The US military emits more CO2 annually than several countries. Winglets have been proven to work well in the commercial sector to reduce fuel burn, therefore reducing both CO2 emissions and fuel cost. The US military doesn't really lack research or production/modification capability, so why have winglets not been installed on non-fighter/attack aircraft? (Think C-130, E-3, KC-135...) They would reduce CO2 emissions and fuel costs, so why not implement them?
I think the main reason is that many of these planes were produced before winglets were common
Look at a list of USAF planes and see what years each one were produced (I am not including every plane, but most of the common ones)
- C-5: 1968–1973, 1985–1989
- C-17: 1991-2015 (has winglets)
- C-130: 1954-present
- E-3: 1977-1992
- KC-10: 1979–1987
- KC-46A: 2013-present (does not appear to have winglets, even though recent)
- KC-135: 1955–1965
But when you look at this story about winglets, there is the sentence
The first 747-400 was delivered to Northwest Airlines in 1989, the first commercial aircraft with proper winglets.
So for the most part you are talking about adding winglets to an existing airframe, rather than aircraft with winglets as part of the design. This increases the expense of adding winglets, which reduces their cost-effectiveness. Look at this article on modifiying the B-52 to use four modern engines and the problems with that (not all of these would apply to winglets).
All the discussions of engineering difficulties and cost-effectiveness are valid arguments but based on my USAF career in program offices and in operating commands, including 4 years in TAC, it boils down to money and priorities.
Once the a/c is operational the program office is concerned with sustainment. They need to provide the engineering, tools, and materials needed to keep the planes combat ready. In addition to providing spares to the operating units, they are also responsible for depot level maintenance. They have a constrained budget and the priorities are set by the operators.
Spare parts are driven by in-field failures. Safety related issues take top priority. Service life extension becomes a concern on older a/c. And the operators have a wishlist of upgrades they would like.
The operator’s top priority is being mission capable. The wishlist will be things that keep the planes operational or improve its ability to perform its mission. There’s never enough money or time to get everything on the wishlist. So the list is a rough plan that projects out 10 years or so. It gets updated every year when they can lock down what they will do next year.
So the marginal increase in fuel economy from adding winglets never comes close to the top of that priority list. In addition to the direct costs of a large mod such as winglets, there is the operational cost of having the a/c spend an extra week in the depot. If a squadron has 24 a/c, that week in the depot equates to almost 6 months the squadron will be short an a/c. That’s a big hit to operational readiness.
When we were retrofitting GPS to the entire AF fleet in the 90’s it was a constant battle to get the operating commands to give us the time in depot to do the upgrades. They wanted the a/c back ASAP. It was about a 10 year effort and it took Congress threatening to pull upgrade funding for any aircraft that wasn’t GPS equipped after a cutoff date.
Any significant upgrades are usually addressed in the development of the replacement a/c. But even that will go through numerous trade offs to (try to) meet schedule and budget.
For example, look at the wingtips of the P-8. They don’t look like the commercial variant (because the commercial variant will snap off if you hang out in icing conditions for hours), but the wingtips it has essentially work the same way.
In general though, the answer to your specific question is that there is no “aftermarket”. Air Force and Army each rely almost entirely on the OEM for engineering design work the Navy does to a lesser extent, and no service has a good, recent record of in-house metal bending.
Winglets work by using the span-wise air flow around the wing tip to generate forward 'lift'. This airflow is highly dependant on the angle of attack, so the winglet needs to be designed for a specific condition or it just adds weight and drag.
Airliners spend almost all their time at cruise height and speed while loaded, so it's possible to design a winglet that works for that case. Military aircraft have to operate in a wider range of conditions, and the improvement in cruise performance might not be worth the reduction in loiter time.
It might also help to consider using the weight of a winglet to just extend the wing. That would reduce wing-tip losses at all speeds, allow the plane to fly higher and reduce take-off and landing speeds. Why isn't it done? Because the designer has already worked out the optimum aspect ratio, and improving performance in one area would reduce it in another.