Author Topic: Just F9R, SEP tugs and propellant depots.  (Read 16164 times)

Offline KelvinZero

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Just F9R, SEP tugs and propellant depots.
« on: 07/11/2014 10:49 am »
Design a lunar architecture around the assumption of a F9R reusable first stage, SEP tugs and propellant depots.

This, SEP and depots seem to work very well together. You can refuel in LEO and again in LLO with fuel you have moved there efficiently with SEP months in advance.

Im not going to propose anything like a complete architecture in the OP, mainly I was interested in the scale that was possible within these limitations. It seems to me to be extremely large. The scheme is to keep the F9R first stage as is but have a modified second stage that could be refueled in LEO and LLO, and act as an EDS and lunar lander.

By my working you could land a 50+ ton payload from LLO to the lunar surface, if you could refuel something with the performance of a F9v1.1 second stage there.

This is using some figures I grabbed off the internet:
F9 v1.1 upper stage:
empty: 6t
propellant mass: 93t
burn time: 375s
ISP: 342s
lunar gravity: 1.622m/s^2
LLO 1.87km/s

(my guess was gravity loss = burn time * lunar gravity = 608m/s)
(edit: apparently a huge overestimate, I guess 200m/s in later examples)

so total necessary delta v = LLO+gravity loss = 2480m/s

using deltaV = ISP*g(earth)*ln(m0/m1) I get:

m0/m1 = 2.09

Anyway, assuming that m0 - m1 = 93t (the propellant) and that the 2nd stage mass is 6t I get a value of 57 tons 78 tons of useful payload.

(edit: corrected a basic algebra error above I think)

Given the above it seems totally reasonable to pursue some sort of lunar architecture with just what you can put on top of a F9R, maybe using a F9H for some large indivisible 50 ton cargo that you have to get up to LEO.

------------- Below this line may be edited frequently: -------------



Here is my first thought of how this lander could be layed out. Other people may have better ideas:

(1) The basic lander is shaped like a F9 upper stage, with 10 tons (potentially 11) for a fairly conventional Dragon v2 on top, lunar landing legs etc. (I hope that is sufficient)

(2) This is rather tall and thin, but it has oversized landing gear creating a bit of a pyramid shape when extended.

(3) The oversized landing gear is justified because in general, in addition to the crew and passengers, it can carry a lot of cargo one way. This cargo is attached to the landing legs around the base of the rocket in either 2 or 4 modules, so they are right at the lunar surface when landed. This cargo could be attached at the LEO depot if perishable, or at the LLO/ EML2 depot to exploit slower, more efficient transport for cargo.

(possibly the landing gear stays behind when the vehicle ascends from the moon)



Here is my attempt at a more rounded architecture, ie including crew and cargo and not one way.

Working backwards:

Returning home (moon surface --> earth surface)
I am using direct return from the lunar surface to the earth's surface. What returns is a fairly standard Dragon v2 and the modified second stage.
Im using 6 tons for the second stage, and 10 tons for the dragon v2, which I hope is a healthy over estimate including draco fuel and a bit of returned cargo etc.
So 16 tons are returned to earth,
Im using 2.8 for the deltaV, and 342 for ISP.
I calculate 20.8 tons for the fuel.

(NOTE: no fuel for propulsive landing of upper stage on earth budgeted for. Potentially, being unmanned, it can aerobrake into LEO and be refueled in order to land and be refurbished. The Dragon returns directly to earth.)

EML2 --> Moon surface
So the lander must have that much fuel once it reaches the lunar surface. Im assuming full refueling at EML2, implying 93-20.8 = 72 tons of fuel can be used in landing.
Im using deltaV from EML2 to Moon surface = 2520m/s + 200m/s gravity loss = 2720m/s
I get 57.6 tons landed, including the mass of the stage (6t), the dragon etc (10t), the fuel for return (20.8t).



Conclusion: 20+ tons of one-way cargo can be sent from EML2 to the lunar surface, in addition to what ever cargo the dragon carries and returns.

