The issue is setting up the "glide path". Since there is no air, it would be an orbit that is tangential to the "landing strip. So you want to go close to the moon going at more than 3500 mph?
Well, by the time a colony exists, the lunar gravity field should be well mapped so how much more dangerous is approaching within tail hook distance to the surface in a 3500 mph orbit
Quote from: aero on 06/24/2014 06:58 pmWell, by the time a colony exists, the lunar gravity field should be well mapped so how much more dangerous is approaching within tail hook distance to the surface in a 3500 mph orbit wrong, gravity field mapping is a minor/secondary effect. The issue is precision of the "deorbit" burn that sets up the orbit "glideslope" that intersects with the landing field. It can't be done accurately enough to ensure alignment vertically, laterally and short/long. The corrections for the errors end up being orbital corrections in reality. Since it is a low point in the orbit, the corrections are at the worst spot in terms of effectivity. Laterally adjustments are plane changes, long/short adjustments are orbit axis changes. These will be huge in terms of propellant. This makes the concept highly unlikely that it is viable. And there still is the risks flying low that fast.
Certainly valid technical problems to be solved.
Don't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.
The "short" landing problem can be solved using the vertical lift thrusters given sufficiently advanced knowledge of the distance short. The "long" problem looks like a "go around" to me. Isn't vertically just another version of short and long? As for lateral, how much lateral velocity do you suppose the tail hook-arrester cable can accommodate?.
Quote from: JasonAW3 on 06/24/2014 07:56 pmDon't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.What do you mean by "spiral?" The lander inserts into a near circular elliptical orbit with the perilune at and aligned with the landing strip. It does not loose speed until it snags the arrestor cable. It gains a little speed as it looses altitude around the orbit.
Quote from: aero on 06/24/2014 08:06 pmQuote from: JasonAW3 on 06/24/2014 07:56 pmDon't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.What do you mean by "spiral?" The lander inserts into a near circular elliptical orbit with the perilune at and aligned with the landing strip. It does not loose speed until it snags the arrestor cable. It gains a little speed as it looses altitude around the orbit.Basic aeronautics; Gravity is a downward force that also acts as a form of drag.Even without an atmosphere, velocity will be lost due to the lunar gravity. The arc would tend to be like that of a cannon ball. While the cannonball will go farther, gravity will still pull it down in an arc. the speed that it gains is in a downward fashion while forward momentum is bled off by gravity.
Even without an atmosphere, velocity will be lost due to the lunar gravity. The arc would tend to be like that of a cannon ball. While the cannonball will go farther, gravity will still pull it down in an arc. the speed that it gains is in a downward fashion while forward momentum is bled off by gravity.
Quote from: aero on 06/24/2014 07:50 pmCertainly valid technical problems to be solved. The point is there is no ned to solve them. There is no benefit to the idea.
Quote from: aero on 06/24/2014 07:50 pmThe "short" landing problem can be solved using the vertical lift thrusters given sufficiently advanced knowledge of the distance short. The "long" problem looks like a "go around" to me. Isn't vertically just another version of short and long? As for lateral, how much lateral velocity do you suppose the tail hook-arrester cable can accommodate?.And what size are the vertical lift thrusters and how much propellant is needed.A tail hook-arrester cable can accommodate no lateral velocity if the vehicle is not over the strip laterally
Basic aeronautics; Gravity is a downward force that also acts as a form of drag.
Ok, Explain why orbits decay, even around airless moons.
Quote from: JasonAW3 on 06/24/2014 08:53 pm Explain why orbits decay, even around airless moons.Apart from tidal effects (if the orbiting object is large enough) or radiation effects, they don't.
Explain why orbits decay, even around airless moons.
...and perturbations from uneven gravity field caused by masscons. The Moon has only certain inclinations which are stable.
Quote from: JasonAW3 on 06/24/2014 08:53 pmOk, Explain why orbits decay, even around airless moons.Apart from tidal effects (if the orbiting object is large enough) or radiation effects, they don't.
Quote from: R7 on 06/24/2014 09:15 pm...and perturbations from uneven gravity field caused by masscons. The Moon has only certain inclinations which are stable.Thanks for reminding this, although to be honest I never understood completely why masscons should ALWAYS bring to the satellite hitting the surface (and not just making the orbit unstable) as some sources say. If someone could explaining me that, even if OT, it would be appreciated.
They won't, not in the way you're thinking - there is no constant force from masscons that makes something hit the Moon. What they will do is *perturb* the orbit - cause it to change very slightly in inclination, apoapsis, periapsis (separately, modifying eccentricity), by small amounts (meters) every orbit, which will gradually build up to larger amounts (kilometers). Since it's airless, low lunar orbits (for the purpose of higher resolution) are already *very highly* circularized, at *very low* altitude, and any deviation from that circle which does not represent a substantial increase in energy on both sides of the lunar surface, will result in the craft smashing into the surface eventually, just from the quasi-random (on long timescales) walk in orbital parameters.The notion of a *bidirectional* magnetic mass driver & mass decelerator is one I have not encountered before, and I think it's very intriguing - I see no reason why it couldn't work, eventually, with the right infrastructure. Do keep in mind we're talking about far, far future stuff here - this is not a small project, just perhaps smaller than a space elevator.
