President Obama, already the most traveled chief executive in the modern era after internationalist former President George H.W. Bush, is expected to run up an Air Force One tab of at least $24 million this year alone, according to the National Taxpayers Union Foundation.

Already Obama, just back from a trip with his family and a huge entourage of staff to several African nations, has run up a $15.2 million bill just for Air Force One in the first six months of 2013. Security and hotel costs can dwarf the jet's cost. Some estimates of the president's Africa trip alone top $100 million.

NTUF's Douglas Kellogg told Secrets that Obama's Africa Air Force One bill to taxpayers was $6,654,345.

But unlike those costs which are hard to estimate and often kept secret, Air Force One costs can be calculated based on published hourly figures provided by the Pentagon.

So far, the president has spent 18 days abroad, is on a pace to surpass is 20 days abroad in 2010 and 2011 and is approaching his high of 40 days in 2009. Total costs so far for Air Force One calculated by NTUF are $15,219,433.

NTUF said that the president's near 18-hour flight back from Tanzania Tuesday night marks his 14th overseas flight this year.

"Estimating the potential costs of additional trips the president already has planned for later this year is imprecise, however, using the median cruise speed of Air Force One applied to distances for trips to Moscow, Indonesia, and Brunei, brings the total for President Obama's 2013 Air Force One costs to over $24 million," said NTUF.

The latest estimate from NTUF provided to Secrets comes just days after the the taxpayer's watchdog group released its latest study of presidential travel, "Up in the Air." That study found that Obama has nearly caught up to Bush's travel despite a tough and long reelection campaign in 2012. "Obama nearly spent as many days abroad as the current single presidential term leader, George H.W. Bush, despite focusing for nearly half his term on the longest and most expensive campaign in history," said NTUF.

"We can only imagine how expensive these travel costs truly are, and how far President Obama will travel with no campaign this term," said the group.

Paul Bedard, The Washington Examiner's "Washington Secrets" columnist, can be contacted at pbedard@washingtonexaminer.com.