JOHT 69: Jetpacks

[stickmaker]

The Joy of High Tech

by

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

Where's My Jetpack?

The Bell rocket belt made its debut in 1961, well after such devices were explored in popular entertainment. Performance of the real thing was impressive, except for the very short flight duration of about twenty seconds. That was disappointing, but people figured this was a prototype, and further development would greatly increase this. Unfortunately, physics and chemistry being what they are, that length of time was not greatly improved, even to this day, for devices using the same method of propulsion. The Bell rocket belt was and is spectacular, but has little practical use. However, that hasn't stopped people from building these as well as improved versions.

In 1995 the RB2000 rocket belt was announced. Using the same basic technology but constructing the device of modern materials and using computer modeling in the design, this rocket belt was lighter, had higher thrust and could fly for about thirty seconds. Unfortunately, one of the developers went rogue and stole the prototype. Multiple crimes and accusations of crimes back and forth complicated the issue. The rocket belt hasn't been seen since. 

Other companies have built similar rocket belts in recent years. While they do work, and do have better performance than the Bell original, they still are mostly used for promotional stunts. 

A rocket is a horribly inefficient device for flight deep in a gravity well, and creates a huge amount of noise while operating. Even if it had to carry its own oxidizer - instead of burning the fuel with air - a modern helicopter could hover for many minutes on the amount of propellant which a rocket vehicle the same starting mass would burn in seconds. However, a rocket is a very compact and light way of generating thrust. Those features are what you want in a rocket belt. It's just that if you also want endurance, you're out of luck. 

High-test peroxide (or HTP) is the propellant used in the Bell-type rocket belt. 100% HTP decomposes to produce just under 47% free O2 by mass, and has a density about 45% higher than pure water. Specific impulse is low at 163, and the decomposition releases a little under three million Joules per kilogram, which isn't great. Because of that low energy density, adding more propellant only works if you also reduce weight elsewhere, since the standard rocket pack is already about as heavy as a single person can practically walk around in. So, there's not a lot of room for improvement in that area. Maybe we should check into using something with a higher energy density. 

However, there's a very good reason for sticking with high-test peroxide: It is about as innocuous as you could hope for in a monopropellant. Yes, with prolonged contact it can literally dissolve you, but there are no noxious fumes, the decomposition products are water and oxygen plus heat, and if you splash some on you and wash promptly and thoroughly with water, you'll just have a patch of itchy, white skin for a while. Compared to things like the hydrazines, nitromethane and even ozone, it's a pleasure to work with. As long as your plumbing is clean. (Dr. John Clark claimed that if you named a random substance there was at least a 50% chance it would catalyze HTP decomposition. That includes people.)

So, for a rocket propellant, high-test peroxide is safe, cheap and easy to work with. The tradeoff being a low energy content. However, the decomposition product is hot, oxygen rich steam. Combine that with a suitable fuel and you've got a bipropellant rocket. Now you're talking about a significant improvement.

Kerosene has a density of about 75% that of pure water. It is even more innocuous than HTP, has a good energy density when combined with a suitable oxidizer, is stable under a wide range of conditions and - the kicker - burns well with HTP. 

For the purposes of this article you can assume that "kerosene" is any of a variety of fuels from actual kerosene through diesel to RP-1. My numbers are rounded, and the results general enough to apply to any of those. 

Combining kerosene with high-test peroxide in a rocket has multiple benefits, the primary one being greatly increasing the energy available. 100% HTP + Kerosene has a maximum specific impulse of nearly 300, with an O:F mixture ratio of 7:1 by mass. The combustion of kerosene with HTP produces nearly 46 million Joules per kilogram. That's about fifteen times what HTP alone gives. So there's also a small but significant amount more than burning kerosene with pure oxygen gives, due to the energy of decomposition from the HTP. That increase of energy content does not mean you will get fifteen times the thrust or flying time or whatever. All other factors being equal, you might be able to get a tenfold increase in time aloft by throttling back. Might.

Of course, ten times thirty seconds is still just five minutes. While there are useful things you can do with five minutes of flying time, there aren't many.  

H2O2/Kerosene rockets are hypergolic (that is, they don't need an independent ignition system but ignite spontaneously when combined) have good energy density, good overall performance and good reliability. The mixture is non-cryogenic so it doesn't need special refrigeration or insulation. If you can safely store and handle high-test peroxide the chemical hazard of the combination is low. The exhaust is even low-polluting. Most such rockets use a two-stage process, with the HTP decomposed before injection into the combustion chamber. The kerosene autoignites, due to the heat and pressure there. Alternatively, a trace amount of catalyst can be added to the kerosene, and a one-stage combustion process used. 

However, designing, building and operating a compact and lightweight bipropellant rocket with this mixture may be beyond the level of expertise of those who might build and/or use a rocket belt. You might have to include such complications as regenerative cooling of the combustion chamber and exhaust nozzles.

For a rocket pack, using a single combustion chamber would mean directing hot exhaust gasses - much hotter than the superheated steam of the standard HTP rocket belt - down long pipes. Keeping those from burning through - or burning the pilot - would be very difficult. 