Well obviously the vertical lift thrusters on the "Eagle" were enough for the mass of the descending "Eagle." That was Apollo 11 wasn't it?
I estimate benefit using Isp = 400 seconds for the lunar vehicles. The benefit is the savings of a half tonne of prop in low lunar orbit for every tonne of lander + payload set down on the moon. Prop in low lunar orbit is expensive as it takes another half tonne of prop to lift every tonne of tanker + prop to low lunar orbit. Unless the prop comes from the Earth which is very much more expensive, but I'm suggesting an existing lunar colony.
Hooking a cable across a runway is ridiculous, like trying to catch a APDS tank round, it'd cut through any solid object. You'd have to either use magnetic fields or a gas, and I've no idea how a gas is supposed to be contained on a runway in a vacuum..
Not on the first mission or the second, but if a lunar colony were established ...
Quote from: aero on 06/24/2014 08:43 pmWell obviously the vertical lift thrusters on the "Eagle" were enough for the mass of the descending "Eagle." That was Apollo 11 wasn't it?Yes, and since you have a vertical landing thruster, there is no need to have a landing strip, colony or not.
You haven't shown that it saves propellant.
Also, your lander has to be overbuilt and heavier to take the g loads that a regular lander wouldn't see.
The reasons the idea is non viable are the fore mentioned and additionally, the lander can only be used in one spot, and it can only land from one orbital inclination*. * this is a killer. More propellant is going to be used to get in that specific inclination. It is like trying to fly from USA to London, and only being able to fly from Miami vs NYC because the London runway lines up with Miami. Whereas vertical landing sites can be reached from many inclinations and directions.
Quote from: Alf Fass on 06/24/2014 08:55 pmHooking a cable across a runway is ridiculous, like trying to catch a APDS tank round, it'd cut through any solid object. You'd have to either use magnetic fields or a gas, and I've no idea how a gas is supposed to be contained on a runway in a vacuum..Remember, I started this thread with:QuoteNot on the first mission or the second, but if a lunar colony were established ...That does put this topic into the future. Do you think that maybe, just maybe cable and arrester technology might be advanced in the years intervening between now and this "future?"What about light weight cables made of carbon nanotubes? They don't exist at the moment but can you say for sure that they or something better won't exist in 25, 50 or 100 years?And can you say that that something better would not be able to withstand the forces involved? Saying that it absolutely never ever could work is what is ridiculous. It's kind of like saying we've already got rockets and they work so don't consider anything else. Ignore that it may have potential to reduce recurring cost.
the energy involved, E=1/2MV20.5x1,700,0002J/kg= 1445000000000 J/kg, the energy release would melt ANYTHING.
1. Well maybe, but I suspect that trajectories coming to the moon can be planned with orbit insertion into the correct inclination to align with the landing strip2. Apollo did it successfully at both ends, after all, the recovery fleet was there when they returned from the moon.3. I further assert that such landing strip will make that colony location the central hub for traffic arriving at the moon.
The added ruggedness of the lander over a propulsive lander does eat into the prop savings on re-launch of the lander. I doubt it totals 100% more structural mass which is what it would need to be to wipe out the savings.
So you think the Lunar colony will be dispersed all over the surface without a concentrated central hub location?
Quote the energy involved, E=1/2MV20.5x1,700,0002J/kg= 1445000000000 J/kg, the energy release would melt ANYTHING.Aren't you using a few to many zeros? V is like 1,700. m/s, so E = 1,445,000. J/kg = 0.401 kW-hr.
landing strip length is doubled to 11 miles. Same thing with time, decelerate at 14 g's for 11.4 seconds, or 7 g's for 23 seconds. 3.5 g's takes 46 seconds and 22 miles. Pretty rugged cargo and lander that.
Quote from: aero on 06/24/2014 10:04 pmQuote from: Alf Fass on 06/24/2014 08:55 pmHooking a cable across a runway is ridiculous, like trying to catch a APDS tank round, it'd cut through any solid object. You'd have to either use magnetic fields or a gas, and I've no idea how a gas is supposed to be contained on a runway in a vacuum..Remember, I started this thread with:QuoteNot on the first mission or the second, but if a lunar colony were established ...That does put this topic into the future. Do you think that maybe, just maybe cable and arrester technology might be advanced in the years intervening between now and this "future?"What about light weight cables made of carbon nanotubes? They don't exist at the moment but can you say for sure that they or something better won't exist in 25, 50 or 100 years?And can you say that that something better would not be able to withstand the forces involved? Saying that it absolutely never ever could work is what is ridiculous. It's kind of like saying we've already got rockets and they work so don't consider anything else. Ignore that it may have potential to reduce recurring cost.You're not getting the energy involved, E=1/2MV20.5x1,700,0002J/kg= 1445000000000 J/kg, the energy release would melt ANYTHING.It did occur to me though that in theory if you used wheels spun up to the landing speed on magnetic bearings with generators in the hubs that you could in theory turn most of that kinetic energy into electricity. But that level of complexity for the return would I think be beyond insanity.