Introduction

The last time anyone went to the moon the primary purpose of the trip was to win a race. Everything else was secondary to that and there was no other purpose, at least for those holding the purse strings. This had the consequence that everything was sacrificed to the altar of getting there FIRST. The approach chosen was to make the trip with a single launch carrying everything needed to get there, land and return. The result was a 3000 Tonne rocket to deliver 45 Tonnes to lunar orbit and return a 5.6 Tonne command module to Earth. Leaving nothing of value anywhere after the mission was complete.

For the intended purpose of beating the Soviet Union to the landing this design was probably optimal. For the purpose of travelling to and from the moon for any other purpose it was a terrible approach.

Proposed here is an approach optimised for travelling to and from the moon at a minimum cost and maximising the return in space going infrastructure and resources. Attempting this approach in the 1960s would almost certainly have lost the race for the USA, but might well have yielded a moon base before the century was out.

The key components of this approach are:

• A reusable lunar lander that remains in lunar orbit when not in use
• A resuable orbital transfer vehicle (OTV) that remains in Earth orbit when not in use
• A solar electric tug (SET) to haul material from Earth orbit to lunar orbit
• Commercial launches of up to 50 Tonnes on Falcon Heavy

The spacegoing vehicles only need to be launched once, after that only fuel, crew and supplies are launched for subsequent missions. The details below are one possible set, there are many easy variations to the designs following the general principles outlined above.

All vehicles should be capable of fully automated operation, except for loading, unloading and (re)fueling. There is no reason to depend on the kind of skill Neil Armstrong needed to land safely.

Each mission will require several launches on the 50 Tonne capacity Falcon Heavy at a launch cost of $100m per launch. This compares very favourably with the multi (45) billion dollar projected cost of each SLS launch.

Chemical rockets may be liquid or solid fueled, for calculation purposes we simply assume an exhast speed of 3000m/s. We also require that all vehicles can be refueled on orbit (Earth or lunar). A solid fuel cartridge may be the most effective design for bulk accelerations along with some kind of restartable liquid fueled motor for fine tuning and landing.

The figures in brackets are for a proof-of-concept ultra small mission that collects and returns a few kilograms from the lunar surface using a tiny solar tug to take a tiny lander there and return with the samples. With a total launch mass of just over 200kg it would be the cheapest ever moonshot.

Vehicles

Solar Electric Tug

This is a key factor in keeping costs down, it will use low thrust trajectories to ferry bulk materials and the lunar lander itself to lunar orbit cheaply but slowly. Each trip may take weeks to reach lunar orbit which is fine because nothing living will travel that way and we're not trying to win races.

These vehicles should be cheap and light, they consist of PV panels, low thrust Xenon ion engines and payload attachments. They will make one way trips to the moon, which will result in them being available in lunar orbit as power sources (the PV panels) or bulk return vehicles or even to carry things to elsewhere in the solar system.

The total delta-V for a moon orbit transfer is around 4km/s, an acceleration of 1mm/s/s will achieve this in 4 million seconds or 46 days. This implies a force of 50 Newtons pushing the 50 tonne mass. The CAT thruster claims 2mN for 10W (or 5kw/N) so 250kW would provide the necessary 50N which amounts to 1000m^2 of solar panels. The only thing is no other thruster seems to claim anything like this power to thrust ratio.

In the table below a conservative exhaust speed has been chosen deliberately a more realistic estimate might be 50% higher. This will be a general principle throughout the rest of this document, conservative performance figures and generous dry mass allocations.

These vehicles will mainly be used to transport fuel to lunar orbit, in this use they will be launched complete with payload. They should be refuelable on orbit by pumping Xenon into the fuel tank and they should have power takeoff attachments to allow them to be used as power sources. Because of the low thrust payload attachments can be simple and light.

Tugs will be fully automatic, although they will require manual work to attach and detach payloads as well as to refuel if they are reused.

Lunar Lander

This needs to be robust and reliable rather than the absolute minimum mass design used in the Apollo missions. The engine(s) must be restartable indefinitely and have good fine thrust control. For a estimation purposes a dry mass of 10 Tonnes is used (double that of the Apollo lander).

This vehicle spends its entire life in lunar orbit, on the moon or travelling between the moon and lunar orbit. It will be ferried to the moon using SET which can also carry 25 tonnes of fuel at the same time, the alternative would be to burn 30 tonnes of fuel to get it there dry.

The five tonne payload assumes the same mass is landed and returned to lunar orbit. It could also deliver 10 tonnes to the surface and return empty or anything in between.

Starting in lunar orbit with a full payload and 50 tonnes of fuel it will land with just over 18.3 tonnes of fuel remaining and achieve orbit again with just over 2 tonnes of fuel remaining. This is based on a generous delta-v of 2km/s between lunar orbit and the lunar surface.

Orbital Transfer Vehicle

This is primarily for people and the supplies they need while traveling. Everything else can be moved much more cheaply, albeit slowly, using the tugs. Like the lander it must be robust and reliable. It can use a combination of liquid fueled precision rockets and solid fueled boosters. Using chemical rockets instead of solar electric propulsion enables it to take the quick, high thrust trajectories, similar to those used by Apollo, and necessary for carrying people. Unlike the Apollo command and service module this should be relatively roomy and comfortable. It cannot have been pleasant lying in spacesuits packed like sardines.