A dual combustion chamber setup would burn the fuel and oxidizer at locations slightly above where the nozzles are on a standard HTP-only rocket belt. This setup greatly increases the cost and complexity, since you're doubling the number - and halving the size - of the combustion chambers. Also, if - something admittedly unlikely - one chamber lights and the other doesn't...

A simpler approach would be to take one of the modern HTP rocket belts and add what might be called afterburners to the nozzles. (You would have to move the "eyelid" redirectors to the ends of the afterburners and probably enlarge them, but the rest of the control authority comes from pivoting the whole chamber-to-nozzle assembly plus a touch of body English, so would not need any changes.) These afterburners would have misting nozzles for spraying kerosene into the stream of hot, oxygen-rich steam the system already produces. You'd probably need an ignition source, since the HTP is already decomposed, at a lower pressure and somewhat cooler than in a rocket of the type described above. Combustion would also be less complete, with a corresponding reduction in performance. However, this setup is much lighter and cheaper than the true bipropellant rocket arrangement described above. Also, if you can make this a ducted rocket you will add atmospheric air to the working medium, increasing thrust. 

You would, of course, need an additional tank for the kerosene, and a larger nitrogen tank to also push the kerosene into the afterburners. Good safety interlocks would be necessary, too. For example, you'd need check valves to make sure fuel and oxidizer don't mix except where you want them to, since they share a pressure source. Flame detectors in the afterburners would be a good idea. If one lights and the other doesn't, a safety system would shut off both afterburners. This would leave you with the original HTP rocket belt for an emergency landing if necessary. 

For added reassurance, a ballistically deployed parachute - the sort of thing used on some ultralight aircraft - could be mounted at the top of the device. This would allow successful use at very low altitude. You'd want another interlock with this addition, to shut off the rocket belt completely if the parachute is deployed. 

Why hasn't this been done? This "afterburner" setup would increase thrust and burn time, but not by nearly as much as a true dual-fuel rocket would. You would stretch the flight from about thirty seconds for a modern rocket belt to a couple of minutes, maybe less. I imagine that most people who have examined the idea have decided that level of improvement is not worth the effort. 

There have also been true jet packs. In 1969 Bell flew a unit constructed around a small turbojet built for the purpose, the WR19. This flying machine had a much greater duration than the rocket belt, up to 25 minutes. However, it was even heavier than the rocket belt, and was much more complicated and expensive. It also required a starter cartridge to get going. Though this jet belt was a dead end, engines developed from the WR19 went on to power cruise missiles.     

Turbojets have come a long way since then, and have actually entered the hobbyist realm. There are even jet-powered ultralight aircraft. These most commonly use a jet engine made from the modified power core of an airliner auxiliary power unit. Something similar could be used for jet packs, though you'd probably want two or even three of the small jets. Additionally, there are now commercially available small jet engines. A design could have one engine to provide main lift and two or more for additional thrust to be used for propulsion and steering. Such designs have actually flown, and show much promise.

A separate approach is the ducted fan. Martin Jetpack has spent thirty years working on one person powered flying rigs. Their current model can fly for half an hour on premium gasoline, at up to 46 MPH. They appear to be the current leader in the field, but objective evaluations of both product performance and company performance are not easy to find. However, even at first glance the Martin Jetpacks are unsatisfying. To get the thrust and efficiency they need they use large diameter ducted fans, driven by compact piston engines. These are more like vehicles you get into than backpacks you put on. 

There is at least one project to build a modern version of the jet pack using small commercial turbojets. This would result in a much lighter and more compact design than the Martin. However, current turbines with a similar thrust to the WR19 are smaller, lighter, more reliable and fuel efficient, and cheaper. Some even have electric start.

In a publicity stunt, a French pilot crossed the English Channel on a "hoverboard." Not quite a jetpack - or not _just_ a jetpack - it still may be showing the way for the future of individual VTOL jet-powered flight. Though for long distances the small, jet-powered wings used by some parachutists seem more practical. As well as faster.  

The main limitation of the traditional jetpack is how the turbine core is positioned. To minimize turns and restrictions in the jet exhaust plumbing, the turbine is mounted with the inlet at the bottom. The exhaust goes out the top, is split, and the two streams are turned down, one on either side of the pilot's center of mass. This means there is a risk of the turbine ingesting some of its exhaust. The chance of ingesting debris kicked up from the ground during takeoff and landing is also far from trivial. Neither event would be healthy for the turbine, or the pilot.

A partnership between Martin Aircraft Company and Avwatch is working to develop a first responder solution for the US Department of Homeland Security and Department of Defense. These are based on the various jetpack designs, and again are fairly bulky and heavy. These are in effect miniature substitutes for helicopters, to be used - as just one example - in rescue work. How practical and effective this will be remains to be seen. 

So, there you have a brief overview of individual rocket- and turbine-based flying devices, plus a piston based one. You also have an answer as to why the more compact rocket and jet versions are not now and probably never will be practical for anything but stunts. Sorry about that. 

Comments

[stickmaker]

Keep in mind that pulse jets need forward movement through the air to develop full thrust. (Though as the Mythbusters showed, they still have significant static thrust.) They are also very, very loud. (Like a large tuba played by an angry giant.) :-)