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Merlin engine
The Merlin engine is at the heart of SpaceX’s Falcon rockets, but it may be adaptable elsewhere. (credit: SpaceX)

Is the Merlin engine the workhorse of future spaceflight?


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“Those Merlin engines are fantastic,” offers Tony Stark to Elon Musk in a cameo for the summer movie Iron Man 2. The brief exchange, occurring as it does in the Monte Carlo Hotel de Paris prior to the Monaco Grand Prix, invites the space enthusiast in all of us to draw an analogy between the two industrialists—and for those inclined to stretch a cameo further than anyone should, between the aforementioned Merlin engines and the Formula 1 racers about to take to the track.

In the long run, the Merlin engine Iron Man so admires turns out to have much more in common with an engine familiar to every weekend mechanic over 40: the Chevrolet small block.

Formula 1 racing is often promoted as the most elite racing environment in the world, akin to building and racing rockets, and it generates the bills to prove it. The F1 engines and the cars they power are engineered to the finest tolerances achievable. They are the physical embodiment of the concept “state of the art” and as such are astronomically expensive.

And yet it may be that in the long run, the Merlin engine Iron Man so admires turns out to have much more in common with an engine familiar to every weekend mechanic over 40 and every dirt track dotting the landscape in locations somewhat less glamorous than Monaco and Cape Canaveral, the Chevrolet small block. If history bears out the comparison, it could bode well for the aspirations of those seeking more affordable access to space, particularly in the nascent space tourism industry.

In the automotive world, and the car-crazed culture that is America, the Chevy small block is pure Americana. From old reliable work trucks to blown out Saturday night racers, the small block has been installed, pulled, rebuilt, and modified by more people than perhaps any other engine. Built from 1955 until 2004 in a production run of an astounding 90 million engines, and offered throughout GM’s lineup as well in myriad marine and industrial applications, where it is still being built today, the small block came to define nearly a half century of automotive power. Listed by Ward’s AutoWorld as one of the ten greatest engines of the 20th century, the small block remains a favorite of auto enthusiasts and racers of all types due to four primary attributes: simplicity, affordability, reliability, and adaptability. Certain engines hold legendary status not because their performance is at the extreme margins but because they are so versatile and reliable that they have shaped entire industries, or more accurately, industries have been built around them.

In most of the commercial world, it is common to develop your product without having to develop the engine to power it. The ready availability of a common set of industrial engines in different power ranges is one of the fortunate facts which allow both individuals as well as companies of all sizes to produce a bewildering array of machines nearly as quickly as the mind can imagine them. Even in applications where the final product is a highly specialized machine built in very limited production numbers, the power plant itself is generally common to the point of mundane. This, however, is obviously not the case it applies to space launch systems.

While the early decades of launch vehicle development were characterized by vehicle specific engines which consumed much of a particular rocket’s development budget, in recent years, designers have begun to look for “off the shelf” solutions. Notable examples include Lockheed Martin’s use of the Russian RD-180 engine for its Atlas 5 booster, as well as Orbital Science Corporation’s application of refurbished Russian NK-33 engines to its Taurus 2 launch vehicle. No doubt OSC hopes the engines are not bad karma, as they were also the engine of choice for the now defunct Kistler K-1—which was itself a COTS entry prior to that company’s bankruptcy—not to mention the failed Soviet N-1 booster program whose termination resulted in these engines being sequestered in the first place.

Expendable launch vehicles such as these, however, offer scant hope of ever providing the basis for affordable space tourism under any definition. Such hopes instead currently depend either on SpaceX achieving its ultimate goal of making the Falcon 9 fully reusable, or the eventual evolution of fully reusable orbital systems, presumably from the suborbital efforts currently underway.

It may be difficult to overstate just how vital access to an off-the-shelf, flight-ready engine could be to development effort of a first-generation reusable orbital system.

If any of the first-generation suborbital space tourist companies actually succeed in establishing profitable business operations, then it is reasonable to assume the next step in the pursuit of affordable access to orbital space for private citizens will begin in earnest. Along these lines, Virgin Galactic’s Will Whitehorn has established the goal of five years of suborbital operations with SpaceShipTwo as the starting point for the development of an orbital system for his company (see “Virgin Galactic and the future of commercial spaceflight”, The Space Review, May 23, 2005). One critical step for Virgin Galactic, and any other company taking up the challenge of orbital space, will be the selection of an engine system capable of safely and affordably bridging the considerable gap between suborbital and orbital velocities. The Merlin 1C might be that engine.

In terms of basic specifications, the Merlin offers many of the features a first-generation tourist-class orbital system might require. At 512,000 newtons (115,000 lbf.) sea level thrust, the Merlin is small for a first stage engine, thus the clustering approach required for the Falcon 9. However, for second or orbital stage applications, its diminutive size places it in the correct range.