The fuel capacity is generous for a one way trip to the moon (minimum would be 60 Tonnes). This allows for variations and a little lattitude in the payload. It should be possible to part fill it for lower payloads. A variable fuel capacity based on attaching tanks may be desirable to enable it to be used for longer duration journeys such as Mars or the Asteroids.

Re-entry Vehicle

This is the simplest component in many ways, with many options including using existing designs such as the SpaceX Dragon or a hypersonic glider. For the purpose of calculations a launch mass of 10 tonnes is assumed. The only real requirement is that it can dock with the OTV and return a sufficient payload to earth safely.

Missions

General

Preparation

Unlike the Apollo missions where everything launches together on a single rocket this system requires some preparatory launches and movements before it is ready for use by people. It is necessary to lift the lunar lander and transfer it to lunar orbit, implying lifting a SET to carry it. With that in place it is necessary to lift the OTV to earth orbit.

For each mission it is necessary to lift the fuel to take the OTV to lunar orbit. Also it is necessary to lift and transport to lunar orbit the fuel required for landing and the return of the OTV which will arrive in lunar orbit with only the reserve fuel.

For the peace of mind of all concerned it is desirable that all the resources to be ferried to the moon by SETs arrive before anyone leaves in the OTV.

Lunar Lander Deployment

This can be achieved with a single launch carring the following.

• SET fully fueled - 15 tonnes
• Lander - 10 tonnes
• Fuel (in lander) - 25 tonnes

Once launched the SET carries the lander with 25 tonnes of fuel to lunar orbit.

Fuel Deployment

Carrying fuel in a chemically powered rocket is a very effective way of wasting fuel all of which has to be launched at $2000 per kilogram. So let's not do that.

A landing and ascent to lunar orbit requires 50 tonnes of fuel. The return from lunar orbit to Earth orbit requires 70 tonnes of fuel. A total of 120 tonnes of fuel must be available in lunar orbit. Initially there is 25 tonnes of fuel in the lunar lander.

Three SETs can carry 105 tonnes of fuel which alongside the 25 already there is 130 tonnes of fuel. So there will be 10 tonnes stockpiled in lunar orbit after the first mission, along with whatever is left in the lander's tanks.

A fully fueled SET masses 15 tonnes, the payload of fuel is 35 tonnes. So three launches will suffice to position the fuel in lunar orbit.

At this point there are also four SETs in lunar orbit. They can be left there as resources to be used in lunar orbit (recharging batteries in the OTV and lander for example) or taken down to the lunar surface to provide power there or returned to earth orbit if Xenon is made available or even used later for low thrust missions to other destinations.

OTV

First Mission

Summary

First mission launches - resources in lunar orbit

1. Lunar Lander to lunar orbit - 25 tonnes fuel, 1 LL
2. 35 tonnes fuel to lunar orbit - 60 tonnes fuel, 2 SETs, 1 LL
3. 35 tonnes fuel to lunar orbit - 95 tonnes fuel, 3 SETs, 1 LL
4. 35 tonnes fuel to lunar orbit - 130 tonnes fuel, 4 SETs, 1 LL
5. OTV + 40 tonnes fuel - 130 tonnes fuel, 4 SETs, 1 LL
6. People, equipment, supplies (10 tonnes), Re-entry (10 tonnes), 30 tonnes of fuel

Second mission launches - resources in lunar orbit

1. 35 tonnes fuel to lunar orbit - 45 tonnes fuel, 5 SETs, 1 LL
2. 35 tonnes fuel to lunar orbit - 80 tonnes fuel, 6 SETs, 1 LL
3. 35 tonnes fuel to lunar orbit - 115 tonnes fuel, 7 SETs, 1 LL
4. 35 tonnes fuel to lunar orbit - 150 tonnes fuel, 8 SETs, 1 LL
5. 50 tonnes fuel to Earth orbit
6. People, equipment, supplies (10 tonnes), Re-entry (10 tonnes) 30 tonnes of fuel

Incremental Development

Rather than a complete mission with all the equipment and risking people it is feasible to first launch and test the lander and its autopilot. This would require launching the lander fully fueled and an SET with a larger Xenon tank to ferry the lander, a total mass of 80 tonnes which would require two launches and end with a tested lander in lunar orbit with dry tanks.

Conclusions

An average of 5.5 launches per trip to the moon and back carrying 10 tonnes each way means launch costs averaging $550m per trip with each trip leaving usable resources (SETs) in lunar orbit.

Two launches would take enough Xenon to fuel 7 of the SETs and leave two more SETs in lunar orbit. Additional OTVs or landers could be deployed with a single launch each.

Two missions for $1.1B in launch costs. If we spend $5B on developing and building the vehicles and $400m on the payload and fuel for each mission then we get 26 trips to the moon and back and some change from the $45B estimated for a single SLS launch.