LEO --> EML2
Im choosing to send the vehicle with a full tank and no additional cargo, and arrive with fuel to spare.
(we could also have sent it with the 20 tons of cargo attached and still have a bit of fuel to spare)
deltaV = 3.45
m0/m1 = 2.8
m0 = 93(fuel)+16(2ndStage+ dragon etc) = 109
m1 = 109/2.8 = 39
spare fuel on arrival= 39 - 16 = 23 tons.

(Oops!  probably needed deltaV=4.0 to keep travel time to 5 days, reducing that spare fuel to 17t. This doesn't apply to the unmanned portion, which could perhaps have been even a bit lower.)

Some missions:
(A) All Chemical, A dragonfull of crew and 20 tons cargo to the lunar surface
= 414 IMLEO + lander launch

(B) All Chemical, Dropping the 20 tons cargo, just sending dragonfull of crew.
= 257t IMLEO + lander launch

(C) Same as (A) but exploiting the ARM SEP tug for unmanned portions
= 217t IMLEO + lander launch

(D) Magical Asteroid ISRU fairy
= 20t IMLEO (thats the lunar cargo) + Lander launch.
« Last Edit: 07/15/2014 05:23 am by KelvinZero »

Offline Nilof

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #1 on: 07/12/2014 08:47 am »
Making a lunar ferry and lander from an already flight-proven upper stage and using depots is a very viable strategy. Masten Space System has been doing some work on a centaur-derived lander called XEUS.

I thought I'd add a couple notes on orbits:

-Nodal precession makes it impossible to permanently align the orbit of a LEO depot with the orbit of the moon. So using a permanent LEO depot as a staging point for cis-lunar missions means you will have to eat a delta-v penalty for the inclination change. Since you can do it near apoapse, i.e. include it in the lunar capture burn, the delta-v cost isn't too high but doing that will limit the number of launch windows to two per month. This can be circumvented entirely by using temporary staging points where the transfer stage is being refilled directly by tankers launched from the ground.

- Most low lunar orbit are not stable due to lunar mass concentrations, and will decay very quickly. There are exceptions though, with four "frozen" low lunar orbits at inclinations 27º, 50º, 76º, and 86º which are stable. Unless you went for the 86º one and you are close to the poles, you will have two launch windows per month. You can solve the problem of infrequent launch windows by using L1 or L2 as staging points, at the cost of increased delta-v distance between the lunar surface and the depot. So another tradeoff.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #2 on: 07/12/2014 09:59 am »
Yeah my sole reason for using LLO was to get the most impressive sized single object down to the lunar surface, 57 tons, slightly more than you could get to orbit with a FH anyway. There are all sorts of other reasons to avoid aiming for this number. But it is nice to know what chunks you can handle without a bigger launcher.

Hey this is embarrassing.. I think I might have messed up with some basic algebra at the end. Is it more like 78 tons?

Given m0/m1 = R and..
m0 = Mc+Mp, (mass landed, mass of propellant) and..
m1 = Mc, (just mass landed)
then Mc = Mp/(R-1) = 93/(2.1-1) = 84 tons landed
..giving 78 tons of cargo, assuming 6 tons for the F9v1.1 upper stage.

Someone really has to check my math! Not just this bit either.

Online sdsds

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #3 on: 07/12/2014 03:32 pm »
It's fascinating to look at what could be accomplished with a specialized second stage lofted by the F9R first stage. But I wonder: since the second stage is an integral part of the launch vehicle, is this something that only SpaceX could do? After all, SpaceX doesn't offer suborbital F9R launches....
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Offline gbaikie

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #4 on: 07/12/2014 06:37 pm »
Design a lunar architecture around the assumption of a F9R reusable first stage, SEP tugs and propellant depots.

This, SEP and depots seem to work very well together. You can refuel in LEO and again in LLO with fuel you have moved there efficiently with SEP months in advance.

Im not going to propose anything like a complete architecture in the OP, mainly I was interested in the scale that was possible within these limitations. It seems to me to be extremely large. The scheme is to keep the F9R first stage as is but have a modified second stage that could be refueled in LEO and LLO, and act as an EDS and lunar lander.

By my working you could land a 50+ ton payload from LLO to the lunar surface, if you could refuel something with the performance of a F9v1.1 second stage there.

