Is it possible to join the united states air force if I have sickle cell disease?

No. This would be found on the initial screening. Flying with sickle disease could be incapacitating and might even be lethal. If all else failed, the condition would certainly arise early in service time and you would be discharged for falsification of the application.
No. You have to. Remember that the regs are in place to ensure not only your safety but the safety of your unit. If one of you becomes ill, it affects everybody. You also have to remember that you have to assume you will deploy to hazardous, austere environments where health decompensation happens to everybody. Dehydration w/ SCD will precipitate an excruciatingly painful crisis. Thanks for your interest to serve.