Delta IV launch vehicle
The Delta IV launch vehicle, in its Heavy configuration, is the most powerful launch vehicle currently available, capable of bringing 28,790 kg to a Low Earth Orbit . The launcher is very reliable, having suffered only a single failure, and is completely American and provides lots of upgrade potential. Delta IV is not exactly cheap at this moment, with a price tag of approximately $370 million, but the main reason for this high price tag is the very low flight rate; only once a year for Heavy, with about 3-4 launches per year for the complete Delta IV family. This low flight rate is because of lack of demand, caused by the competition of Atlas V. Increasing this flight rate by two launches and six additional cores every year would reduce the per-unit cost by a decent amount.
The reason for picking Delta IV for this comparison was the upgrade potential and the fact that all the infrastructure for launching large amounts of cryogenic propellant to orbit are all already included; something that does not exist yet for Falcon Heavy. Delta IV is also being used for an Orion flight test next year, and is therefore likely closer to being adapted for launching Orion than Falcon heavy or Atlas V heavy is. Delta IV Heavy already exists, unlike Atlas V Heavy or Falcon heavy. Also, unlike Falcon, upgrades to Delta can be partially paid for by the Air Force. All in all, upgrading Delta IV for the job could likely be done quicker and cheaper than either Falcon Heavy or Atlas V. However, they remain strong alternatives, and in case of Congress going nuclear, it's up to NASA to decide which one is better.
These alternatives, as well as Atlas Phase 2 and others, can easily be copy-pasted in this paper mission, and I see no reason why it couldn't work. Delta IV was simply assumed here for reasons mentioned earlier.
The exploration gateway
For exploration, some infrastructure to support it will be needed. By far my favorite proposal to do exploration is Boeing's Exploration Gateway plan. This option allows for lower cost access to the lunar surface, and provides a great place to do deep space research and as a staging point to asteroids and Mars. In this post I will focus on the lunar architecture, like I usually do. It can be expanded to Mars later.
The plan I'm presenting here would not dump Orion, but rather, it would dump SLS. As much as I personally support SLS, it's one of the more likely to be canceled and it's more expensive than Orion, and potentially also more replaceable.
Assembling the station
The Gateway station plan includes a small space station to be assembled behind the moon, at the EM-L2 point. For this plan, the space station would consist of two main smaller components which can be launched in two launches with the Block 1 SLS. The first launch would carry the Science/Power module, the second one the Node and Utility module. They would be docked at the L2 point and a third SLS brings the Orion spacecraft there to man the station and start doing science. The architecture assumes SLS can lift 27 tons to TLI; Delta IV Heavy probably can't even get half of that there in a single launch, so a slight change in architecture will be required.
|ACES compared to Centaur and DCSS|
In order to reach the 27 TLI goal, a dual launch is obviously required. However, even with a fully fueled DCSS in orbit, the TLI capacity falls a few tons short. In order to reach the goal, an upgrade to Delta IV will be required. The ACES (Advanced Common Evolved Stage) allows Delta IV to lift a whopping 37 tons to LEO . It carried 41 tons of propellant and has an empty mass of ~3.6 tons, and uses four RL-10 engines with a specific impulse of 461.5 . Using a propellant drop tank, it can refuel itself in LEO; such a drop tank could realistically carry about 34 tons of propellant, allowing the ACES to refuel itself to about 83% of maximum capacity. With a propellant load of 34 tons and 1% prop residuals, the ACES stage can send 28.8 metric tons to a TLI trajectory with a delta V of 3200 m/s. While this is not using the most accurate numbers in existence, it nonetheless provides 1.8 metric tons of margin, as well as the not-insignificant launch margin from the payloads (ranging from 1.6 to 4.6 tons of margin).
To recap, in order to assemble and man the station, Delta IV would launch six times:
1. DIVH Launches drop tank, upper stage refuels in orbit
1. DIVH Launches drop tank, upper stage refuels in orbit
2. DIVH launches Science and Power module
3. (see 1)
4. DIVH launches node and Utility module
5. (see 1, 3)
6. DIVH launches an Orion spacecraft with a crew of four astronauts
The parts would rendezvous and dock at L2; Orion, the Utility and Science module all have their own propulsion system with sufficient delta V to dock.