This is using some figures I grabbed off the internet:
F9 v1.1 upper stage:
empty: 6t
propellant mass: 93t
burn time: 375s
ISP: 342s
lunar gravity: 1.622m/s^2
LLO 1.87km/s

(my guess was gravity loss = burn time * lunar gravity = 608m/s)

so total necessary delta v = LLO+gravity loss = 2480m/s (roughly) ?

One should avoid such a large gravity loss in regards to the Moon.
Just as guess it should be about 100 m/s.
LLO orbit is not 1.87km/s.
Spacecraft LRO:
"LRO orbit is nominally 50 km, polar, with 2 hour period. Orbital velocity is 1.6 km/sec."
http://ilrs.gsfc.nasa.gov/docs/2007/lrolr_mcgarry_0709.pdf
I suppose one could have perigee velocity at 1.87km/s if it's highly elliptical.
In Apollo the delta-v used to go from LLO to surface was about 1.8 km [that includes gravity loss].

Generally to have lowest gravity loss leaving or landing on the Moon, the zero to 1 km/sec velocity must adequate acceleration. Or with leaving earth the zero to say 4 km/sec must have adequate acceleration.
Or this in the range where rockets generally have most the their gravity loss.
Or if landing on the Moon, if lands with low and constant thrust one will have excessive gravity loss.
So generally, at LLO, you do burn that puts the perigee intersecting surface, and as get near to crashing, one applies thrust so as to avoid hitting the surface a high velocity.  So ideally or excessive in terms of lowering gravity loss [which isn't a big deal] one have very high thrust- say 20 meter/sec/sec.
So say 1600 m/s divided by 20 is 80 seconds and after the 80 second you end up less than 1000 meter above the surface, and this in less than 1000 meter above the surface one makes up the remaining velocity one must lose.
So that is path have less than 100 m/s of gravity loss- but it would be fairly scary the more one wanted to reduce gravity loss. And leaving the Moon means one has start at least 5 m/s/s acceleration. I believe with Apollo was around 1 gee acceleration [9.8 m/s/s].
Or if you only accelerate at 5 or less meter per second per second, one will have a significant amount of gravity loss.
Now what you doing is using a second stage, and thrust needed on any second stage does not need to be high, to prevent gravity loss- one does have much gravity loss leaving earth after one gotten to around 4 km/sec. Plus you putting a large payload on it.
So one could modify it, by increasing engines- say 3 engines instead of one.


« Last Edit: 07/12/2014 06:52 pm by gbaikie »

Offline Burninate

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #5 on: 07/12/2014 08:12 pm »
It's fascinating to look at what could be accomplished with a specialized second stage lofted by the F9R first stage. But I wonder: since the second stage is an integral part of the launch vehicle, is this something that only SpaceX could do? After all, SpaceX doesn't offer suborbital F9R launches....
A second stage with no payload would possess 0.9 TWR.  This is prohibitive for, say, launching straight up from the pad.  The second stage's thrust would be outweighed by gravity for a while, though I think it would be able to get to positive TWR before it actually reached apex, after it's drained some fraction of the tank.

However - Lop the second stage off and attach a payload directly to the first stage*, running it until it's dry, and suborbital flights work just fine for extremely large payloads.

This is a configuration that *might* make some sense for very heavy F9H flights to LEO.  If the second stage dry mass is not so much of an issue due to the enormous bulk of the payload, and thrust is scarce, then the central core could act as the stage that gets the payload to orbit.

*Obviously this would require some amount of additional engineering.

EDIT: An SLS-class payload, 70T, would get 7658m/s out of this arrangement at 300s Isp.  A 60T payload, 7986.  A 50T payload, 8361m/s.  This is close enough that if the payload already has a sizable engine for other purposes (EDS?), it could provide the last kick without requiring a high TWR.

