For a layman like I am, both the planes look massive and similar in size and shape, as if a near copy of each other. Do the similarity only end in the size and shape? Do they not perform equally well in similar roles? While Boeing 747 is used for commercial airlines, why isn't Lockheed C-5 Galaxy? Similarly, why is C-5 galaxy popular for Military and 747 isn't?
C-5 Galaxy was designed to military requirements that for cargo aircraft include:
- operating from unpaved runways (I am not sure in case of C-5, but probably yes)
- operating from airfields without much support
- operating from short runways
- paradrop capability
These basically imply a high-wing design so that engines are high (to minimize risk of foreign object damage) and the deck is lower for easier loading, front and rear cargo door with loading ramps so loading is possible without any ground infrastructure and some excess thrust for sufficient performance on short fields.
However that design is less efficient. It has higher operating cost. The air force is willing to pay it, because it needs the capabilities it requested. But commercial operations don't need those things, because it has enough well equipped airports to operate from. So the higher cost of operating an L-500 (as civil version of C-5 would be called) makes it uninteresting.
B-747 only shares one design feature with C-5, the raised cockpit that allows full-height (well, almost) cargo door in the nose, but otherwise it is optimized to minimize operating cost, making it great cargo plane for operations between airports.
On a side note, there is some commercial demand for transporting heavy or large cargo to less equipped airports. These are currently covered by ex-soviet-military An-124 aircraft that look very similar to Galaxy, are a bit larger, are certainly able to land on unpaved runways and were simply available when Soviet Union collapsed.
A commercial jet, designed to carry people, like the '747 or any other jetliner, need to fullfill a series of requirement about:
- noise (both inside and outside)
- safety (emergency exits, oxygen masks and other safety devices)
- dimensions (space between gates at airport are a design contraint)
Also, the areas around windows and doors require reinforcement in the fuselage.
All those things are unnecessary on a military transport. Other features are usually required like:
- operating from unpaved runway (need special landing gears)
- mid-air refueling (too dangerous for a jetliner)
- being able to drop cargo mid-air (requires a ramp on the tail of the aircraft)
A lot of considerations can be done, but converting or modifying aircrafts isn't an easy thing. One big exception are tankers. There's only one KC-747 Tanker and is flown by the Iranian Air Force.
While Boeing 747 is used for commercial airlines, why isn't Lockheed C-5 Galaxy?
The C-5 was created on a proposal made directly to the military to fill a heavy lift role. It is more rugged than a 747 and includes things that only military operations call for like countermeasures and probably lot of other classified systems. It's uniquely suited for evasive maneuvers and down range operations that a 747 wasn't designed for. It's specifically a military aircraft with no civilian version.
Similarly, why is C-5 galaxy popular for Military and 747 isn't?
The 747 is popular for military use, but mostly in the form of contract work. The military hires private companies to move cargo with 747s all the time. The military does have some 747s of their own too. The E-4 is one example. And there's always Air Force One.
I doubt any commercial operator would want to use a C-5. The 747-400 series is roughly contemporary with the C-5B. Compared to the 747-400F freighter, the C-5B:
- is slower (cruise speed of Mach 0.79 vs Mach 0.85);
- has a smaller internal diameter (19ft vs 20ft) and isn't much longer (247ft vs 231ft externally);
- has a cripplingly short range when fully loaded (2,760mi without refuelling isn't even enough to get you from London to Moncton, let alone London to New York, vs over 5,000mi);
- carries less (122t maximum payload, vs 128t for the 747-400F)
- requires a cockpit crew of four, vs two;
- uses huge amounts of fuel (the 747 gets twice times the range from fuel tanks that are only 10% larger).
The main benefits of the C-5 are that its high wings and low landing gear make it easier to load: you don't have to lift everything fifteen feet in the air to get it into the plane. and I think the C-5's cargo bay is full-height for its whole length, whereas the 747 has reduced height at the front under the cockpit. That also makes loading a 747 something of a sliding-blocks puzzle, since any full-height load has to go in through the side door and be turned, which limits the length of any item; the C-5 has a tail ramp that lets you load an object of almost any length that will physically fit. The C-5 can also use lower quality, shorter runways.
Airlines, even cargo airlines, don't carry many long, tall, heavy loads, and they fly to and from well-maintained airports with nice runways, so don't need the advantages the C-5 gives. That just leaves the disadvantages.
In his book 747, Joe Sutter (the project lead for the initial 747) mentions that the idea of using the C5 as a transport was floated by then president Lyndon Johnson as an alternative. The US was experiencing a period of high economic inflation in the mid 1960's, and Johnson was putting the brakes on corporate expenditures so as not to drive prices even higher.
Sutter points out that the C5 simply wasn't economical enough to consider as a civilian transport in a competitive market. Its fuel usage was too high, and it had a slower cruising speed than most civilian airliners of that day.
Being a military aircraft, the C5 is designed to operate under different conditions than civilian airliners... short and/or rough field capability (heavier landing gear), surviving battle damage, etc... All of that adds weight, making the aircraft less economical to meet requirements that it won't encounter in civilian use.
Plus, military transports aren't designed with passenger comfort in mind, as anyone who has ever spent a few hours in the back of a C130 or C141 can attest to. More weight gets added to address that situation, and that means more fuel usage.
With the military, the taxpayer foots the fuel bill, and has no choice in the matter. Nor do the soldiers riding on the transports have any choice. In the commercial airliner field, the customer foots the bill, and definitely has a choice... another airline, flying a more economical and comfortable aircraft. It was the fuel efficiency of the 747, as compared to the smaller turbojet airliners of the 1960's, that made it a great success.
These are essentially the same reasons that the current C17 and A400 aren't used as an airliner. Not as efficient as existing airliners, and designed for situations that commercial airliners won't encounter.
Ironically, the 747 project was considered a sideline at Boeing when it first started up in the 1960's... all of the attention, and the best engineers, were on the 2707 supersonic transport.
The C-5 Galaxy is not for sale
The USAF jealously hoards certain aircraft. They are shared with foreign militaries only very reluctantly and never Civilians.
Think of the A-10 Warthog (contrast with the Su-25 which Russia sells to pretty much everybody who wants CAS), B-52, F-111 (shared only with Australia, and only near its end of life), F-14 Tomcat (its only share being a complete disaster), and indeed C-5 Galaxy.
Anyway, it's not competitive
A civilian entity seeking a strategic airlifter has so many options. The 747 is rapidly becoming an aircraft without a market, so they are readily available, and of course Boeing would fall over themselves to sell you a new one. Meanwhile Antonov would not only be absolutely thrilled to sell you the An-124, they can offer you positively insane pricing because of the odd economics in Ukraine and Russia of late.
As a partner, Antonov totally understands you because they are you - having spent 20 years themselves being a commercial cargo carrier and tackling loads absolutely no one else can carry with their own An-124 fleet and An-225. Boeing has 40 years of experience specifically in the 747 supporting commercial customers who are most concerned with total life-cycle cost. Whereas you won't get any love from Lockheed. Their last venture into the finicky commercial market, the L-1011, was a complete disaster and nearly bankrupted the company. They don't even possess a sales and support division to deal with customers who are not in Arlington, Virginia and whose offices are not 5-sided, and that will make you a penny-ante annoyance. They'll charge you "I don't want to deal with you" prices.
The commercial win is to stick with 747 unless you want to take chances with the An-124. Heck, Antonov would love to finish the other An-225 for you, and that's something not Boeing and not even Lockheed can supply.