Future Falcon Heavy upgrades
With each iteration of Falcon upgrades, SpaceX has consistently expanded its envelope to the point that their existing offering is at the same level or better than any other launcher in the world. There are borderline cases (such as high C3 missions), where other launchers could have better capability (such as Delta IV Heavy). Would SpaceX stop there to focus on BFR/BFS in the future? Maybe, but not likely.
Main deficiency of the current design is low ISP in the 2nd stage, so logical approach taken was a design of high energy upper stage using scaled down Raptor engine and wider five meter tank. But engine development is expensive. Using cutting edge fuel cycle makes it even more expensive. Ground segment changes driven by wider stage and new fuel are also expensive. Are there other options that could utilize existing pieces?
One way to move forward is to implement kerolox staged combustion engine, similar to RD-180. Unfortunately, such engines typically need to operate oxidizer rich and at much higher pressures and temperatures. For comparison, chamber pressure in RD-180 is 2.5 times higher than in Merlin 1D (note that the difference might be lower for full thrust versions). Result is that current RD-180 has much lower T/W than current Merlin engine.
In any case, improvements regarding ISP are limited for the current M1Dvac. Also any change in stage dimensions (height, width) will cause major changes of tooling and ground segment infrastructure. Increasing height is not really an option due to already extreme fineness ratio of the existing vehicle. Increasing diameter would break commonality with the 1st stage production and break capability of cheap road transport.
So we can conclude that F9 is currently maxed out with future growth possibilities. What about Falcon Heavy? It has such a huge capacity to LEO that nears SLS. And latest T/W figures for Merlin 1D give it a huge margin during takeoff. Thus it enables much heavier 2nd stage that currently possible.
Now, I know rockets are not legos. But, often rocket designs do follow the Lego principle. Delta IV Heavy. Falcon Heavy. Angara Family. Ariane 6. Antares. All these rockets follow reuse of existing stages and combining them in a new way. Even SLS can be considered just a rehash of the existing designs and engines.
What if we add two additional second stages as 2nd stage boosters, essentially using the same "triplet" configuration as the first state? External stage dimensions stay the unchanged. Road transportation capability remains the unchanged. Total rocket height - unchanged. Manufacturing process - unchanged. Assembly buildings - unchanged. Engines - unchanged. Rocket assembly - unchanged. T/E - almost unchanged. Propellant lines? unchanged. Total propellant tank capacity? unchanged. Total stack mass? Roughly 200mT larger (about 15% more). First stage core + booster reusability? The same. Supersonic profile? Probably similar. And performance? Much better, because we have effectively turned three stages into four stages.
But what it means for payload to orbit? Well, here it gets interesting. Since effectively 2nd stage boosters would effectively turn three stage LV into four stages. It comes close to 70mT for LEO. SLS territory. And to GTO? Almost 30mT. Better than Delta IV Heavy. And all that comes without major development needed, just by reusing existing components. Plus it adds capability of designing an enhanced 5x10x13 m fairing with much larger volume, ideal for launching larger structures, such as antennae or telescope mirrors.
Such upgrade would also provide much higher margins for the upcoming Red Dragon missions. For example, with additional fuel tanks for Draco/SuperDraco propulsion, Red Dragon could first enter eliptical orbit around Mars without aerobraking, deploy additional satellites from the trunk and then start EDL phase of the mission. And all this would be achieved with minimal development and investment effort.
Main deficiency of the current design is low ISP in the 2nd stage, so logical approach taken was a design of high energy upper stage using scaled down Raptor engine and wider five meter tank. But engine development is expensive. Using cutting edge fuel cycle makes it even more expensive. Ground segment changes driven by wider stage and new fuel are also expensive. Are there other options that could utilize existing pieces?
One way to move forward is to implement kerolox staged combustion engine, similar to RD-180. Unfortunately, such engines typically need to operate oxidizer rich and at much higher pressures and temperatures. For comparison, chamber pressure in RD-180 is 2.5 times higher than in Merlin 1D (note that the difference might be lower for full thrust versions). Result is that current RD-180 has much lower T/W than current Merlin engine.
In any case, improvements regarding ISP are limited for the current M1Dvac. Also any change in stage dimensions (height, width) will cause major changes of tooling and ground segment infrastructure. Increasing height is not really an option due to already extreme fineness ratio of the existing vehicle. Increasing diameter would break commonality with the 1st stage production and break capability of cheap road transport.
So we can conclude that F9 is currently maxed out with future growth possibilities. What about Falcon Heavy? It has such a huge capacity to LEO that nears SLS. And latest T/W figures for Merlin 1D give it a huge margin during takeoff. Thus it enables much heavier 2nd stage that currently possible.
Now, I know rockets are not legos. But, often rocket designs do follow the Lego principle. Delta IV Heavy. Falcon Heavy. Angara Family. Ariane 6. Antares. All these rockets follow reuse of existing stages and combining them in a new way. Even SLS can be considered just a rehash of the existing designs and engines.
What if we add two additional second stages as 2nd stage boosters, essentially using the same "triplet" configuration as the first state? External stage dimensions stay the unchanged. Road transportation capability remains the unchanged. Total rocket height - unchanged. Manufacturing process - unchanged. Assembly buildings - unchanged. Engines - unchanged. Rocket assembly - unchanged. T/E - almost unchanged. Propellant lines? unchanged. Total propellant tank capacity? unchanged. Total stack mass? Roughly 200mT larger (about 15% more). First stage core + booster reusability? The same. Supersonic profile? Probably similar. And performance? Much better, because we have effectively turned three stages into four stages.
But what it means for payload to orbit? Well, here it gets interesting. Since effectively 2nd stage boosters would effectively turn three stage LV into four stages. It comes close to 70mT for LEO. SLS territory. And to GTO? Almost 30mT. Better than Delta IV Heavy. And all that comes without major development needed, just by reusing existing components. Plus it adds capability of designing an enhanced 5x10x13 m fairing with much larger volume, ideal for launching larger structures, such as antennae or telescope mirrors.
Such upgrade would also provide much higher margins for the upcoming Red Dragon missions. For example, with additional fuel tanks for Draco/SuperDraco propulsion, Red Dragon could first enter eliptical orbit around Mars without aerobraking, deploy additional satellites from the trunk and then start EDL phase of the mission. And all this would be achieved with minimal development and investment effort.
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