EDIT2: You could also simply fly the second stage half-full.
« Last Edit: 07/14/2014 07:19 am by Burninate »

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #6 on: 07/12/2014 11:31 pm »
It's fascinating to look at what could be accomplished with a specialized second stage lofted by the F9R first stage. But I wonder: since the second stage is an integral part of the launch vehicle, is this something that only SpaceX could do? After all, SpaceX doesn't offer suborbital F9R launches....
Hi sdsds, I guess what is meant is we have already looked at shoestring approaches with modified dragon etc. I was interested in what we could do with a properly designed architecture BUT limited to what we can fit on top of a F9R. It is not so much about getting to the moon faster but imagining a world where the F9R first stage is a proven article, along with SEP and depots. So we are not limited to the upper stage in any way. I feel safer sticking to the performance of the upper stage since it obviously does fit on top of the first stage and I can find numbers.

It looks like in principle you can land extremely large objects, assuming one way and unmanned. I imagine assembling entire 70 ton lunar outposts in LEO, perhaps a donut shape around the base of the upper stage, then tugging the whole thing to lunar obit, refueling and depositing in one go.

A more reasonable goal would probably be to consider modules we can launch in one go. Since the upper stage may never return to earth, or could be refueled in orbit by reusable upperstages, I think I heard somewhere this could be only 15% smaller than current F9v1.1 payloads, so around 11 tons? That could pretty much all be payload so it can far exceed the apollo ascent vehicle that was around 4.5 tons including ascent rockets and fuel.

Offline Alf Fass

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #7 on: 07/12/2014 11:58 pm »
That's a thumbs up from me.
When my information changes, I alter my conclusions. What do you do, sir?
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Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #8 on: 07/13/2014 12:33 am »

One should avoid such a large gravity loss in regards to the Moon.
Just as guess it should be about 100 m/s.
LLO orbit is not 1.87km/s.
Spacecraft LRO:
"LRO orbit is nominally 50 km, polar, with 2 hour period. Orbital velocity is 1.6 km/sec."
http://ilrs.gsfc.nasa.gov/docs/2007/lrolr_mcgarry_0709.pdf
I think you are right. I found that 1.87 with a random google. Wikipedia suggested 1.6, In the solar sail thread Jim posted something that I think said 1.72 or 1.73, so I guess my number is very wrong and we can land even more.. but I dont know how to get such objects into LEO in the first place without lots of fiddling, so I will just leave it as we are not limited by what we can transfer from LLO to the lunar surface.

(edit: here is where I got the 1.87 figure: http://en.wikipedia.org/wiki/Delta-v_budget  )
(edit: here is another wiki page suggesting 1.6  )

I got the gravity loss by multiplying the published burn time for the upper stage by lunar gravity. That is the right approach right? I was just guessing there. I imagine this is an extreme case where you are landing a fully fueled upper stage one way. I guess burn time would look only half as bad if you were landing a smaller payload but reserving enough for returning to orbit. You would be splitting the burn time over descent and ascent.
« Last Edit: 07/13/2014 01:27 am by KelvinZero »

Offline Alf Fass

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #9 on: 07/13/2014 01:53 am »

I got the gravity loss by multiplying the published burn time for the upper stage by lunar gravity. That is the right approach right? I was just guessing there. I imagine this is an extreme case where you are landing a fully fueled upper stage one way. I guess burn time would look only half as bad if you were landing a smaller payload but reserving enough for returning to orbit. You would be splitting the burn time over descent and ascent.

Far too pessimistic, gravity loss leaving Earth is usually less that 1km/s, even though burn times around 500 seconds or more.

The sooner you can switch away from vertical accent the lower gravity loss. Cant say for sure but I'd expect loss of much less than 200m/s
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Offline Nilof

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #10 on: 07/13/2014 02:31 am »
Yes, since the moon has no atmosphere you can skip the vertical ascent phase entirely. You just need enough altitude to turn the lander and at that point a lander capable of 1g acceleration will only need to be pointed 9.5 degrees (arcsin(1/6) ) above the horizon to cancel out gravity. Cos(9.5 deg) = 0.986 so you would only need lose 1.5% of your delta-v to gravity drag at most, and that is an upper estimate since you can point your spacecraft closer to the horizon as you gain speed.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #11 on: 07/13/2014 03:28 am »
Haha.. with my bad algebra (57 should have been 78) high lunar gravity (1.87 could be as low as 1.6) and gravity loss overestimate we are probably getting closer to a round 100 tons one way. Two Falcon heavy launches of just payload.

