To the Moon, Slowly
An Ed Kyle Commentary
April 20, 2010, updated April 26, 2010

Delta 4 Heavy Liftoff in 2009

NASA's now-canceled Constellation Program died because the U.S. government was unable, or unwilling, to pay its costs - and the costs were considerable.  Last year's Augustine Committee estimated that NASA's budget would have to be increased by at least $3 billion per year to even begin to make the program possible.  The Committee recommended shelving the Constellation "Program of Record" in favor of commercial crew launch contracts to provide access to the International Space Station (ISS), followed by development of a new super heavy lift rocket that could be used, later, for "Beyond Earth Orbit" (BEO) missions.

Even Augustine Committee members were shocked by the White House decision, announced in February, 2010, to cancel not just plans for a lunar base, or plans for a NASA-designed Ares I crew launcher, but to cancel all of it - the crew vehicle, the heavy lifter, the spacecraft, and the entire thought of sending astronauts to the Moon.   Only ISS, and plans for commercial crew launch to ISS, survived.  Some hoped that the Administration had cancelled Constellation in favor of a better "Plan B" to reach the Moon, but there was no "Plan B". 

In retrospect, the problem with Constellation was that it tried to do too much with the resources available.  Constellation planned to send two complete, multi-billion dollar crew missions to the Moon each year.  To accomplish this required development of two completely new launch vehicles, one of which would have dwarfed any previous rocket ever seen on the planet.  It required development of two completely new crew-carrying spacecraft, one to fly and one to land.  It required new rocket engines, new launch pads, new escape systems, and so-on. 

Given the current budget realities, can any human lunar exploration program be conceived to fit the monetary limitations?  Augustine thought it was possible, by canceling Ares I, delaying development of the super heavy lifter, and then performing numerous, so-called "Flexible Path" non-landing missions for years until more funding could be freed up to develop a lander.  NASA's current plan seems to aim down this path, but with no firm goals or timelines for going beyond Earth orbit or for landing anywhere.  President Obama offered no plans to return to the lunar surface at any time in the future. 

I want believe that that astronauts can walk on the Moon without breaking the bank. 

A traditional way to fit a big government program, usually an overrun program, into a limited budget is to slow the program down - to build fewer fighter jets or submarines than originally planned, for example, or to spread the manufacturing out over more years.  It was not possible to slow down the Constellation Program in this way, since its launch rate was already at a minimum given its huge fixed costs.  Billions of dollars would have been needed each year to support the manufacturing and launch infrastructure of Ares I, Ares V, Orion, and Altair, whether or not any astronauts walked on the Moon. 

A lower-cost, slower-rate program might be possible if those fixed costs were slashed.  What is the most obvious way to cut fixed costs?   Use existing launch systems.  Using those systems to fly less frequently to the Moon would spread the costs over time, allowing the program to fit within NASA's budgets.  Rather than two landings per year, NASA might perform one landing every two years, for example, dropping the grandiose plans for a lunar base in favor of sortie missions.

Existing launch systems are much less capable than the planned super-heavy Ares V, but given today's budget realities, existing rockets seem to offer the only readily affordable path.

Delta 4 Heavy on Cape Canaveral Space Launch Complex 37B

The United States already has a heavy lift rocket named Delta 4 Heavy and a nearly comparable roster of Atlas 5 Medium models.  The production capacity and launch infrastructure already exists for both rockets, and both are sorely underused.  Delta 4 Heavy, in the process of being upgraded with RS-68A engines, will soon be able to lift more than 27 tonnes to low Earth orbit (LEO).  Atlas 5-551 can lift about 19 tonnes to LEO.   A planned, but never-flown 552 model, with a second RL10 engine on its Centaur upper stage, would be able to lift more than 20 tonnes.          

The Apollo 17 S-IVB+IU stage weighed 16.2 tonnes after its trans-lunar injection (TLI) burn, including residuals. It carried about 75.13 tonnes of propellant into its initial parking orbit, for a total 91.24 tonne stage weight in orbit. It boosted a combined 48.601 tonne CSM/LM spacecraft mass from LEO toward the Moon. Thus Saturn V had to lift roughly 140 tonnes to the initial low earth parking orbit.

For argument's sake, let's back up from that 48.601 tonne Apollo TLI mass for a minute and try to re-create Apollo.

