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I have a captured planet/moon orbiting HD 37124 c, known as HD 37124 c m XVIII, and I was wondering: since the moon has 1.7 Earth masses, 1.1 Earth radii, a density of 7.038 g/cm^3 and a gravitational pull of 1.41 Earth gees, would it be possible for any technological civilisation to reach this moon's orbit with conventional rockets and conduct space exploration of other (if any are possible) moons with their own crew or would it mostly be probes and satellites leaving this planet with conventional rocket launches, rather than local crews or orbital stations? Could they even build them at all?

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  • $\begingroup$ Your question is quite confusingly worded. Could you unpack it a bit and explain in smaller bites? $\endgroup$ – John Dallman Jul 8 '16 at 22:16
  • $\begingroup$ One moment please. $\endgroup$ – Future Historian Jul 8 '16 at 22:17
  • $\begingroup$ @JohnDallman Done. $\endgroup$ – Future Historian Jul 8 '16 at 22:18
  • $\begingroup$ I would assume the planet they are starting from is larger than earth in order to hold the moon in its orbit? $\endgroup$ – Bellerophon Jul 8 '16 at 22:23
  • $\begingroup$ @Bellerephon This is the parent planet: en.m.wikipedia.org/wiki/HD_37124_c. $\endgroup$ – Future Historian Jul 8 '16 at 22:24
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My answer to another question: "As we increase the size and mass of a world, at what point does it become impossible for a rocket to achieve an orbit or escape velocity?", answers this one quite well.

Depending upon what you're willing to pay for, it is well within the our technical capabilities to build a rocket that would accomplish this mission.

  • The Earth's mass is the limit for a single stage chemical rocket to achieve orbit.
  • A 3 stage chemical rocket, with performance similar to those we already have, could achieve orbit around a planet with Saturn's mass.
  • A single stage nuclear pulse rocket could achieve orbit from a planet with mass around 6 times that of Jupiter.
  • A 3 stage nuclear pulse rocket could achieve orbit from any planet and even low mass stars.

As others have pointed out, some of these solutions would be very costly indeed.

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It's not as bad as the other posters are making it out to be. Earth's escape velocity is 11.2 km/sec. This world has an escape velocity of 13.9 km/sec. Thus you need another 2.7 km/sec out of your booster. That's not quite another stage on your rocket. It's still going to make the moon project very difficult but certainly not impossible. I think two launches, one with the rocket, one with the fuel with a rendezvous in low orbit would do it.

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Yes, it's possible to take of from this body with chemical rockets and get into orbit around it. It's harder than it is from Earth, but there isn't a cut-off - you just need more and more rocket stages for more massive bodies and it gets correspondingly more expensive.

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  • $\begingroup$ So, their version of Saturn V needs to have one or two additional stages, correct? $\endgroup$ – Future Historian Jul 8 '16 at 22:42
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    $\begingroup$ Yes, but if you want the same payload as the Saturn V, the extra stages are at the bottom, and hence bigger and more expensive. $\endgroup$ – John Dallman Jul 8 '16 at 22:44
  • $\begingroup$ Oh.........well, that is bad. :/ $\endgroup$ – Future Historian Jul 8 '16 at 22:48
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With conventional rockets you may be out of luck.

Even though it is technically possible to construct a rocket system with multiple extra stages, you need to consider that the expense would be astronomical.

Earth rockets with 3 stages are able to get about 10% of the total launch mass into LEO. Yet, you are launching from a planet that is 1.7 M0 (M0 is Earth mass). No ground based chemical launch system ever designed for use on Earth could reach LEO at 1.7 M0 - the best designs are only about 10% payload. You would need quite a few more stages, and each additional stage is subject to diminishing return as you are lifting additional dead weight for the separate rocket stage mechanism, such as booster separation, fuel pumps, fuel tanks, combustion chambers, etc.

The most efficient practical chemical launch fuel is liquid hydrogen and oxygen, the same as that used on the 2nd and 3rd stage on the Saturn 5. But the first stage used a highly refined form of kerosene. Why, it is because liquid hydrogen is a low density fuel and comparatively large (and heavy) tanks are needed.

Your conventional rocket would require at least 10 stages, with the early stages burning kerosene and the upper stages burning hydrogen. This would be outlandishly expensive even to orbit very modest payloads. Cost could easily be 100 times as expensive in terms of $/kg. payload.

Just because it is theoretically possible, does not mean it would ever happen.


You could build a build a 5-stage rocket that gets you into orbit, but the two additional stages would be respectively about 5 and 25 times the size of the largest stage required for lifting from earth.

Since the largest stages would have to be so huge, it would be far more effective (and efficient) to create more stages that supply less delta-v to achieve. You would have to be more efficient at producing multiple stages, as you would multiply costs greatly by trying to build a 5 stage rocket with the final stages weighing in at 10 and 50 million kg. Clearly such a design would require more attention to increasing the efficiency of multiple stages.

A 10-stage equivalent would still be huge of course, but the stage sizes would be more manageable (at the expense of the additional complexity of more stages). In earth gravity, such a design is not needed and a large number of stages is not cost effective.

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