Apollo on steroids
Twenty-first century moon technology
How does NASA's new lunar programme, Constellation, compare with the 1960s moonshot? E&T examines the technologies.
The lunar landings represented, in 1969, a pinnacle of exploration. The political challenge laid down by US President John F Kennedy just eight years and two months earlier, had been met.
Four decades later, the lunar landings still represent a pinnacle. Although 11 men followed Neil Armstrong to the lunar surface, the US Congress soon cut funding for the programme, and no one has scuffed their boots in the grey dust since December 1972. The Moon remains devoid of human presence and, as Buzz Aldrin poetically described it, an example of "magnificent desolation".
But things are looking up for prospective moon-walkers. In January 2004, US President George Bush announced, in what became known as 'the Vision speech', a plan to return astronauts to the Moon by 2020. "With the experience and knowledge gained on the Moon," he said, "we will then be ready to take the next steps of space exploration: human missions to Mars and to worlds beyond". Vision indeed!
The similarities between the spacecraft being designed under the Constellation programme and the Apollo hardware of the 1960s are immediately evident. The Orion spacecraft that will transport four astronauts to lunar orbit has a conical crew module and a cylindrical service one; the Altair lunar lander is a kit of struts, spheres and cylinders on four spindly legs. Indeed, when NASA administrator Mike Griffin unveiled the Agency's lunar exploration plan in September 2005, he urged his audience to "think of it as Apollo on steroids".
Although some were disappointed that these 21st century spaceships owed less to the technology portrayed in 'Thunderbirds' and 'Star Trek' than to an era when desktop computers, mobile phones and satellite TV were themselves science-fiction, there are good engineering reasons for the apparently 'retro' design.
For a start, the Orion Crew Exploration Vehicle (CEV) is required to transport astronauts from Earth and return them from space at the end of their mission. The most obvious shape for a vehicle launched by a cylindrical rocket is a similarly-proportioned cylinder with a pointed end. Likewise, the shape and form of a lunar landing vehicle like Altair is, to an extent, irrelevant, because it operates only from lunar orbit to the surface and back, entirely in a vacuum. To add the sleek aerodynamics of many a science-fiction rocket would be a waste of materials and an unmanageable strain on the mass budget.
Mass is a key driver for any space mission, simply because a given rocket has a limited payload capability. The overall mass of the Orion spacecraft, as quoted in NASA's Exploration Systems Architecture Study (ESAS), is broadly similar to that of the respective Apollo modules. The Orion 'Block 2' lunar spacecraft (Block 1 variants are for Earth orbit missions) is predicted to have a mass of just over 23t, while the Apollo command and service modules totalled around 30t. There are, of course, many differences between the designs, but a significant positive impact on the mass budget for the new spacecraft is the use of composite materials that did not exist in Apollo's day.
The real difference, however, comes with the lunar landers. The Apollo lunar module (LM) was designed to carry two men to the surface for, at most, a three-day mission. The all-up mass of the Eagle LM that performed the Apollo 11 landing was some 14.6t, more than 10t of which was propellant. By contrast, the Altair lander is baselined at 45t, about 30t of which is propellant.
The much larger vehicle is perhaps the most encouraging aspect of the new lunar programme, because it implies a commitment to more than a politically-driven, flag-planting visit. Indeed, Altair has been specified to carry a crew of four (on a seven-day surface sortie) and a payload of more than 2t, which suggests a commitment to at least a semi-permanent infrastructure.
Mass on the Moon
Recognising that mass would be one of the major challenges in any lunar lander design, rather than start with a wish-list of specifications, NASA elected to pursue what it called a "minimum functionality" approach. "We consider a minimum capability vehicle, with minimum requirements, then 'buy back' safety, reliability, functionality and capability," explains John Connolly, head of the lander team at NASA's Johnson Space Center.
"It's a bit like buying a car," he continues. "You start with the basics, then decide whether to add GPS, a stereo, or leather seats." A similar approach, called zero-based vehicle (ZBV), was initiated for the Orion CEV.
The approach is not intended to produce anything like a final design, because it ignores contingencies, such as extra propellant for missions aborts, and omits redundancies. "It's a single string implementation," says Connolly.
What it does, however, is to provide two 'data points' that are critical in the early design stages. The first is related to launch vehicle performance: put simply, if the rocket can't deliver a 'minimum capability' spacecraft, there is no way it will be able to deliver a fully functional one. "This provides an early indication of a need to revise the transportation architecture," says Connolly. The second point establishes a safety and reliability baseline from which to make informed trade-offs between cost and risk.
Risk mitigation is an important concept, which the space industry addresses by close monitoring of subcontractors, careful parts procurement and rigorous testing regimes. Another solution is to provide subsystem redundancy, which effectively means carrying spares, but this has an obvious and significant effect on the all-important mass budget.
According to Connolly, the new approach "recognises redundancy as one of the options available to mitigate risks", but challenges the notion that redundancy is "the correct or preferred solution for all risks". We are used to specifying "two-fault tolerant systems" in our spacecraft, he says, but engineers are inclined to "spread redundancy over the systems like peanut butter!".
Having briefed industry on its plans for the lunar lander in September 2008, NASA issued a Request for Proposals last January, with a view to contract awards "in the spring". What Altair's industrial prime contractor thinks of the new approach remains to be seen.
Meanwhile, work is forging ahead on the Orion CEV, which is needed well before Altair because of its other role as a space station crew delivery vehicle. American industry giant Lockheed Martin secured the prime contract in August 2006, beating a Northrop/Grumman team for what NASA described as "one of the most significant NASA procurements in more than 30 years".