In addition to being of manageable size, the Merlin is a comparatively simple engine by design. The Merlin runs on arguably the simplest combination of fuels, liquid oxygen and rocket grade kerosene, and utilizes a less complex combustion process (open cycle as opposed to staged) to do so. Compared to liquid hydrogen based systems, RP-1 offers a distinct advantage in terms of overall ease of system design and operational handling. This relative simplicity manifests itself in reduced cost and complexity in terms of propellant piping, seals, valves, and insulation, as well as the ability to incorporate smaller fuel tanks when contemplating airframe designs. Hydrogen is not without its advantages, and in the long run may be the fuel of choice, but for first-generation systems with limited development funds, its use requires an expense and infrastructure that are likely out of reach.

In selecting one basic engine to be used throughout its family of launch vehicles (with the exception of the Falcon 1 second stage Kestrel engine), SpaceX engineers contained both design and production costs as opposed to alternative approaches. An equally important aspect of the single-engine strategy for SpaceX, however, is that it allows for a rapid buildup in experience base, as ten engines are utilized in each Falcon 9 flight. Assuming that both its current manifest holds, and recovery and reuse are still a number of years in the future, SpaceX is going to fly in excess of 200 Merlin engines in the next five years alone. If recovery and reuse efforts are ultimately successful, SpaceX will have afforded itself the nearly unique opportunity to examine and refine engines post-flight with an eye towards future improvements. Quite simply, the Merlin engine is on track to become one of the most frequently flown and well-understood engines of the space age.

Like the Chevrolet small block, the Merlin may require an exceptionally long life span if it is to play a role outside the Falcon family of launch vehicles.

While the Merlin is not (sorry about this) magical, it does possess two very important properties that set it apart from a number of possible contenders. First, it actually exists, and has successfully flown in space. Second, it is under production now and apparently will be so for some time to come. If the history of manufacturing teaches us anything, it is that high volume and long production runs are a good thing, and in the right environment frequently translate into product improvement, affordable pricing, and increased reliability. These are three of the traits that made the Chevrolet small block a legendary engine, and, if combined with adaptability, might do the same for the Merlin.

It may be difficult to overstate just how vital access to an off-the-shelf, flight-ready engine could be to development effort of a first-generation reusable orbital system. Even with a successful suborbital vehicle in the garage, the financial and technical challenges of designing and testing a small orbital vehicle will be daunting. Having access to an existing engine with a known performance range and a strong flight history could lower the bar from nearly impossible to just really, really, hard. Rather than struggling through an engine development program, new vehicle designers, and the those footing the bill, could focus on the laundry list of other challenging issues such as airframe, reentry, landing, and the constant battle against weight growth.

Why would Musk wish to license his technology to other parties? After all, SpaceX itself is probably the single company closest to being able to offer a partially reusable platform for orbital tourism at the present time. If the company is able to fulfill its Commercial Resupply Services contract without major mishap, then it is difficult to believe Musk will refrain from flying a crewed version of Dragon regardless of the outcome of the current congressional debate.

Additional revenue is one simple reason for making its engines available on the open market. SpaceX is sure to face financial challenges over the coming years, and an additional revenue stream, even a small one, could not hurt. Low-cost access to new markets is another reason. If SpaceX sticks to its core Falcon launch vehicle business and declines altogether to pursue different platforms, the company might reason it is better to have its engines powering alternate space launch systems through a partnership arrangement than risk being left out in the cold in the event somebody produces a LEO-capable craft with their own engines. By offering a product it has already developed, the Merlin engine, SpaceX could position itself to become a player in multiple facets of the space tourism industry as it matures without engendering much additional risk or expense.

The final, and most compelling, reason that SpaceX might want to make the Merlin available to other emerging launch systems is Elon Musk’s clear desire to play a transformational role in opening up access to space to the general public. Musk is quite vocal on this point, and made it as recently as this July in an email imploring SpaceX supporters to contact Congress to vote no vote against HR 5781, a NASA authorization bill. In the e-mail, the SpaceX founder asserted passage would effectively kill the commercial crew initiative, and with it “the only hope for the average citizen to one day travel to space.” Musk has already indicated a willingness to work with potential competitors, stating that SpaceX “is interested in launching other companies’ manned spacecraft” and would offer launch pricing which did not internally favor their own efforts (see “Big plans for SpaceX”, The Space Review, November 14, 2005).

Virgin Galactic president Will Whitehorn expressed a similar sentiment even more explicitly at a ceremony unveiling the design for WhiteKnightTwo in 2008. Whitehorn stated that his company wished to make the SpaceShipTwo technology “open source architecture like Linux,” and would welcome approaches from outside sources in developing new applications. A similar statement from Musk regarding the eventual availability of the Merlin engine might be the small development now which opens the more door to much more interesting possibilities in the future.

Like the Chevrolet small block, the Merlin may require an exceptionally long life span if it is to play a role outside the Falcon family of launch vehicles. After all, the aforementioned NK-33 engines have waited more than 40 years to fulfill their potential, with perhaps another decade or more to go until they have all flown. In terms of longevity, the all time winner is undoubtedly the venerable RL-10, which was ground tested in 1959, first flown in 1963, powered the DC-X test vehicle, remains the heart of the Centaur upper stage, and appears to have credible future as a deep space engine. It seems we are safe in taking the long view of things. After all, the Chevrolet small block found ultimate greatness not just through the genius of its creators, or in its original design specifications, but because the design itself was capable of evolving and being adapted by those who were not even born when the original engine first roared to life.


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