Author Topic: Landing on the Moon using a tail hook?  (Read 13321 times)

Offline aero

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Landing on the Moon using a tail hook?
« on: 06/24/2014 06:36 pm »
Not on the first mission or the second, but if a lunar colony were established, what would be the potential of preparing a very long lunar landing strip with arrestor cables to decelerate lunar landers equipped with a tail hook? The analogy of aircraft carrier landings comes to mind.

As I envision it the lander, equipped with landing skids or wheels, would approach the landing strip in a Moon grazing orbit with landing engines oriented downward (horizontal lander) to avoid slamming into the surface when the tail hook engaged the arrester cable. The cable would be very long and the lander would skid to a stop before the end of the runway. Maybe 2-3 g's deceleration? How long would the strip need to be and would it be possible to do a wave off and go around? What technology would be needed before such a technique could be considered? Are there long enough, smooth enough, high enough places on the moon that such a landing strip could be built and what delta V savings might be achievable?
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #1 on: 06/24/2014 06:49 pm »

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?
« Last Edit: 06/24/2014 06:50 pm by Jim »

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #2 on: 06/24/2014 06:58 pm »

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 than approaching tail hook distance at 350 mph glide slope to an aircraft carrier. Both are fatal if you're to low. 

I would allow that we might want a little longer tail hook for use on the moon.  ;D
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Offline JasonAW3

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Re: Landing on the Moon using a tail hook?
« Reply #3 on: 06/24/2014 07:01 pm »

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?

Excellent point!  a tail hook would either be ripped off of the vehicle, slice through the cable or both.

However, using an Maglev system to slow down, (Using large ring magnets that you'd fly through)  Might work, but it would cause a lot of issues by magnatizing the incoming craft.
« Last Edit: 06/24/2014 07:02 pm by JasonAW3 »
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Offline mheney

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Re: Landing on the Moon using a tail hook?
« Reply #4 on: 06/24/2014 07:14 pm »
Fighter jets are doing a lot less than 350 mph when they land ...  I did a quick google, and 130 knots / 150 mph is closer to the actual landing speed ...

Energy goes up as velocity squared, so you're looking at 20+ times the velocity, so 400+ times the energy.

3500 mph is 1565 m/sec.  If you decided that you'd accept a 10G deceleration (98 m/sec^2), you'd take 16 seconds to decelerate to zero.  Your average speed is 780 m/sec, so you'd travel about 12.5 km before stopping.  At a more reasonable 5 Gs, you're looking at 32 seconds and 25 km to stop.

That's a LONG arresting cable ....  (and we haven't even looked at the forces on the cable, which needs to go almost
instantaneously from rest to 1565 m/sec without snapping ....)

 

Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #5 on: 06/24/2014 07:15 pm »

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

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.

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #6 on: 06/24/2014 07:50 pm »

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

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.

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?

I did a little calculation and found that a 5.5 mile (29,000 feet) landing strip would need 14 g's deceleration and since distance is inversely proportional to deceleration for a fixed initial velocity, at 7 g's decal, 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.
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Offline pagheca

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Re: Landing on the Moon using a tail hook?
« Reply #7 on: 06/24/2014 07:51 pm »
Funny (and therefore interesting, at least for a an arm-chair rocket scientists like me) idea.

If a mass driver exists on the Moon, the numbers involved (acceleration, energy, accuracy requirements, etc.) are more or less the same.

At this point I guess that a mass driver, or a hook mounted on a mass driver, could be a better solution.

I don't mean the mass driver is easier or doable. Just remarking that a tail hook is the same class of approach to the problem of accelerating/decelerating in vacuum, except unwinding a cable several km long, rather than using a rail and magnetic forces or friction to land at that speed means probably gigantic wheels and h/w involved.
« Last Edit: 06/24/2014 08:01 pm by pagheca »

Offline JasonAW3

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Re: Landing on the Moon using a tail hook?
« Reply #8 on: 06/24/2014 07:56 pm »
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.

Actually, you'd most likely be better off with a sort of magnetic catch net to decelerate the craft as it came down vertically.  No actual net, but magnetic fields to reduce the speed of descent.
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #9 on: 06/24/2014 08:04 pm »

Certainly valid technical problems to be solved.


The point is there is no ned to solve them.  There is no benefit to the idea.

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #10 on: 06/24/2014 08:06 pm »
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.

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.
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #11 on: 06/24/2014 08:07 pm »

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?
.

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

Offline JasonAW3

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Re: Landing on the Moon using a tail hook?
« Reply #12 on: 06/24/2014 08:17 pm »
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.

