-size and extended to full size booster. That way SpaceX could justify varying cost structure (SSTO, half-size, full-size BFR) and get flight heritage and happy paying customers very early in the development cycle.
So BFR upper stage is, by itself, very capable vehicle that can replace existing Falcon fleet and open new revenue streams. Full BFR is really needed just for large and BEO payloads. Therefore SpaceX could utilize upper stage as a booster and/or SSTO for real, commercial missions while doing testing of critical components of the system in space. And that capability is enabled simply by its size that given large margins for any current space applications.
While new design is more in line with super-heavy lift vehicles of SLS, Saturn V and New Glenn, it is intended to be fully reusable architecture from the start. With full re-usability and 150mt LEO capacity and huge cargo space, it obviously targets SLS as a direct replacement. Moon is (finally) represented as a worthwhile destination of equal value as Mars. It obviously targets any cis-lunar infrastructure such as Deep Space Gateway and Moon Village concepts.
But the most important part of the presentation - how to fund and pay for ITS was not really explained well. While Elon Musk made it obvious that BFR is intended to completely replace Falcon 9 and Falcon Heavy and Dragon 2, it did not explain how do they plan to launch satellites to different orbits.
Any BFR mission should not be priced more than existing Falcon based mission, which really puts an upper price range between 50 million USD (for LEO/SSO/GTO satellite missions) and 140 million USD (for BEO/GEO/ISS missions).
Upper stage is on the critical path for the development and it will need to perform a full envelope testing of the re-entry and precision landing. Doing it purely as a Grasshopper style, will require a lot of time and financial resources.
If SpaceX would replace four vacuum Raptors with sea-level engines, it would have a lift-off force of 1040t. That is more than the mass of fully fueled 2nd stage without any cargo (which is projected to 1185t). But with tanks not full, it could provide up to 860t of fuel and delta-V of roughly 7.9km/s. The payload should include additional kick stage motor to provide additional 2km/s to reach LEO or 6km/s to reach GEO.
Here the issue is vast volume of the BFR cargo space of 825 m3 which is not used for much. It seems strange to even consider putting even the largest geostationary satellites into such a large space, that is five times larger then current fairings on EELV class vehicles.
But once vacuum Raptor engines are replaced with their sea level equivalents, up to 4 additional Raptors can fit inside the the same engine area. Which brings lift-off thrust up to 17MN, so the total stack can weight up to 1600mT. Enough for full propellant load of 1100mT, and 400mT of payload delivered to 3.4 km/s delta-V. That is not much, but that 400mt could easily fit four Falcon 9 upper stages. Or 17 Atlas V second stages (Centaurs). Or three iCPS (SLS upper stages). Ok, maybe length would be an issue there.
But why bother with packing a second stage inside the cargo space? Why not dedicate most of this space to additional propellant tanks that will transfer the propellant in-flight into the main tanks? If we add 400mT of additional propellant and 15mT of additional mass for tanks and engines, the same stack can reach LEO with effective delta-V of 10km/s. Fully reusable SSTO with couple of tonnes capacity. More than Vega, or even Soyuz. Maybe even Atlas V in smallest (401) configuration. And it will still have more available space in cargo bay that the current Falcon 9 fairing.
That fairing volume is significant problem for any large constellation deployment, since satellites (especially for LEO) will usually be quite constrained with available volume. So larger volume can mean larger solar arrays. And larger solar arrays mean either more power or use of cheaper, but less efficient silicon based cells.
And if upper stage can perform as SSTO, it can also do a suborbital flights between the cities, just like the whole BFR. Of course, with at least 15 times less capacity. But there is no staging. And there is no mating of upper and lower stage. Which leads to rapid turnaround. There might not be a market for hundreds of passengers wanting to pay tens of thousands of dollars for 30-minute travel between Shanghai and New York. But what about tens of passengers per day? Maybe doing a hop from Hawthorne to Adelaide and back in the same day? Private jet costs 200.000USD per day. But it takes at least 24 hours sitting and living in an airplane to do a round trip. Paying ten times as much for one hour flight? Sounds good if you are wealthy. Because time is a scarce resource for anybody.
Once the second stage is operational and has flown a full envelope to LEO and back multiple times (using it for low cost commercial launches, possibly in the cost range of the small launch vehicles such as Rocket Lab's Electron), it can be extended with BFR first stages of varying length, starting from the half-size and extended to full size booster. That way SpaceX could justify varying cost structure (SSTO, half-size, full-size BFR) and get flight heritage and happy paying customers very early in the development cycle.
So BFR upper stage is, by itself, very capable vehicle that can replace existing Falcon fleet and open new revenue streams. Full BFR is really needed just for large and BEO payloads. Therefore SpaceX could utilize upper stage as a booster and/or SSTO for real, commercial missions while doing testing of critical components of the system in space. And that capability is enabled simply by its size that given large margins for any current space applications.
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