Returning from the moon

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bdewoody

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Assuming that at some point in the future we will realise that the moon should serve as our earth orbiting space station how easy or difficult will it be for a manned vehicle returning from the moon to enter earth orbit instead of a straight re-entry? It seems to make more sense to me to build a vehicle that would transport crew and cargo from earth orbit to the moon and back to earth orbit and then transfer to a specialised ascent/descent vehicle to get to the ground. But to do this the returning vehicle from the moon would have to slow down to orbital velocity.
 
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Captain_Salty

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im guessing the extra fuel you'd have to carry to slow down would be too prohibitive. I don't know if they were planning a straight re-entry on return from mars without some braking burns.
 
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Astro_Robert

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I guess it must be a slow Monday if some of the more technical folks have not commented on this.

My understanding is that since Lunar surface gravity is so much lower than Earth's (~ 1/6th I believe) that it takes less energy to get to low Earth orbit from the Moon than it does from Earth. However, I just have vague memories, and no hard numbers to back this up.

As far as a comparison to a more direct re-entry, I believe that would place extra thermal stress upon the heat shield due to higher incoming velocity. It is my understanding that the astronauts returning from the Moon usually had an Earth Orbit Insertion manuever to bleed off some of this velocity so as not to cook themselves during re-entry.

As far as my attempt at a soft answer I believe the difference is that a transfer to a low Earth orbit station/vehicle would be comparable in terms of mass of fuel/heat shielding trade off verses having a single vehicle go from Lunar Surface thru re-entry. From a vehicle design standpoint it may even be easier to stop at low earth orbit if there is enough traffic to warrant development of such infrastructure.

I think that if we do wind up with serious traffic to and from the Moon then your idea would bear fruit.
 
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MeteorWayne

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It would require quite a bit of propellant to return from the moon to LEO. Right now, we don't even have the ability to get a manned craft to the moon. It requires no propellant for a ballistic reentry of a capsule. As for a flying brick, that remains to be seen when one is designed for the trip.
 
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bdewoody

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I was kind of thinking that since escape velocity is lower leaving the moon a returning vehicle would not have to be going all that fast on a return trip and could possibly do some aero braking to slow to earth orbital velocity.
 
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MeteorWayne

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It's all gravity my friend. Even if your speed relative to the earth is 1 mm/yr at the start, when you hit the atmosphere you are travelling 11.2 km/sec (25,000 mph). Whereas from earth orbit, it's only ~ 7 km/sec (17,500 mph). And since ke=1/2m*(v^2) that speed diffence means 2.5 times the energy to disappate.

Wayne
 
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Astro_Robert

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Yes there is a serious fuel cost to slow down for orbit, but there are always tradeoffs. My understanding is that the Apollo craft (and by extension most future craft) performed a manuever to slow down and adjust their re-entry angle prior to hitting the atmosphere on their return trip.

The Apollo vehicle had to be structurally robust enough to withstand re-entry g-forces and stress levels. It had to have robust (ablative) thermal protection systems to protect from the heat of re-entry. It had to have a parachute system. It had to have certain redundant systems and floatation devices. There are probably more things as well. All of these things take mass, which could have been used instead for propellant to slow down to a stable orbit and rendevuos with a landing craft.

As to how exactly the mass balance works out I don't know, but I don't believe that a ballistic re-entry from a Lunar return is necessarily better in all cases. Carrying a little bit of extra fuel vs a lot of structure and thermal protection system mass could be a favorable trade.
 
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MeteorWayne

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Well, it's not a little bit of extra propellant, it's a lot. You need to lose 6500 mph, the same as you gained to go from LEO to the moon. I recall it was a pretty long burn on the way out.

And IIRC, the Apollo made no maneuvers to slow down on the return home, after the burn behind the moon for the return trip, they were ijn free fall. They did of course adjust their trajectory to refine the entry angle, since it was critical, but not to slow down.
 
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MeteorWayne

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I need to correct myself. Of course, since the moon as well as the spacecraft are both in the earth's gravity well, you don't need to reach escape velocity of 11.2 kps (25,000 mph) on the way out, and you don't have that much speed coming back.

Edit: This is bad info...see later post- MW --->>>(FWIW, the TLI burn from earth orbit to head to the moon, using the S-IVB, lasted about 6 minutesand increased the speed by ~ 113 mph. The TEI burn on the return home, using the CSM Engine lasted ~ 200 seconds and only increased the velocity by 40 mph.)
 
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kg

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MeteorWayne":1a8tbbf3 said:
FWIW, the TLI burn from earth orbit to head to the moon, using the S-IVB, lasted about 6 minutesand increased the speed by ~ 113 mph. The TEI burn on the return home, using the CSM Engine lasted ~ 200 seconds and only increased the velocity by 40 mph.
0 to 113 mph in only 6 minutes?...
Did they need to burn any fuel to insert themselfs into lunar orbit?
 