Lunar surface missions
|The Boeing HLO lander|
A humble change to the architecture is to use Orion's SM instead of ATV as the LTV. ATV production will stop after number 5, and there is not chance for revival. Orion's SM, which is ATV derived, will be able to operate independently from the crew module (required by law) and is European anyway. One of the big reasons to use a modified ATV is to allow for international cooperation to share the costs, but if ESA provides and maintains the Orion SMs for this mission it will do that just as well. Assuming a lander mass of 21.4 tons with 6 tons empty mass, as given in , an Orion SM with 316 second Isp and 4115 kg empty mass (Oxygen, nitrogen and water are not needed) can do this job of transferring the ship from HLO to LLO (571 m/s ) with a propellant load of 7620 kg, which fits within the 7907 kg maximum prop load Orion can hold . The tanker would still be a single vehicle though; Orion SM with a fuel tank for 15.4 tons of hypergolic fuel would (barely) exceed the 28.8 ton TLI capacity of two Delta IV rockets. It's still a possibility though, but with very little margin, only ~200 kg, which NASA likely wouldn't risk.
For lunar missions, the Utility module would use its propulsion system to transfer the station from L2 to High Lunar Orbit, where it will remain for the rest of the lunar campaign. After this, the lunar lander is launched fully fueled, followed by the LTV (small enough for a single ACES Delta IV H) and a crewed Orion spacecraft. The crew would dock with the station, transfer to the lander, and would descend to the lunar surface from where they can do a 14 day science mission with 300 kg of scientific instruments. After the mission, they ascend to LLO, where they will dock with the LTV and return to the station. At the station, the crew enters their Orion spacecraft and heads home. For follow-on missions, a tanker vehicle and Orion are sent to the station, taking in total four launches per follow-on mission.
It would be possible to launch Orion to the station in one go by adding 6x GEM boosters to the Delta IV booster. This would increase payload to about 45 tons . Using Orion and a drop tank at the same time, it could refuel to about 25 tons of propellant. Orion could be short-fueled to about 17 tons for L2 missions, in which case ACES has sufficient delta V to get Orion to the station. A single Delta IV would have about 19 ton TLI capacity, enough for Orion or any other crew or cargo vehicle currently planned. This upgrade would save 1 launch per mission on the manifest, but would require additional upgrades to Delta IV which might end up costing more.
The advantages of a Gateway station
The architecture described by Boeing, and the modifications I made, have a few significant advantages over normal exploration. First, there's technical/economical advantages: the Gateway allows the lunar lander, by far the most expensive part of this architecture, to be fully reusable and used for many times. Unlike letting the thing float in LLO, it can easily be refueled and can be repaired at the station, should maintenance be required. It is also an efficient staging point for missions to asteroids and Mars; while those will likely require bigger launchers in the 80 ton class, the groundwork for such missions could be laid with near-term launchers.
Another advantage is the international aspect of the plan. The costs are split over several participating countries. For example, the US provides the LV, the Node and Utility Module, the Orion crew module, and commercial resupply. Russia provides the Science and Power module and does most of the lunar lander. They also provide crew access and HLV capacity later on via Angara 7, PTK NP, and Sodruzhestvo (Zenit super Heavy). Europe contributes to the lander, the LTV and provides a LV for commercial resupply. The advantages of such an approach are obvious. America can provide their part within the current exploration budget, while the high cost of a lander of ~$7 billion  is split over two space agencies rather than one.
All in all, an international Gateway station program allows for a flexible, low cost exploration program supporting a wide range of missions, and Delta IV with the ACES upper stage upgrade is very well suited for laying the groundwork for an exploration program and supporting the exploration of cislunar space.
 "THE SPACE LAUNCH SYSTEM CAPABILITIES FOR ENABLING CREWED LUNAR AND MARS
EXPLORATION", IAC-12- D2.8
 Done using the Vis-Viva equation: 232.6+(1972-1633.9)=571 m/s Delta V
Most images are taken from this presentation: http://spirit.as.utexas.edu/~fiso/telecon/Raftery_12-12-12/Raftery_12-12-12.pdf