08102024, 05:19 PM  #101  
Join Date: Feb 2005
Location: Berkeley, CA

Re: thinking about spacecraft design
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08102024, 05:23 PM  #102  
Join Date: Jun 2005
Location: Lawrence, KS

Re: thinking about spacecraft design
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Bill Stoddard I don't think we're in Oz any more. 

08102024, 05:32 PM  #103 
Join Date: Feb 2005
Location: Berkeley, CA

Re: thinking about spacecraft design
Honestly, unless there's a really compelling reason to visit the planet, people will probably just not do so, as it's an absolutely atrocious planet to try and get on or off of, but given the limits on rocket options, might be worth considering nonrocket options such as a momentum exchange tether (it's a formidable materials science problem with the sheer amount of deltaV required, but might still be less effort than rockets).

08102024, 07:15 PM  #104  
Join Date: Jun 2005
Location: Lawrence, KS

Re: thinking about spacecraft design
Quote:
__________________
Bill Stoddard I don't think we're in Oz any more. 

08122024, 03:06 PM  #105 
Join Date: Jun 2005
Location: Lawrence, KS

Re: thinking about spacecraft design
To carry things forward, I wanted to work out orbital mission plans for various planets. Doing the calculations in mixed units was just insane, and doing everything in metric proved tediously time consuming, so I worked out a set of canonical units, in which the radius, mass, and density of Earth are all 1.00. Here are the calculations I came up with:
Start with the planet's radius and density. The planet's mass is equal to density times the cube of radius. The surface gravity is the product of radius and density, in G. The escape velocity is the square root of (mass divided by radius), times 7 mps. For a standard circular orbit at an altitude of 0.3 (or 1911 km), and thus an orbital radius of planetary radius plus 0.3, the orbital period is the square root of (orbital radius cubed, divided by mass), times 5050 seconds. The orbital speed is the square root of (planetary mass divided by orbital radius), times 4.73 mps. For a grazing elliptical orbit, with apoapsis at the circular orbit, and periapsis at the planetary surface, the semimajor axis is the planetary radius plus 0.15. Each of the two orbital velocities is the square root of (2 divided by orbital radius, minus 1 divided by semimajor axis, quantity times planetary mass), multiplied by 4.95 mps. Then you can subtract the apoapsis speed from the circular speed, and add the periapsis speed, to find how much deltaV is needed for landing. For takeoff, you need downward thrust to compensate for planetary surface gravity. This calls for a thrust angle of arcsin(planetary surface gravity/spacecraft acceleration). You then divide the periapsis speed by the cosine of this angle to find how much deltaV you have to expend to get into an elliptical orbit, and add the deltaV you need to speed up to circular orbit speed. Assuming an acceleration of 2.00G, I came up with deltaV of 0.67 mps for both landing and takeoff on a tiny world (0.09G); 5.51 mps for landing and 5.82 mps for takeoff on a small world (0.64G); and 7.30 mps for landing and 10.12 mps for takeoff on a large world (1.40G). So that gives me estimates of what performance is needed to make each of these worlds a viable site for missions (assuming that the colonists aren't going to be shipping masses of fuel/reaction mass down to the respective planetary surfaces!).
__________________
Bill Stoddard I don't think we're in Oz any more. 
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