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MeteorWayne

Guest
No 17,500 (remember they were in orbit) to 17,613 in 6 minutes. Edit: Acutal velocity after TLI was 24,545.6 mph. Searching for some discrepencies here.

And yes, they had to burn propellant to slow down to be captured into lunar orbit. The trajectory they were on was what is called free return, which means they would have passed behind the moon and returned to earth had they not slowed down. I'll try and find details on that burn. Here they are: The engine burned for 358 seconds to slow the CSM/LM down to be captured into lunar orbit; an 11 second burn was done later to circularize the orbit.

(Just to nitpick, the proper term is propellant, not fuel, since fuel is only half the equation, the oxidizer is the other part, and you have to carry both...with respect to shuttle_guy for pointing that out so often)
 
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MeteorWayne

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OK, here appear to be the correct numbers. I think I've found the most authoritative source (also possible I misread some units or converted them incorrectly previously)

In earth orbit, the velocity was 17,432.6 mph. After the 347 second TLI burn by the Saturn IVB the velocity was 24,236 mph for a gain during the burn of 6803 mph.

At the moon, the first burn was 358 seconds, the circularizing burn 11 seconds.

The burn to head back to earth was 151 seconds and left the CSM with a velocity of 5856 mph.

Only one correction was made during the coast back to earth lasting 10 seconds and changing the velocity by 3.3 mph.

At atmospheric contact, the velocity was 24,678 mph.
 
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bdewoody

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But assuming a vehicle being used after a lunar outpost is manned and operating with the capacity to manufacture and transfer fuel there for the return to earth in the long run it would be more economical to keep said vehicle in space and the amount of fuel required would not take up it's entire useful load.
 
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EarthlingX

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I will try with some simple logic, will see how bad it works :
- to get from Earth to Moon orbit you need some amount of energy, which is probably very close, if not the same as amount to get back;
- in Moon orbit transfer propellant, brought there by a slow boat, and you have it enough to get back to where you came from;
- if lander is brought to the lunar orbit by a slow ride, even less mass to get there fast;
- same works for the way back, no lander, just transfer vehicle, with much less needed propellant than it would need if it would have everything included;
- transfer vehicle can be control-habitat-propellant-propulsion, like Orion-lite, BA-330, tanks, and one of the upper stages with restartable engines;
- it can be parked at ISS for maintenance works and re-fittings;
- only lander gets down, comes back empty, or with the next crew.
 
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pathfinder_01

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bdewoody":q0vy2f4k said:
But assuming a vehicle being used after a lunar outpost is manned and operating with the capacity to manufacture and transfer fuel there for the return to earth in the long run it would be more economical to keep said vehicle in space and the amount of fuel required would not take up it's entire useful load.
Depends on how the trips are conducted.

Electrical propulsion methods are slow but efficient. On a lunar trip they are too slow for use of people (i.e. 3 weeks to 1 year (or more) is too much time esp. in the Van Allen belts.). A vasmir rocket could carry all the propellant it needs for multiple trips back and forward to the moon. The other hassle is they require big power sources (i.e. big solar panels\nuclear).

There are also some low delta V methods for chemical rockets of getting to the moon (not sure about from). However the trip might take 100 days.

The big problem is propellant. The best propellants in terms of ISP are cryogenic and will boil off. The Saturn V of yore only had 4 hours before it would lose too much propellants. Ares V would have used a blanket to up the amount of time it could stay in LEO to 5 days. If the boil off problem is solved it become possible to store higher ISP propellant in space. The best propellant in common use is lox\loh. It is probably possible to keep lox in space (oxygen liquefies pretty easy), but Loh is the problem. Methane could be easier to store in space and can be made at mars but it is a middling in terms of ISP and thrust. Less potential ISP than lox\loh and less thrust than lox\kerosene so no earth rockets use it. Kerosene could be a good propellant in terms of storability, but it cokes up the engine.

There are also hypergolic propellants. They are space storable, but give considerably less performance than the cryogenics (meaning your craft will require more mass of propellant to do the same thing).
Of course ISP and thrust are properties of the engine (not the propellant) and the mass of other things required to use said propellant (i.e. insulation for fuel tanks) can detract from performance.

So if you have enough propellant on both ends it becomes possible to travel back and forth. However getting tons of propellant to the other end is not simple.
 
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bdewoody

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This ain't Mars I'm talking about it's the Moon. 100 days to the moon, I could get there quicker using a solar sailing vessel.
 
