I'm also a federal employee with visions of early retirement before I'm eligible for a pension.
The federal pension system under FERS is currently based on your years of service, so there is definitely a penalty of sorts for working fewer years. After I work 10 years, it will theoretically pay 10% of my high-3 salary after I turn 57 if I opt for the (recommended) deferred retirement. Combined with anticipated SS benefits, a frugal person might survive off of just those two income streams after age 57, TSP be damned.
So the difficulty for an early retiring fed is thus bridging the gap between retirement and the pension onset. The typical strategies are the same as for everyone else here:
1. Roth IRA contributions (but not earnings) can be withdrawn tax and penalty free at any point, though you pay the penalty of forsaken estate planning benefits inherent in the Roth.
2. Rule 72t withdrawals from the TSP will provide some tax-free income before age 59.5, though typically not very much even using the Fixed Annuitization or Fixed Amortization Methods. You can also make a regular withdrawal from the TSP at age 55 or later without paying the 10% penalty.
3. Current law will not allow conversion of regular TSP balances to the newly available Roth TSP, but since 2006 you CAN convert from the TSP to your Roth IRA and pay the taxes in the year of the conversion. This means you should be able to quite working (lowering your tax bracket), convert one year of living expenses from TSP to Roth IRA every year while paying a low tax rate, then after five years start withdrawing that money from the Roth IRA tax free. This little loophole seems to allows you tax free savings and growth in the TSP, low tax conversion to Roth IRA, and then tax free withdrawals from the IRA, netting you low-tax penalty-free TSP withdrawals before retirement. The only hiccup is that you need to have five years of living expenses stashed elsewhere, like a taxable account or from straight up Roth IRA contributions.
This little income pipeline only needs to last you until you start taking real retirement benefits like your pension and social security, if they are sufficient to live on. Alternately, if they are almost-but-not-quite sufficient then you could draw down a big chunk of your TSP this way and leave a little behind to supplement your income after you start collecting pension and SS.