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.
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Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #13 on: 06/24/2014 08:25 pm »
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.

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.

I don't think that's right. If it were right then to maintain LEO would require continuous boosting beyond boosting to counter aerodynamic drag and it does not.
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Offline pagheca

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Re: Landing on the Moon using a tail hook?
« Reply #14 on: 06/24/2014 08:40 pm »
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.

As long as the spacecraft has not other forces acting upon it, it's in orbit, and therefore subject to the laws of gravity only. You may imagine an orbit tangential to the surface at the point it must be hooked. Than you need some adjustments. But there is not necessarily any ballistic arc involved like you mentioned. Everything depends on the regular trajectories laws.

You may figure it out by temporally inverting a mass driver launch under vacuum: at the point the spacecraft reaches orbital speed, some longitudinal thrust may make it lift. No vertical thrust necessarily needed.

You temporally invert this and you get exactly what you can and you can't do.
« Last Edit: 06/24/2014 08:52 pm by pagheca »

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #15 on: 06/24/2014 08:43 pm »

Certainly valid technical problems to be solved.


The point is there is no ned to solve them.  There is no benefit to the idea.

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.


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?
.

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

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?

And, as I recall the Eagle landed long, but I don't know how far laterally the trajectory was from the target landing area. If the lander trajectory is laterally displaced from the landing strip that is another case where a "go around" is a solution. It would require a new landing trajectory calculation but calculations are cheap.
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Offline pagheca

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Re: Landing on the Moon using a tail hook?
« Reply #16 on: 06/24/2014 08:49 pm »
Basic aeronautics;  Gravity is a downward force that also acts as a form of drag.

If you mean "gravity drag" as for rocket launch from the Earth, this is not necessarily true.

On Earth we launch vertically because we have a dense atmosphere. Therefore, we can't accelerate like an aircraft at more than 1-2 Mach without having to gain high altitude before getting from 2 to Mach 25.

But there is actually no reason to launch vertically in vacuum if you have a veeeeeery long runway (no, Apollo LMs haven't...). That's the point of having a mass driver. In this case there is NO gravity drag, because the gravity drag - at least this is what I understood - is just a way to take in account the fact that you have to lift the fuel (i.e. not only accelerate it) on top of the dry mass of the rocket when you have a vertical component of the speed at launch.

If you don't - as for a mass driver, gravity drag is not required.

Hope to have been clear (and right...).
« Last Edit: 06/24/2014 09:04 pm by pagheca »

Offline JasonAW3

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Re: Landing on the Moon using a tail hook?
« Reply #17 on: 06/24/2014 08:53 pm »
Ok,


    Explain why orbits decay, even around airless moons.
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Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #18 on: 06/24/2014 08:55 pm »
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..
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Offline pagheca

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Re: Landing on the Moon using a tail hook?
« Reply #19 on: 06/24/2014 08:55 pm »
Ok,


    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.
« Last Edit: 06/24/2014 09:00 pm by pagheca »

Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #20 on: 06/24/2014 08:59 pm »
It was alway funny when they all started getting excited about orbital decay when the engines were off-line in the original Star Trek series.
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Offline R7

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Re: Landing on the Moon using a tail hook?
« Reply #21 on: 06/24/2014 09:15 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.

...and perturbations from uneven gravity field caused by masscons. The Moon has only certain inclinations which are stable.
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Offline pagheca

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Re: Landing on the Moon using a tail hook?
« Reply #22 on: 06/24/2014 09:21 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. I say that because the (only) force involved is conservative. If someone could explaining me that, even if OT, it would be appreciated.

« Last Edit: 06/24/2014 09:29 pm by pagheca »

Offline Burninate

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Re: Landing on the Moon using a tail hook?
« Reply #23 on: 06/24/2014 09:31 pm »
Ok,


    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.
+1.

The 'decay must be caused by gravity' thing is a disturbingly common belief.

If you'd like a basic introduction to orbital and launch mechanics, I would strongly recommend Kerbal Space Program.
...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, plus conservation of orbital energy (which is reflected in sacrificing periapsis for apoapsis).

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.
« Last Edit: 06/24/2014 09:42 pm by Burninate »

Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #24 on: 06/24/2014 09:44 pm »

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.

Thanks for that, I had wondered if mascons can bring a lunar satellite down if they could also pump it into a higher orbit? But what you're saying is that it isn't a change in the semi-major axis of the orbit, but one in the eccentricity.
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #25 on: 06/24/2014 09:49 pm »

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?

Yes, and since you have a vertical landing thruster, there is no need to have a landing strip, colony or not. 


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.