Although the cylindrical service module and conical crew one owe their basic shape and aerodynamic design to Apollo, the technological developments of the past 40 years make them entirely new. Among the many improvements are the computers and other electronic systems, some of which were in their infancy in the 1960s (see 'Guidance by Rope and Gates').
It is self-evident that Orion will be far more reliant on software than its predecessor, but a number of key mission elements drive the need for a more autonomous spacecraft. Chief among them is the requirement for the crew of four to descend to the surface, leaving Orion unoccupied in lunar orbit for at least a week. On the Apollo missions, one crewmember remained behind while his two colleagues conducted their surface explorations.
Lockheed Martin is responsible for software development, while its subcontractor Honeywell is building the avionics systems and crew interfaces.
An important part of Orion's command and data-handling architecture are the self-checking pair (SCP) processors, which will handle all safety critical tasks such as manoeuvring, sequencing and resource management. According to Orion program manager Cleon Lacefield, an SCP comprises "two identical single-lane computer systems (CPU, flash, RAM, I/O and power) implemented on a single circuit card" that compares its partner's activity with its own. This allows the crew, ground controllers or the Orion avionics package "to detect and isolate failures in real time", explains Lacefield, "without depending on complicated hardware or software fault masking or voting".
So this time, rather than relying on Earth-based tracking stations, manual observations of stars using a handheld sextant and keypad-loading of an antediluvian guidance computer, lunar travellers will have the convenience of automatic star trackers, an autonomous rendezvous and docking system and, of course, GPS. Still, if operations aboard the International Space Station are anything to go by, no-one will venture far beyond Earth orbit without their laptop!
The new generation
Most of the Apollo engineers and astronauts are now retiring, retired or deceased, while even those motivated by Apollo towards science, technology and engineering careers are middle-aged or beyond caring. NASA's next manned lunar mission is planned to occur around 2020, by which time not one under-50 will have personal recollections of those seminal events in space exploration.
Nevertheless, a new generation of engineers, scientists and explorers, who know the Apollo programme as cultural history, are taking the baton of lunar exploration in preparation for Constellation and the programmes that will follow.
Apollo was not without its failures - the Apollo 1 fire and the unforgettable Apollo 13 - so the new 'Generation-Moon' would do well to learn from history.
Guidance by Rope and Gates
Today, the 'core rope memory' used in the Saturn V guidance system is, literally, a museum piece. It was constructed from hundreds of tiny doughnuts of ferrite material (the magnetic cores) and a mass of fine signal wiring (the rope), which appeared to string the cores together like microscopic washers. This read-only memory operated by passing a current along a wire to magnetise a core ring, either in the north-south direction for a 'one' or south-north for a 'zero'. It had a storage capacity of just 460kbit, a tiny fraction of the megabyte memory of a turn-of-the-century floppy disc, but the Saturn V was 'state-of-the art' in the 1960s. Its guidance system incorporated 80,000 components and could perform 9,600 operations per second.
NASA selected MIT's Instrumentation Lab to design and develop the Apollo guidance, navigation and control system, the key component of which was the Apollo Guidance Computer. Since integrated circuits did not become commercially available until 1961, they could offer little in the way of a reliability record or 'heritage' for Apollo, but MIT convinced NASA that they were worth the risk.
The ICs themselves were single logic gates, available in three classical variants: AND gates that turn 'on' if all inputs are 'on'; OR gates that turn 'on' if any input is 'on'; and NOR gates that turn 'on' if all inputs are 'off'. Although it would have been simpler to use a selection of different gates, MIT chose to make the circuits from three input NOR gates because the single-part solution improved reliability. Some 5000 gates were used in each Apollo computer and something like 60 per cent of the total US production of ICs was being used on Apollo prototypes by the summer of 1963. This is one reason the Apollo programme is credited with encouraging the development of the computer industry.
The initial proposal for the Apollo lunar module (LM), which specified a mass of about 10t, was based on the predicted lift-capability of the Saturn V rocket in the early 1960s. However, according to Joseph Gavin, director of space projects at LM contractor Grumman Aircraft Engineering, the LM's mass had increased to nearly 13.5t "within six months of the start of the design programme".
The preoccupation with the LM mass budget was understandable considering the effect it had on the overall Apollo-Saturn system. Each kilogram of inert mass lowered to the lunar surface and returned to lunar orbit required an additional 3.25kg of propellant in the LM's tanks, so each extra kilogram of LM structure added 4.25kg to the payload of the Saturn V. The resulting knock-on effect on the Saturn's propellant budget added about 50kg for each kilogram landed on the Moon.
Although the performance of the Saturn V was eventually improved sufficiently to allow LMs in excess of 16t to land on the Moon, an unparalleled mass reduction effort had to be instigated to slim down the lunar module. According to Gavin, "weight reduction became a way of life at Grumman, eventually becoming a part of the day-to-day language" to the extent that its Super Weight Improvement Program produced the verb 'to swip', where "swipping meant scrutinising every rib, panel, valve and switch in the lunar module to determine where extra weight savings could be made".
For example, the original five-legged design was reduced to four, crew seats were eliminated and the ladder was structured for a 1/6g environment. Aluminium shear webs, or stiffeners, were 'chemically milled' to reduce their thickness and cabin walls were thinned to 0.25mm in some places. They were so thin, says Gavin, that you could "pierce them with a screwdriver".
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