OK, back to what I __really__ enjoy doing. Sorry this is so long, but there's a lot of stuff here.

**The Objective:**

The objective is to extend the lunar capabilities of the currently-conceived next generation of launch vehicles, specifically the Ares-V HLLV. The key to this objective is to reduce the number of Earth launches required for each lunar mission from the present 1.5 launches (1 Ares-I and 1 Ares-V) to a single Ares-V launch. The real goal here is to minimize the cost of each mission, thereby making lunar missions more affordable and thereby, hopefully, more pleasing to the eyes of the Congress.

The basic assumption is that we are able to harvest lunar oxygen, either from the common regolith or from water ice at one or both poles, and use it as a propellant for a reusable LS - LLO shuttle. I will start a separate thread later to study the possibilities that open up if it turns out to be feasible to harvest both lunar hydrogen __and__ oxygen.

**Mission Profile:**

- A single HLLV launch. The stack consists of the Booster stage using 2 5-segment SRMs, the Core stage using 5 RL-68's, the extended EDS using 1 J-2X, a full LH2 propellant tank for the Lunar Shuttle, a Cargo module destined for the lunar surface, and the Lunar CEV (4-person CM plus Lunar SM).
- The initial EDS burn delivers the payload to TLI on a direct-ascent profile.
- The second EDS burn inserts the payload into a co-planar lunar orbit. The EDS is expended.
- The reusable lunar shuttle leaves the lunar surface carrying its ascent propellants (LOX + LH2) plus the LOX required for the descent back to the surface. All LOX is derived from lunar resources.
- The shuttle enters LLO and performs whatever plane-change maneuver is necessary to rendezvous with the CEV.
- The Cargo pod and LH2 tank are transferred from the CEV to the Shuttle. [Alternate: if it turns out to be less risky to keep the same tank on the shuttle and transfer the fuel, then so be it. It makes no difference to the mission capacity.] The empty LH2 tank is expended.
- Presumably the crews swap vehicles.
- The SM does the TEI burn to return the Command Module back to Earth.
- The shuttle does another plane-change maneuver, de-orbits, and lands back at its base.
- The shuttle refuels with lunar LOX for its next trip.

**Capacities:**

The current Ares-V HLLV can deliver 65.4 metric tons (net) to TLI on a direct-ascent profile. "Current" is a moving target - I am using the version with 5 RS-68s, 2 5-segment SRBs, and an EDS with a single J-2X. As the booster evolves these numbers will change accordingly.

The direct-ascent profile delivers a higher mass to TLI than does the profile which includes a stop-over in LEO. Since this is a single-launch mission there is no EOR to carry out, so the LEO stop-over is unnecessary.

I am assuming the EDS tankage is extended to carry enough propellant for both TLI and a co-planar LOI. Adding a separate LOI stage would only marginally increase the net payload delivered to LLO and would necessitate development and crew-certification of yet another stage. I don't think it trades off well.

Insertion into a co-planar LLO requires a ΔV of 850 m/s. The mass ratio for such a burn using the EDS is 0.8247. Given a mass at LOI ignition of 89.4mT (65.4mT net + 24mT extended EDS dry mass including residual propellants), the total mass inserted into LLO is 89.4 X 0.8247 = 73.7mT.

The EDS is expended, leaving 49.7mT in LLO. This manifest is a full LH2 propellant tank for the Lunar Shuttle, a Cargo module, and the Lunar CEV (4-person CM plus Lunar SM). As currently conceived, the lunar CEV masses 20.7mT, which leaves **29mT **of LH2 plus tankage plus cargo in LLO waiting for delivery.

**The Lunar Shuttle**

The Lunar Shuttle is a single-stage reusable vehicle designed to depart the lunar surface with a full round-trip (LS-LLO-LS) load of LOX derived *in situ*, reach LLO, perform the necessary plane-change burn to match the CEV stack's co-planar orbit, pick up cargo plus a full round-trip (LLO-LS-LLO) load of LH2, perform the necessary plane-change maneuver to return to its base, de-orbit, and land. The shuttle should be designed to carry a crew of five to support the mission where the same pilot flies it to LLO with a crew of 4 returning to Earth, and returns to the lunar surface with a new crew of 4.

The baseline dry mass of the shuttle is 23mT. I am assuming this vehicle is a growth product of whatever the final LSAM turns out to be and not an entirely new vehicle. So for now I used the basic LSAM structure, cabin, engines, etc, eliminating only the separate ascent stage, and scaling the propellant tanks to the required size. These are place-holder numbers only. Later optimization trade-offs will include the number, size, and placement of the LH2 tanks, especially if tanks are physically swapped in LLO.

This mission requires a ΔV of 2,400m/s each way, assuming the worst case of a base at the Aitken Basin near the South Pole. Lunar bases elsewhere have lower ΔV requirements, which require less hydrogen, and therefore allow more cargo to be lifted from Earth. Based on this, the mass ratio for each half of the trip μ = 0.58.

Propellant calculations are more complex this time because we are adding mass at each end of the trip.

At departure from the lunar surface,

M

_{0}= M_{v}+ M_{ao}+ M_{ah}+ M_{do}

where M_{ah} is the mass of LH2 consumed during Ascent, etc.

Upon arrival in LLO,

M

_{1}= M_{v}+ M_{do }= μ • M_{0}

In LLO the shuttle acquires the cargo (Mc) plus the descent hydrogen (Mdh) and ascent hydrogen (Mah). The mass at ignition to depart the co-planar orbit then is:

M

_{2}= M_{v}+ M_{c}+ M_{do}+ M_{dh}+ M_{ah}

And, finally, the mass upon landing is:

M

_{3}= M_{v}+ M_{c}+ M_{ah}= μ • M_{2}

Solving simultaneously, and assuming Mv = 23mT, we find that for Mc = 18mT, we require a LH2 load of 10.2mT in LLO, for a total payload delivered to LLO by the Ares-V of 28.2mT, just within the 29mT limit we calculated above.

**Getting it There:**

Given these parameters, the lunar shuttle can be built on Earth and lifted to LLO by a single HLLV launch. The total payload is the dry shuttle mass of 23mT plus 26mT of LOX, within the 49.7mT LLO capability of the Ares-V + Extended EDS. Once in LLO, a launch of the HLLV/CEV stack described earlier carries a light load of 10mT of LH2 and 14mT of cargo which the shuttle will take to the surface on its maiden voyage.

**Conclusions:**

Harvesting lunar LOX is a crucial step towards dramatically lowering the cost per lunar mission. The trade-off is 1 Ares-V launch to place the shuttle in LLO initially versus 1 Ares-1 launch plus the cost of an expendable LSAM per mission. If the life of the shuttle is as low as 2 or 3 LS-LLO-LS round-trips, the total program cost will benefit. If the life of the shuttle is as long as 10 or more trips, significant cost savings can be realized. Replacing short-life shuttle components, such as individual engines, either in LLO or on the lunar surface, can dramatically lengthen the useful life of each vehicle.

Ron