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High Frontier (3rd edition)» Forums » Rules

Subject: Thrust for Landing rss

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Matt Watkins
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Rules state that I have to have net thrust higher than the site size to land. The question is: when do I calculate the thrust? Logic tells me I should be able to switch thrusters and recalculate net thrust (but not TMPs) for landing just before entering a lander burn or attempting to land. I shouldn't have to consider the wet mass of my fully fueled LEO rocket stack when determining if I can drop a lander on Luna in the same year.
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Rich James
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You calculate thrust and TMPs before you move your rocket. If you need to switch thrusters to land, then stop short of the site/lander burn and next turn switch to the more powerful thruster.
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Adam Gastonguay
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You calculate thrust at the beginning of a turn (or really, at the beginning of the rocket's movement to be precise). If you're using, say, a solar sail to get around, you can use the slow sail to get you right up to Luna, or Mercury, or Titan or whatever, then you can switch to a chemical rocket to land you. Remember, though, that you can't end your turn on a Lander Burn, so you have to have enough fuel to use your chemical rocket for those burns too, if the site is big enough.

ninja'd
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Bruce Silzer
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Here is a queest on thrust which came up when we were foing a refresh off the rules.

A solar thruster (thrust fo 1) approached to land on a 1S planet assited by and ESA boost. This meets the > size rule to land.

Now the same thruster wishes to lift off. Again it has the required thrust BUT here is the question.

Given the nature of a solar thruster in the real world can it even lift off. I cannot invision such a structure lifting off from any body.

What opinions can be offered?

NOTE: Up unitl this game the solar thrusters have been used only as transport to in the rae occssional burned as part of braking. This use has never cropped up. Applying the LITERAl meaning of the rules landing and lift off appear to be possible.
 
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Matt Watkins
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A size 1 planet has a gravity more than 1000x less than Earth's. (0.75 milligee according to the scale reference notes.) You can't really orbit such a body; you do stationkeeping. I can imagine a solar sail performing a slow rendezvous and setting up station-keeping, then developing the thrust necessary to impact the surface at low velocity. It could perhaps collapse the sail and use a harpoon to facilitate landing, then take off using a small charge, or even a spring (can a tensioned spring survive the temperature, vacuum and radiation of space travel?), and deploy the sail at sufficient distance. For reference, escape velocity at the surface of Phobos is about 12 m/s (~40 kph). A 20 ton sail would weigh about the Earth equivalent of 10 kg there, so you'd have to have a thrust capable of launching 10 kg about 15 m in the air on Earth to reach escape velocity on Phobos, which sounds difficult (I couldn't do it by hand), but could be done with a gram of TNT.
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Francisco Colmenares
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In other words perfectly feasible to use the Solar Sail to "take off", the hard part of landing on a size 1 world is actually staying on it. You definitely need to anchor yourself to the surface in some fashion.
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Bart Rachemoss
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Silver City
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Masterdragon wrote:
Given the nature of a solar thruster in the real world can it even lift off. I cannot invision such a structure lifting off from any body.

Escape velocity is independent of direction. If you ignore friction and so forth you can escape from a planet tangentially just as well as vertically. So if you situate your craft on the highest hill or peak you can find and you just see the ESA lasers or the sun at the horizon then they can give you thrust tangential to the surface of the planet.

PS: do not look directly at the 60 MW lasers with your remaining eye.
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