Using a modern upper stage designed just for the TLI task, and powered by a more efficient RL10 cluster (450 sec specific impulse rather than the 421 sec J-2 ISP), would reduce the parking orbit requirement from 140 tonnes to only 111 tonnes.  The TLI propellant mass would be reduced to only 56 tonnes from Apollo 17's roughly 77 tonnes.


Atlas 5-551 at Canaveral's Complex 41 in 2006

Five Delta 4 Heavies would be able to beat that 111 tonne total.   A new cryogenic propellant depot, or refuelable upper stage, would be needed.   But this plan requires surge launches to minimize propellant boil-off.  Costly new launch pads would be needed, increasing the cost of launching Delta 4.  Surge launches obviously don't match with the "slow-rate" mission concept. 

Which brings me to storable propellants and what I've called "Plan C". 

Storable hypergolic propellant, used to power the Titan II missile and its follow-on space launchers, seems less in-fashion in the West since the retirement of Titan 4 and Ariane 4.   But Proton, Dnepr, Rokot, China's "CZ" series, and India's PSLV and GSLV still use such propellants.   Thousands of on-orbit satellites, including Western satellites, depend on storables. They orbit for years, for decades even, using the stuff.  ISS uses it. Soyuz and Shuttle and Shenzhou use it. Nearly half of the "commercial" payload mass orbited each year *is* storable propellant. Just within the past year probably 70-90 tonnes of this versatile propellant was orbited by the world's rockets.

Russia's Briz M upper stage pump-fed engine provides the world's most efficient storable propellant specific impulse, as I understand it, at about 326 seconds. The best in the U.S. is, I believe, the out-of-production pressure-fed Delta 2 upper stage at about 319 seconds.


Aerojet's Now-Retired LR91/AJ11 Titan Upper Stage Engine Might Have Been a Good Starting Point for a Storable Propellant TLI Stage Propulsion System

If the U.S. could develop a 326 sec ISP storable propellant TLI stage engine, a 48.601 tonne TLI mass would require use of a 100 tonne gross mass TLI stage loaded with 92.2 tonnes of storable propellant (something only roughly the size and mass of a loaded Titan 2 first stage). The total LEO mass requirement would climb to nearly 149 tonnes, 9 tonnes more than for an Apollo mission, but the cryogenic boil-off concerns would vanish. Six Delta 4 Heavy launches would now be needed, but without a launch surge requirement the flights could occur at a more leisurely, affordable pace, from an existing, already built-and-paid-for launch complex, using rockets manufactured in an already-existing factory.


Relative Size of TLI Stages.  Storable Propellant TLI Stage Would be About Same Size as Titan II First Stage on Left

If one lunar mission were performed every other year, NASA would only have to fund three Delta 4 Heavy launches in any given annual budget.  That is less payload to LEO than that now provided by NASA's soon-to-be-retired Shuttle fleet - and for less money (at least on the launch-cost side)!

NASA would need to develop a refuelable storable propellant stage and an engine for it.   An engine capable of producing about as much thrust as the now-abandoned Titan IV second stage would be needed.  The stage would need an on-orbit long-duration "kit" with solar arrays and batteries to heat propellant.  Automatic docking and refueling techniques would, of course, be vital - some type of precursor test-flight program would be needed to prove the systems in LEO. 

Alternatively, if advanced, ultra-low long-duration boil-off on-orbit propellant storage methods can be devised, use of high-energy cryogenic propellants could be contemplated - but such methods won't pay unless propellant can be stored for many months, or even years, in space.

Freed of launch vehicle development costs, NASA could focus on crew carrying trans-lunar and landing spacecraft years earlier than with any alternative plan.  Freed of the need to maintain a massive, costly launch complex, NASA could retire Complex 39 and share EELV costs with the U.S. Air Force.  Without launch surge requirements, NASA could gradually build up lunar missions in LEO at whatever rate afforded by existing budgets.  

Since the purpose of deep space human exploration is national prestige, the number of landings performed in any given time-frame is almost irrelevant.   Just as the quadrennial Olympics garner worldwide attention, the general public would more likely notice one landing every-other year than it would a continous stream of missions.

Earth's Moon is nearly big enough to be considered a planet in a twin-planet system.  It is the largest moon relative to the size of the planet it orbits in the solar system.  It is only three days travel from Earth.  Forty years ago, twelve men explored a few square kilometers of a surface that is multiple times larger than the United States.  To bypass the Moon because "we've been there" makes no sense to me, especially when it seems possible to go with funding and rockets that exist.