The Bremen Drop Tower | Hacker News Reader

The Bremen Drop Tower

zarm.uni-bremen.de

83 pointsTomte35 comments

There's a great Tom Scott video about the tower https://www.youtube.com/watch?v=4aCMDQsx740

Oh, and there's a user's manual!

https://www.zarm.uni-bremen.de/fileadmin/images/droptower/do...

How does it compare to zero-G planes (vomit comet)? These planes can provide much more than 10 seconds of weightlessness, do it a dozen of times per flight without exceeding 2g of acceleration. Much bigger volume, much bigger payload, people can use it, it looks better in every aspect. By comparison, the drop tower can do three 10-second experiments per day, and the acceleration/deceleration look quite violent.

If you want to get in and have a trip, it costs around $5-10k. So judging by this price, it doesn't seem that the expense is high enough to justify building an apparatus that looks quite expensive.

I'm sure that there is a very good reason for drop towers to exist, I just don't know which one.

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.

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.

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

> 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.

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 ;-)

One of the applications for these sorts of microgravity conditions is the creation of large quantum superpositions, especially doing interferometry (roughly, double-slit experiments) with Bose-Einstein condensates.

https://physics.aps.org/articles/v6/23

Hey, that's my experiment! :-) So great to see that here on HN.

I would think many countries have disused mineshafts that might be good candidates for a far longer drop

It isn't sufficient to just have a deep hole in the ground. You need a vacuum too!

I had to poke around to find this: "Before the experiment, 18 high-performance pumps make sure that the drop tube is almost free of air containing only one ten thousandth of the normal air pressure. Due to the vacuum, the air resistance is so low that the Bremen Drop Tower can provide one of the best quality of microgravity - in some aspects even better than on the International Space Station (ISS)"

> in some aspects even better than on the International Space Station (ISS)

Woa, does anyone know in what way this tower provides less gravity than the space station?

I think they're implying that if you want to test to effects of microgravity, you don't want something like 14 psi of air pressure happening at the same time, because interactions with the air flows might confound whatever you're trying to test.

The station is likely better in terms of gravity, but in terms of test conditions, you're somewhat limited by the astronauts being there.

High-quality vacuum is fairly readily available at the ISS.

The centre of mass of the ISS follows a fixed orbit round the earth (to first order approximation that is, occasional thrust is required to maintain this trajectory). Moving away from the centre of mass, by definition, puts you into a slightly different orbit. This perturbed orbit has a different trajectory and orbital speed. This will manifest itself as very small reactionary forces relative to the ISS structure, in other words what you and me call gravity. In plain English, except for at the exact centre of mass position within the ISS (Or any point exactly on the trajectory traced by it) objects will appear to drift.

In the microgravity tower, translations within the experimental container will not result in divergent trajectories, so no drift will occur.

I think you've just answered for me why the ISS is effectively flat and two-dimensional rather than three. Also this strongly suggests that the station has a spin moment corresponding to its orbital period, about 90 minutes.

Perhaps the ISS has more vibration from all those life support systems?

Correct, that's the main problem there. Demanding experiments are even carried out when the astronauts are sleeping, they also tend to vibrate ;-)

Wouldn't it be easier to add some rope pulling the capsule to compensate for the air resistance? Or would it add too much "noise" because of the imperfections of such system?

I was thinking a electromagnetic rail controlled by precise sensors combined with very powerful gyroscope would also work. Probably still need to evacuate some of the air.

They're building something similar in Hannover: https://www.hitec.uni-hannover.de/einsteinelevator.html

Sure, the rope could pull an evacuated chamber, inside of which another chamber could fall without much noise. (assuming you could handle the velocity and stretch in that much rope. thrusters are probably easier)

The longest vertical shaft you're likely to find is around 3km, which gives you a little over 20 seconds of freefall and a couple seconds of violent deceleration.

But the problem is, you're dealing with acceleration squared. The Bremen tower already does almost half that time (9.3s with the slingshot), so it's probably not a big enough difference to warrant the effort and travel.

Now you just need to retrofit a zero-gravity environment into the disused mineshaft..

So then something akin to a vacuum chamber in a freefall miner's lift then? Surely cheaper than evacuating the whole chamber.

If that vacuum chamber is falling through air, then it's not in freefall.

So then you accelerate the lift to the correct speed

Great to see this on hacker news. I can see the tower from my office.

It's a Johnny-come-lately. Every day going to work I used to drive by the "Shot Tower" in downtown Baltimore, USA (it's still there, I'm not), built in 1828 to make various types of spherical ammunition, including cannonballs. Wikipedia lists some even older shot towers. The Baltimore tower was for many years the tallest structure in America.

https://en.wikipedia.org/wiki/Shot_tower

Shot tower != drop tower tough. I'm not familiar with shot towers, but based on the wiki page they just seem to be tall towers that are used to produce ammunition.

Drop towers are used to conduct scientific experiments and require a lot of precise engineering to work properly. So for example they need to have a double-wall to keep the inner tube still, and have vacuum pumps to remove air from the inside.

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