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. 
« Last Edit: 06/24/2014 09:58 pm by Jim »

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #26 on: 06/24/2014 10:04 pm »
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..

Remember, I started this thread with:
Quote
Not 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.
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Offline QuantumG

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Re: Landing on the Moon using a tail hook?
« Reply #27 on: 06/24/2014 10:23 pm »
How about a gradually slopping tunnel with magnetic deceleration? Maybe you use your hook to swing the vehicle into the mouth of the tunnel at the beginning.

Sounds like a fun math problem.

Human spaceflight is basically just LARPing now.

Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #28 on: 06/24/2014 10:53 pm »

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?

Yes, and since you have a vertical landing thruster, there is no need to have a landing strip, colony or not. 


You are certainly right if it is "Not colony." If it is "colony," then the jury is still out. Just because we have one way that works doesn't imply that there may not be a better way.


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.

Quote

You haven't shown that it saves propellant.



The whole point of the landing strip and coming in at orbital velocity is to eliminate the need for the half tonne of prop per tonne of landed mass needed for the deorbit burn. That is the horizontal delta-V to be supplied by the arrestor cable instead of propellant.
Quote
Also, your lander has to be overbuilt and heavier to take the g loads that a regular lander wouldn't see.
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.
Quote

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.
So you think the Lunar colony will be dispersed all over the surface without a concentrated central hub location?

Well Jim, interpreting you freely, you are saying that the Lunar Elevator is the next logical step. The Lunar Elevator does not require any de-orbit delta-V burn, no huge landing strips crisscrossing the lunar surface, no technologically advanced cables or guidance and control systems, and multiple anchor points can service the distributed colony, and all for less than a $Billion to start. (See the lunar elevator thread)

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 strip (Apollo did it successfully at both ends, after all, the recovery fleet was there when they returned from the moon. I further assert that such landing strip will make that colony location the central hub for traffic arriving at the moon.
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Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #29 on: 06/24/2014 10:57 pm »
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..

Remember, I started this thread with:
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Not 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=1/2MV2

0.5x1,700,0002J/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.
« Last Edit: 06/24/2014 11:01 pm by Alf Fass »
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Offline aero

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Re: Landing on the Moon using a tail hook?
« Reply #30 on: 06/24/2014 11:09 pm »
Quote
the energy involved, E=1/2MV2

0.5x1,700,0002J/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.
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Offline QuantumG

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Re: Landing on the Moon using a tail hook?
« Reply #31 on: 06/24/2014 11:17 pm »
Yeah, a railgun to accelerate something to (or decelerate something from) 1.6 km/s is pretty trivial. For example.
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #32 on: 06/25/2014 12:02 am »

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 strip

2.  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.

wrong on all accounts.

1.  That will severely limit the number of launch opportunities.  There will be days with no launch opportunities.

2.  No, Apollo did not landing on a strip, it landed at points which are accessible from various angles.

3.  Bad assertion.  The landing strip limits the operability of logistics to the colony

The idea is bad all around.  It is like having a helicopter than only can land at one place.   What is done for emergencies? or if supplies are needed elsewhere?
« Last Edit: 06/25/2014 12:02 am by Jim »

Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #33 on: 06/25/2014 12:04 am »

 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.

It would add more than 100% and it is not just relaunch propellant.  There is deorbit and maneuvering propellant that have to deal with a heavier lander.

Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #34 on: 06/25/2014 12:07 am »

So you think the Lunar colony will be dispersed all over the surface without a concentrated central hub location?


Either way, it is better that the logistics vehicles can approach from all directions (orbits)

Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #35 on: 06/25/2014 12:11 am »
Quote
the energy involved, E=1/2MV2

0.5x1,700,0002J/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.

oops!

Still when solid materials collide at 1700m/s the two surfaces are no longer solid as the heat energy released at the point of impact exceeds the melting point of the materials.
When my information changes, I alter my conclusions. What do you do, sir?
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Offline Jim

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Re: Landing on the Moon using a tail hook?
« Reply #36 on: 06/25/2014 12:14 am »
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.

You are ignoring the curvature of the surface of the moon.  The moon drops 65 feet over 5 miles.

Offline Vultur

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Re: Landing on the Moon using a tail hook?
« Reply #37 on: 06/25/2014 01:32 am »
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..

Remember, I started this thread with:
Quote
Not 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=1/2MV2

0.5x1,700,0002J/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.

Wouldn't that mean a speed of 1,700,000 m/s? That seems about three orders of magnitude too high.