Roll on the day we are landing two 50-ish ton diggers with each mission, but not today.

Here is my first thought of how this lander could be layed out. Other people may have better ideas:

(1) The basic lander is shaped like a F9 upper stage, with up to 11 tons for crew and two-way cargo on top. Possibly this is in the form of a Dragon and trunk, or otherwise crew could board only when it reaches the LEO depot.

(2) This is rather tall and thin, but it has oversized landing gear creating a bit of a pyramid shape when extended.

(3) The oversized landing gear is justified because in general, in addition to the crew and passengers, it can carry a lot of cargo one way. This cargo is attached to the landing legs around the base of the rocket in either 2 or 4 modules, so they are right at the lunar surface when landed. This cargo could be attached at the LEO depot if perishable, or at the LLO/ EML2 depot to exploit slower, more efficient transport for cargo.

(possibly the landing gear stays behind when the vehicle ascends from the moon)

Offline gbaikie

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #12 on: 07/13/2014 03:44 am »

One should avoid such a large gravity loss in regards to the Moon.
Just as guess it should be about 100 m/s.
LLO orbit is not 1.87km/s.
Spacecraft LRO:
"LRO orbit is nominally 50 km, polar, with 2 hour period. Orbital velocity is 1.6 km/sec."
http://ilrs.gsfc.nasa.gov/docs/2007/lrolr_mcgarry_0709.pdf
I think you are right. I found that 1.87 with a random google. Wikipedia suggested 1.6, In the solar sail thread Jim posted something that I think said 1.72 or 1.73, so I guess my number is very wrong and we can land even more.. but I dont know how to get such objects into LEO in the first place without lots of fiddling, so I will just leave it as we are not limited by what we can transfer from LLO to the lunar surface.

(edit: here is where I got the 1.87 figure: http://en.wikipedia.org/wiki/Delta-v_budget  )
(edit: here is another wiki page suggesting 1.6  )
I would say, it can't be much less than 1.6 km. 
Or 1.6 km/sec is low end of that which is possible.
The wiki number of 1.87 is simply wrong- or I don't know how or where they are getting it- as wild guess maybe it from a lunar trajectory from Earth and including some braking. Or I mean, a sort of powered capture type orbit which would have is lowest point at low lunar orbit distance which then immediately descent to surface [not done before, as far as I know].

Here they are suggesting it's about 1.6 km/sec:
http://forum.nasaspaceflight.com/index.php?topic=29195.0
Here's a more detailed analysis, indicating an aspect of landing one moon require a period of time to allow pilots to see the terrain they going to land on:
http://web.mit.edu/digitalapollo/Documents/Chapter8/lunarlandingsymposium.pdf
E.g:
Summary of Braking Phase -
"The braking phase, lasting about 450 seconds, covers
some 243 nautical miles during which the velocity is reduced from 5500 ftlsec.
to approximately 600 ft/sec., and the altitude from 50,000 feet to about 9,000 feet.
The attitude during the phase is normally such that the thrust vector is close to being aligned
with the flight path angle. In this attitude, the pilot is not able to look in the direction of the
intended landing area. "

So in meters:  5500 ft is 1676.4 meter and 4900 feet is 1493.52 meters per second.
So from orbit, 1493.52 m/s delta-v gets to about 3 km above surface and going at velocity 600 ft/sec [182.88 m/s]. So that's so pilots can see where they are going.

I also recalled they changed the trajectory somewhere around Apollo 14 so it was more efficient [or at least, better in some way].
And if one going to location one had previously gone to before and had things like landing beacons, one could use less delta-v [one would know exactly where one was going to and not need to look where to land].
Quote
I got the gravity loss by multiplying the published burn time for the upper stage by lunar gravity. That is the right approach right?
Well, it gives the most it could be, but basically, no.
Though it does sort of indicate the problem with using Falcon upper stage with one engine.

Now, perhaps, if you wanted to use falcon upper stage with one engine, one could probably use as a stage. It could use low thrust to get about 1/2 way to the surface, and with the final stage having higher thrust to weight ratio. So final stage only needed about 1 km/sec of delta-v. And falcon-9 handling the .6 km + delta-v needed, and then falcon 9 upper stage separates and returns to be re-fuel at LLO.

