I've been reading about the Roth TSP all day and there are a few things I haven't been able to answer. If I stop contributing to my conventional TSP and spend the next few months maxing out a Roth, is that money in a separate pot or does it accumulate with my conventional TSP? After I get home from this deployment do I just stick with the Roth TSP or can I contribute to both?
One caveat-- make sure your deployment still qualifies as a combat zone. There have been a few rules changes in the last six months, and I haven't kept up with them. You'd want to see the latest message that says your deployment area still falls within the dates & boundaries.
Let me clarify that we're discussing the differences between contributions to the conventional TSP and the Roth TSP. (Other posters have confused the Roth TSP with the Roth IRA.) Regardless of what you decide to do with your contributions, either conventional TSP or Roth TSP, you're going to try to maximize the contributions because the combat zone deployment gives you a new limit of $52K in 2014. It's a great financial opportunity, but since it's in a combat zone then hopefully you never get this opportunity ever again. Because of the higher contribution limit, I'd try to max out the TSP contributions (and the Savings Deposit Program contributions) before putting anything into Roth IRAs. Of course if you can maximize your TSP, SDP, and IRA contributions all in the same year then that's even better, but that's rarely achievable.
Your TSP account will track your contributions to both the conventional TSP and the Roth TSP. In addition, your TSP account will track how much of your contributions to your conventional TSP came from a combat zone (tax-free). The advantage of maxing out your Roth TSP with contributions from a combat zone is that you won't have to pay any taxes on the contributions (because your pay is free of tax) and since it's a Roth TSP you won't have to pay any taxes on the (regular) withdrawals.
When you get home from the deployment and resume contributions with "normal" pay, your contributions to the Roth TSP will be taxed. If you did not maximize your Roth TSP contributions while you were on deployment, it might still make sense to contribute to the Roth TSP after you return home because you're going to have very low taxes in 2014 (most of your income was tax-free). You'll pay almost no tax on Roth TSP contributions now and (years later) you'll pay no taxes on withdrawals.
So try to max out the Roth TSP contributions (and the SDP contributions) while you're deployed. When you get home, keep going to maximize the Roth TSP contributions (if you haven't already done so) and then continue to contribute to the conventional TSP. (You may be home from deployment, but the $52K limit applies to all of calendar year 2014.) If you manage to hit the $52K contribution limit to both your conventional TSP and your Roth TSP, then you can contribute to your Roth IRA.
The biggest problem you have with this scheme is juggling all the various accounts and contribution limits in myPay and your TSP account.