Offline Burninate

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Re: Landing on the Moon using a tail hook?
« Reply #38 on: 06/25/2014 01:48 am »
A serious implementation of the idea involves a sizable maglev track across some section, or perhaps the whole length, of the Lunar equator.  A vehicle travels the track at slightly below orbital velocity, propelled by a linear induction motor.  The vehicle holds a big asphalt-topped landing strip attached to a pivot, which swings in the forward/back direction to maintain the pad's net acceleration vector in the "vertical" in relation to the pad, while speeding up or slowing down in relation to the Lunar surface.

The track is stabilized against inductive rails using adjustable-pressure magnetic sliders, in multiple dimensions, a low-gravity implementation of the Inductrack design which would in theory be capable of super-orbital velocities.

An Earth-Moon trip would aim for a LLO injection anywhere between +-7 degrees of the orbital plane, which is the relative inclination of the equator, phased to match the equator.  It would circularize in LLO using thrusters.  It would then lower altitude, and perform a deorbit burn to rendezvous with the vehicle, at a low relative velocity and low relative acceleration, performing a "vertical" landing.

On the other side, it's mounted on the landing pad and takes off "vertically" when the vehicle is up to speed, transitioning to orbit in < 100m/s of burn, with a very low thrust requirement.  Or, with greater difficulty, strengthen the track and work out a release mechanism and you can eject mass out to Lunar escape velocity, albeit inclined relative to the orbital plane, requiring the correct phasing if you want to send it back to Earth.
« Last Edit: 06/25/2014 01:51 am by Burninate »

Offline KelvinZero

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Re: Landing on the Moon using a tail hook?
« Reply #39 on: 06/27/2014 04:31 am »
I have always liked the idea of a maglev around the entire equator since acceleration to from orbital velocity could be as smooth as you like, and the only forces it absolutely must handle are those to suspend the train, which are already practical on earth and would be easier in lunar gravity. It could handle receiving and sending from practically any target in the solar system to anywhere around the lunar equator.

Obviously this assumes a future where such trade exists. A maglev track around the lunar equator that can be even less lifting than ones on earth really isnt that big a project once the moon has the GDP of a small country on earth.

I imagine incoming spacecraft would pick up and leave cargoes in the lowest practical lunar orbit and dedicated maglev carriages would handle stationary on surface<-->orbital speed attached to track<-->lowest practical lunar orbit transitions.

A bit more on topic, I also posted a silly idea here a while back about a landing strip of super fine lunar dust. The incoming vehicle would not touch the dust directly but aim for a sort of air-cushion effect of dust bouncing between surface and a special shield on the bottom of the craft. I prefer going straight to the maglev rather than fiddling around to get this working.

How hard would the ISRU for the maglev be? Could you build something from just big lumps of iron that would give enough lift to a well designed carriage? I was hoping that what was on the surface could be fairly low tech, for slowing down if not launching. Come to think of it perhaps you don't even need magnetic lift. You could squirt some volatiles out beneath some sleds just to avoid actual metal on metal.

Offline Alf Fass

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Re: Landing on the Moon using a tail hook?
« Reply #40 on: 06/30/2014 07:44 pm »
An idea more amusing than practical would be the theoretical possibility of landing from orbit on an ice field, with the friction between the ice and lander creating a vapor cushion (a bit like steam insulating a water drop from a hot plate, Leidenfrost effect.
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Offline colbourne

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Re: Landing on the Moon using a tail hook?
« Reply #41 on: 11/16/2014 05:44 am »
Rather than attempting to hook a cable, an alternative would be to drag a cable from the space craft. I dont actually see this as a sensible solution for manned craft as there are too many unknowns, but it may work to lesson the fuel requirements/impact  speed for freight.

By spinning the cable from the lander it could be positioned so that it will drag before impact.

Offline Moe Grills

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Re: Landing on the Moon using a tail hook?
« Reply #42 on: 11/16/2014 06:54 pm »
 OK! Jets coming in and landing on carriers do so at about, more or less, 150mph?

The fastest land vehicle with wheels did supersonic at about 800 mph. It slowed by chutes.

And as Jim pointed out, no air on the Moon worth spit, so you are in danger of landing HARD (to destruction?) on the lunar surface at low suborbital speeds.
OK! It is possible to fit a lengthy lunar lander with flanking compressed or liquefied-gas bottles and nozzles that shoot out gas downward or laterally and the expanding gases can apply some lifting force to a lift-body frame or foils on the lander. Think of a glider with its own air supply for lifting-force landing on a celestial body with no atmosphere.
Barely possible theoretically, but not practical since you add unwelcome mass with the air bottles, nozzles, lifting foils, etc.

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