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Please forgive me, I'm a stargazer not an astronomer, nor a rocket scientist.

If one could get a complete, fully fueled Falcon Heavy assembly into space, say for example by sandwiching one between two others KSP style, would the high payload and longer burn time be useful for missions beyond Jupiter?

Would the cost and complexity be worth it?

Examples of high payload mass missions where this might be helpful:

  • A large space telescope in orbit around Neptune, where terrestrial signals would be almost non-existent and the risk of debris collision would be minimal.

  • A mission designed to leave the solar system in a relatively short time span, a matter of years instead of decades.

  • A multi pack type of mission, designed to deliver multiple vehicles to the gas giants in one shot.

  • A large, multiuse unmanned "lab" to Jovian orbit designed to assist with sample and return missions or long term in-depth studies.

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    $\begingroup$ The majority of a Falcon Heavy is designed to work in an atmosphere so it would be very inefficient as a deep space vehicle. If we had the ability to send that much mass into orbit, we would send something much better than a Falcon Heavy $\endgroup$
    – Jack
    Jul 29, 2018 at 5:51
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    $\begingroup$ Rocket equation is very harsh thing, unfortunately en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation. For chemical rockets you can use a rule of thumb - to get extra 2km/s we need to DOUBLE the rocket's mass. If we need 2 km/s more - double again, etc. So, increasing the mass of rocket is wery expencive way to move faster. $\endgroup$
    – Heopps
    Jul 29, 2018 at 7:10
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    $\begingroup$ @Heopps oic: $300(s) \times 9.81 (m/s^2) \times \log(2) \approx 2000 (m/s)$ sweet! $\endgroup$
    – uhoh
    Jul 29, 2018 at 7:59
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    $\begingroup$ Neptune is a bad place for a large space telescope, it is too far away. Data transmission is very slow due to the huge distance. Payload is very limited. The use of solar panels for electrical power is very difficult due to the very low intensity of solar light at Neptune. $\endgroup$
    – Uwe
    Jul 29, 2018 at 15:39
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    $\begingroup$ @Coyote you might underestimate power consumption and data transmission rate in deep space. I don't know any numbers from the top of my head, but as soon as your data transmission rate is any lower than what you want to send, you have a Problem. You can encounter this with more powerful transmission hardware, which requires more power. Largescale Radioisotope Generators are a problem as well as the fuel for it is... more than limited. $\endgroup$
    – DaGroove
    Jul 30, 2018 at 10:09

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A Falcon Heavy on the launch pad masses about 1400 tons. Its payload to low orbit is about 64 tons (fully expendable) (both figures from wikipedia). So it would take, not 2, but about 22 Falcon Heavy's somehow strapped together to lift one fully fuelled one into orbit.

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In some ways, you are describing in principle, but not specifically the approach SpaceX intends to use with the Big Falcon Rocket (BFR).

The booster stage (31!!!) Raptor engines sometimes called BFR, boosts the upper stage (7 Raptors, 3 sea level, 4 vacuum optimized) sometimes called the BFS (Big Falcon Ship) to orbit with up to 150,000lbs of payload.

Then for Lunar missions, at least two refueling flights by tanker versions of the BFS deliver fuel and oxidizer. For Mars flights, they deliver 6 loads of fuel and oxidizer.

At the end of the refueling exercise you end up with basically a fully fueled upper stage in orbit with vacuum optimized objects. Payload to Mars then is the same 150,000lbs to Mars orbit.

So a Falcon Heavy specifically is a bad choice, but a fully fueled upper stage can be mighty useful.

Raptor Vacuum is a much better engine than sea level Merlins.

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If "sandwiching one between two others KSP style" means horizontally, so that a FH has two more FH's on either side making nine parallel first stages, then that's an interesting question and as far as I know hasn't been asked. The closest thing would be questions about a "Falcon Quad"; a central body with four more arranged in a quad or cross-shape. For that see answers to Ballpark comparison of a hypothetical Falcon 'Quad' Heavy with cross feeds.

However, if "sandwiching one between two others KSP style" means vertically, then this won't work. A fully-fueled, un-ignited, dead-weight Falcon Heavy would be a Falcon Too-Heavy as a payload for another Falcon Heavy if sitting on the launch pad. However, yes, launched in space, a FH could certainly act as "first stage" for another FH, but presumably that's not what you mean. I think your sandwiching refers to getting a FH into space by launching it atop another FH, and that just won't work.

Another problem as @Jack points out is that the nozzles on pretty much any first stage of any kind are much smaller in maximum diameter than the would ideally be for space operation. Nozzles should expand the exhaust so that the pressure drops to something roughly similar to ambient pressure. You can't pack nine vacuum-sized nozzles in a F9 configuration. They would have to be much farther apart.

See a comparison of atmospheric nozzles and one vacuum nozzle in @RusselBorogove's excellent answer to What are the differences between a standard Merlin engine and the Merlin Vacuum engine?, as well as the question Why is the Merlin 1D Vacuum nozzle bigger than the Merlin 1D nozzle? and answer(s) there.

You might be able to get away with using atmospheric nozzles in vacuum I believe. This is not necessarily as dangerous as using a vacuum nozzle in atmosphere, but the efficiency would be compromised.

Summary: Yes, if you magically got a FH in space and fully fueled, that would be useful. However, you can not do that by putting it on top of another FH.

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    $\begingroup$ Yes, I did mean in a side by side or quad formation with the two on the side lifting the central unit. Given the rest of the information, the central unit could be optimized for vacuum use I suppose, perhaps by offsetting the height to clear the larger nozzles. $\endgroup$
    – Coyote
    Jul 29, 2018 at 7:10
  • $\begingroup$ Remember that the "9" in Falcon 9 means each first stage has nine nozzles for nine engines. Perhaps one of nine could be longer, but that's not much improvement on its own. $\endgroup$
    – uhoh
    Jul 29, 2018 at 7:54
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The thrust to weight ratio of a rocket is very important at liftoff because the rocket has to fight gravity just to leave the launch pad. In addition, every second spent fighting gravity incurs gravity loss.

A rocket already in orbit does not need an especially high TWR and it certainly doesn't need to be > 1. So if you could put the mass of a fully fueled FH into orbit, you'd be better off without all the extra engines of the side booster. Probably a single Merlin would be suitable for most missions. Just let the center core burn a little longer.

Remember that ion engines are useful for interplanetary travel (and even entering geosynchronous orbit) and their thrust is very small compared to a single Merlin.

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