So with it's total burn time of 375 second, one use around 300 second to descent and 75 second to return to orbit- just as SWAG. And/or have shorter tank or more rockets.
Though it could be too complicated.

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #13 on: 07/13/2014 03:51 am »
One question is if the the upper stage is sufficiently throttle-able. I could imagine some super dracos being exploited at the last moment, even what is essentially a dragon v2 (possibly minus heat shield) on top. This might provide additional thrust at end plus interesting abort scenarios if the main engine fails.

---

Ok here is my attempt at a more rounded architecture, ie including crew and cargo and not one way.

Working backwards:

I am using direct return from the lunar surface to the earth's surface. What returns is a fairly standard Dragon v2 and the modified second stage.
Im using 6 tons for the second stage, and 10 tons for the dragon v2, which I hope is a healthy over estimate including draco fuel and a bit of returned cargo etc.
So 16 tons are returned to earth,
Im using 2.8 for the deltaV, and 342 for ISP.
I calculate 20.8 tons for the fuel.

So the lander must have that much fuel once it reaches the lunar surface. Im assuming full refueling at EML2, implying 93-20.8 = 72 tons of fuel can be used in landing.
Im using deltaV from EML2 to Moon surface = 2520m/s + 200m/s gravity loss = 2720m/s
I get 57.6 tons landed, including the mass of the stage (6t), the dragon etc (10t), the fuel for return (20.8t).

Conclusion: 20+ tons of one-way cargo can be sent from EML2 to the lunar surface, in addition to what ever cargo the dragon carries and returns.

(rats, just noticed an error: I did not include fuel for the 2nd stage to propulsively land on earth. It is possible it refuels in orbit I suppose. It is also likely that recovering the 2nd stage from the moon just won't be attempted initially)
« Last Edit: 07/13/2014 11:34 am by KelvinZero »

Offline Burninate

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #14 on: 07/13/2014 06:53 pm »
It's not clearly wrong.  Two things -
1) Gravity losses are undefined in this number, and are TWR-dependent.
2) "Low Lunar Orbit" is a subjective concept.  Stability and margin of safety increase at higher altitudes, with the Lunar mass concentrations causing considerable orbital perturbations.  It was only with the detailed gravitational mapping of the Moon, and computer-assisted discovery of 'frozen orbits' in the past decades which balanced out masscon influence, that a craft has been able to travel at anywhere near mountain-clipping altitude for long, and even then there's a thin atmosphere to deal with, akin to the Earth's atmosphere at ISS altitude.

WP defines LLO as anything under 100km altitude, and notes Lunar mountain ranges reaching up to 6km.  Apollo's parking orbit was 110x110, and it reduced periapsis to 15km only briefly for landing.

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #15 on: 07/13/2014 08:41 pm »
It probably came from Apollo. I decided to go with EML2 to be conservative. In any case I wouldn't put my depot in the lowest possible orbit. That would probably only be relevant if I had another stage that handled to and from the moon to squeeze the largest possible payload size out of the actual lander.

Most sources are probably implicitly or explicitly covering the Apollo lunar ascents. For those, this looks reasonably authoritative:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790072468_1979072468.pdf

It asserts for Apollo 14 the theoretical minimum would have been 6045.3 fps (1842.6 m/s) over ~430 seconds of flight time.

gbaikie also had a similar number for Apollo on this thread.
« Last Edit: 07/13/2014 08:45 pm by KelvinZero »

Offline gbaikie

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #16 on: 07/14/2014 03:26 am »
It's not clearly wrong.  Two things -
1) Gravity losses are undefined in this number, and are TWR-dependent.
2) "Low Lunar Orbit" is a subjective concept.  Stability and margin of safety increase at higher altitudes, with the Lunar mass concentrations causing considerable orbital perturbations.  It was only with the detailed gravitational mapping of the Moon, and computer-assisted discovery of 'frozen orbits' in the past decades which balanced out masscon influence, that a craft has been able to travel at anywhere near mountain-clipping altitude for long, and even then there's a thin atmosphere to deal with, akin to the Earth's atmosphere at ISS altitude.

