There are plenty of very good reasons:

zero-G planes are only mostly zero-g. They fly a parabolic trajectory, so their body actually rotates about the center of mass while flying.

Also, their weightlessness is not as well controlled as a drop tower's.

The cost you've listed is for a single passenger.

You know what's more expensive to build and operate than a pressure controlled tube on the ground? A pressure controlled tube with jet engines operated by pilots.

While there are good reasons for a drop tower over an airplane, namely experiments requiring more precise control, I’m not remotely convinced that the Bremen tower was cheaper to build and operate than a Vomit Comet. For one, a 146m tall tower for which ZARM broke ground in 1988 and inaugurated the catapult in 2004; that sounds incredibly expensive. NASA uses old 727s, long out of production so a cost is difficult to assign but they were $20mm new, NASA’s planes were probably enroute to the boneyard when they acquired them.

The tower has actually been in operation since September 1990. So the time needed for construction was just 2 years and not 16 years. For the first years it was operating as a simple drop tower, without a catapult. The catapult was added later in 2004. Construction costs according to German Wikipedia were more than 24 million DM (12.2 million euros), with a further 4.2 million euros for the catapult. They are doing on average about 400 drops per year. The time in micro-gravity is 4.74 sec for a simple drop and 9.3 sec with the catapult. (source: Wikipedia) According to my estimations this amounts to about 18.72 hours of micro-gravity in 24 years.

That's still expensive, but not outrageously so.

Even if the planes were inexpensive, maintenance and flight costs are high.

Does anybody know why they are limited to three experiments a day?

According to Wikipedia, they need 90 minutes to evacuate the tower before every experiment. Does an individual experiment involve multiple (dozens? hundreds?) of falls?

I'm assuming they have a long list of people who want to book time in the tower.

Hi, drop tower user here :-) Three drops per day is due to 20 min preparation, 90 min evacuation, 30 min to refill with dried air, 20 min recovery. The is indeed long, which was not anticipate when the tower was first built, this the lack of automatic recover systems. Also, many experiments need to repair/change stuff in their apparatus between drops.

Hi, long-time drop tower experimenter here :-) One very good reason to choose a drop tower over the vomit comet is microgravity quality, i.e. residual acceleration and vibrations. The drop tower provides roughly 10^{-6} g, which is a few orders of magnitude better than zero-g aircraft. Another is easy accessibility. You can work on your apparatus until ~2 hours before doing a drop, and you have electronic and mechanical workshops on premises.

Regarding acceleration and deceleration, it's not as bad as it sounds/looks. Most off the shelf component will easily survive, provided you fastened them well enough. Rule of thumb: If it survives shipping by parcel service, it will survive the tower ;-)

I think 10 seconds is long enough for most purposes.

Let's say the drop tower is open 9am-7pm Monday-Friday, so 10 hours a day, for 5 days a week, and 50 weeks a year. That's 2500 hours of drop time a year. Let's say the building can be expected to last 100 years (probably lasts much longer, it's solid concrete after all) and cost 20 million dollars to build. Thus, you get 250k hours of drop time for $20MM, so each hour of drop time costs $80.

[edit] whoops, turns out it's only operational for 30 seconds a day! That makes the price go up about a factor of 1000, so the cost is now roughly $80k per hour.

Let's say you have a vacuum chamber (usually required for zero-g experiments) that takes up 3 seats, and you pick the cheapest zero-G flight I could find at $5000 for one flight, 15 periods of 20-30 seconds of weightlessness each (let's say avg 25 seconds.) That's $15,000 for 375 seconds, or 0.104 hours. So your hourly price is $144,230.

With the revised math, the tower still wins, $80k to $144k, but I'm surprised it can only drop three times a day.

> Let's say the drop tower is open 9am-7pm Monday-Friday, so 10 hours a day, for 5 days a week, and 50 weeks a year. That's 2500 hours of drop time a year. Let's say the building can be expected to last 100 years (probably lasts much longer, it's solid concrete after all) and cost 20 million dollars to build. Thus, you get 250k hours of drop time for $20MM, so each hour of drop time costs $80.

The drop tower does not allow things to be dropped for 10 hours each day. There are only 3 drops per day, so each day gives <30 seconds of drop time.

According to the video on the linked website, ZARM can do 3 drops a day with 4.7 seconds of microgravity per drop.