LO, Courtney and others,
Perhaps I can help here on the subject of F-35B VLBB and margins.
First, it's important to say clearly that any powered lift aircraft (i.e. a zero kts flying speed machine) will have a weight challenge (problem, issue, crisis, the term used depends on your viewpoint). It's just physics and maths. Helicopters have the same problem. Harrier had it, as does V-22. In looking at F-35B, it's sometimes useful to keep that fact in mind. It's not a failure because it has to deal with weight.
Actually, most modern combat aircraft have had severe weight problems during development. However, they've not been reported in anything like the detail offered by the F-35 programme. F-22 had a seriously bad weight crisis, which was resolved by removing many pounds of metal that had been put in to support air to ground requirements. That's why you don't hear much about the Raptor's ground attack capabilities.Typhoon had a thoroughgoing nightmare with weight, went unreported, and again was solved by a major airframe weight reduction programme. Of course, a conventional aircraft can solve lots of max weight issues by just using more runway. Lots of runway, sometimes, but that can give problems when the aircraft wants to deploy to a hot and high airfield.
I've posted before on the causes of the F-35's weight problems, and won't repeat them here. LM did a poor job of controlling weight early on and had to undertake a major programme of work to get the design back in the box. That's on the record, too. It's important to remember that all three variants had the problem - but the STOVL aircraft was the worst affected due to those pesky physics.
Margins. The F-35's VLBB calculations are based on many hundreds of assumptions, and I certainly don't have all the detail - even if I did, it wouldn't be public forum stuff. However, I can tell you that the VLBB assumptions include not only weight growth, but a 'fully degraded' engine at the end of its service life. They also include a 'reserve weight margin' (somewhere over 500 pounds) that the US DoD applied to make LM work harder at weight reduction. They also include a higher weight for the propulsion system (which is Government Furnished Equipment), again to get some margins in place.
What has improved quite a bit over the past year is that the weight of the design has stabilised - there have been changes required as a result of flight test, but the risk of more airframe changes is decreasing as the test programme moves forward. However, the risk hasn't gone away.
These facts don't 'make it all better'. As I said at the start of this post, managing weight on a powered lift aircraft is a very, very hard thing. LM, the DoD and the UK MoD know and understand that and are working the issues.
And guys - let's play nicely, shall we? Differing opinions are good, strong opinions are good. We should all respect that. It's what we post that should matter, not who we are. (And if anyone wants to know who I am, just PM).
Best Regards as ever to all those working hard to make the F-35 a success.
Engines