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pathfinder_01

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bdewoody":3r4ubx8c said:
This ain't Mars I'm talking about it's the Moon. 100 days to the moon, I could get there quicker using a solar sailing vessel.
Think about it this way. The Saturn V third stage needed 119 tons of propellant or so to get to the moon. Also if it was not pushing so much cargo(the CM|LM) it might be possible to go to the moon with less proplent. Now this is over simplistic as the third stage usually just crashed into the moon a few weeks after the mission but remeber it also went about the same distance. The smart move might be to park such a craft in an Lagraine point or stable high earth orbit so that you use a smaller craft to get in and out of orbit or land the moon, but if you could refuel it then it should be able return.

If you could move the propellant without people you could refuel and return to LEO.Who cares how long it took the propellant to get there so long as it is there. The LOW delta V methods are to make sure you don't use way more propellant getting there than you can send. (I.e. those 119 tons pushed 44 tons of cm\lm (its cargo)). With chemical the best you could do is an almost 1:1 ratio of proplent to cargo using those methods.


Another way would be a hybrid approach. Use disposable capsule to get to mobile space station. The station could have better life-support, hold more supplies, and be a lot more functional than a capsule. The station could be moved back to LEO for servicing without the crew. The crew could leave ala Apollo and let the spacecraft take its sweet slow time home.
 
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kelvinzero

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Almost on topic,

I quite like the idea of an earth-moon cycler.

http://www.moonminer.com/lunarcycler-orbits.html

This does not solve how you stop, but it means that for no additional fuel cost, your dinky little capsule can dock with a luxurious and heavily shielded hotel for the journey.

Also, in theory you could combine the cycler idea with a tether or mass driver, thus requiring no propellent to take something from LEO to LLO and back again.
 
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EarthlingX

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This would complicate resupplies, easier with Lagrangian points, i think. You also need more than one to make it functional, maybe some day.

Further destinations make a bit more sense, like Venus/Mars cycler, and i would not start with it - again - later.

Buzz Aldrin has a Mars cycler plan, plenty of links to various pdfs :

buzzaldrin.com : Aldrin Mars Cycler
 
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js117

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bdewoody":36v1im5m said:
Assuming that at some point in the future we will realise that the moon should serve as our earth orbiting space station how easy or difficult will it be for a manned vehicle returning from the moon to enter earth orbit instead of a straight re-entry? It seems to make more sense to me to build a vehicle that would transport crew and cargo from earth orbit to the moon and back to earth orbit and then transfer to a specialised ascent/descent vehicle to get to the ground. But to do this the returning vehicle from the moon would have to slow down to orbital velocity.
This would not require no heat shield to do this, only the descent vehicle would.
 
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kelvinzero

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EarthlingX":3q53vn3z said:
This would complicate resupplies, easier with Lagrangian points, i think. You also need more than one to make it functional, maybe some day.

Further destinations make a bit more sense, like Venus/Mars cycler, and i would not start with it - again - later.

Buzz Aldrin has a Mars cycler plan, plenty of links to various pdfs :

buzzaldrin.com : Aldrin Mars Cycler
Well I disagree, but it is not a big issue for me.

Firstly you do not need more than one to make it functional. Some tether ideas require more than one but this is just a large object in an orbit that carries itself between the earth and the moon say once a month with no additional fuel cost.
A tether could be added to this cycler to sling objects from the cycler into an orbit, but that is a bonus for later. Lets forget the sling and just discuss the advantages of a cycler plus small vessel over just the same small vessel, as used by Apollo.

A monthly launch window is no great hassle and the advantages are huge.

* A better space hotel. Even before considering actually landing on the moon, a cycler seems like a natural step up after after LEO space hotels becomes practical but before tourists actually start landing on the moon. Its always good if you can start making money developing one requirement at a time.
* Relatedly, of course when you actually do start going to the moon commercially, This will be vastly more comfortable than going in a cramped apollo style command module.
*radiation protection. Small vessels are currently vulnerable to solar flares. Maybe we will solve this but this is not proven yet. Currently visiting the moon is a bit of a lottery.
*A cycler would take you to the moon slower, say a couple of weeks. This might sound like a disadvantage but I think it means cheaper, because less speed means less propellent, cheaper rockets and so on.
*many more redundancies for safety. The small vessel has to be as light as possible, obviously. However the cycler can contain redundancies of everything, even spare small vessels and their fuel. If a small vessel is crippled by a malfunction, or a rocket failure puts them on a bad orbit, instead of an apollo-13 scenario, the cycler would probably be able to rescue them with ease, and even more likely to at least send supplies one way.
*additionally, the small vessel itself is a redundancy. If you fail to dock with the cycler, you are not in a much worse situation than you would have been on any given apollo style trip. The slower velocity probably makes a turnaround more practical also.



Forget the tether/mass driver aspect for now
 
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JonClarke

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Nearly all of the dV needed to enter Earth orbit on lunar return can be achieved by aerocapture.
 
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