WP defines LLO as anything under 100km altitude, and notes Lunar mountain ranges reaching up to 6km.  Apollo's parking orbit was 110x110, and it reduced periapsis to 15km only briefly for landing.

If was instead lunar orbit rather than lunar low orbit, 1.8 km/sec would be fine. But saying LLO and 1.87 km/sec is suggesting a certain degree of accuracy.
1.8 km/sec would be more accurate than 1.87 km/sec- or in other words, if one carries something to lower decimal point one is implying a degree of precision.

Offline jongoff

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #17 on: 07/14/2014 05:03 am »
Making a lunar ferry and lander from an already flight-proven upper stage and using depots is a very viable strategy. Masten Space System has been doing some work on a centaur-derived lander called XEUS.

I thought I'd add a couple notes on orbits:

-Nodal precession makes it impossible to permanently align the orbit of a LEO depot with the orbit of the moon. So using a permanent LEO depot as a staging point for cis-lunar missions means you will have to eat a delta-v penalty for the inclination change. Since you can do it near apoapse, i.e. include it in the lunar capture burn, the delta-v cost isn't too high but doing that will limit the number of launch windows to two per month. This can be circumvented entirely by using temporary staging points where the transfer stage is being refilled directly by tankers launched from the ground.

I'm not sure if this is correct. My understanding was that with most LEO depot options you get windows on average every 6-9 days, which is more like 3-5x per month. Even twice a month is plenty frequent for the near term... If it ever cramps your style too much, you can add another depot in the same inclination but with its RAAN offset by say 180 degrees.

~Jon

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #18 on: 07/14/2014 05:19 am »
I have a concern that my delta-v values for EML2 etc may assume unreasonably long travel times. Should I use different numbers for reasonable travel times? (This might also become a reason to use lunar orbit possibly)

I would also like some numbers to use for unmanned cargo. I know there are slower, more efficient trajectories to the moon.. were they perhaps about 25% cheaper? Im also really hazy. Im Hazy if SEP would help, or what numbers to use if SEP is assumed.

Currently we are looking at getting 93t of propellant, 20 tons of cargo, and the vehicle+crew (16t) to EML2, the last being sent without exploiting low energy trajectories or SEP.

Offline KelvinZero

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Re: Just F9R, SEP tugs and propellant depots.
« Reply #19 on: 07/14/2014 06:35 am »
I decided to paste the evolving plan into the OP.

Just added this step:

LEO --> EML2
Im choosing to send the vehicle with a full tank and no additional cargo, and arrive with fuel to spare.
(we could also have sent it with the 20 tons of cargo attached and still have a bit of fuel to spare)
deltaV = 3.45
m0/m1 = 2.8
m0 = 93(fuel)+16(2ndStage+ dragon etc) = 109
m1 = 109/2.8 = 39
spare fuel on arrival= 39 - 16 = 23 tons.

Assuming we send propellant by sending a full upper stage and delivering the remaining fuel,
100t in LEO --> 28t at EML2, and assuming the same ratio for cargo

So, so far we need
* 93 tons of fuel in LEO,
* 70 tons of fuel delivered to EML2 --> 250t in LEO
* 20 tons of optional lunar cargo delivered to EML2 -->71t in LEO

so thats about 414 tons in leo, or about 42 F9R launches (plus the lander+crew launch)
to deliver a dragonfull of crew and 20 tons cargo to the lunar surface, all elements potentially reusable.

(edit: dropping the 20 tons of cargo, that 70t becomes 46t and total IMLEO becomes 257t or 26 F9R launches, (plus the lander+crew launch))

The depots are pretty much equivalent to the upper stages so I think they are already included. In fact, since everything is chemical you probably dont need EML2 and could launch both the full lunar vehicle and the "depot" at once.. Or you could investigate SEP and possibly bring IMLEO down.

rats: should probably rework this with gbaikie's numbers:
deltaV of 4km/s for fast (5 day) to EML2
deltaV of 3.2 for slow (180 day) to EML2.
« Last Edit: 07/14/2014 10:57 am by KelvinZero »

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