By Gregory Mitchell (July 1990)
I would consider myself to be an amateur space scientist; furthermore I have worked as a professional engineer for a subcontractor to Marconi Aerospace and Bell Industries for more than ten years and prior to that I owned my own company making special effects for stage and screen plus some military experience, so I can say that I have both professional and amateur experience in space science and its applications.
Currently, that is at the present state of the art, we have a small number of companies, who make rockets and such and a small number of customers, by and large the military, so the cynical view prevails. By this I mean that big industry is only concerned with expensive billion dollar solutions, because at present the market is limited: there would be no interest in low cost solutions of the type that would be implemented by an amateur scientist.
If we consider the lesser goal, 'The Exploration of Subspace' rather than the major goals of putting something into orbit or reaching escape velocity, there is much that the amateur can do. For the purposes of the following discussion, I would define subspace as all that between 50 and 150 kilometers of altitude. In other words, above the highest altitude that can be reached by research balloon and the lowest practical altitude for an artificial satellite.
Although there are airplanes that can ascend to about 30 kilometers, and sounding rockets that can reach 100 kilometers to my knowledge there are no vehicles in the official inventory that can spend a significant amount of time in subspace, so this area is wide open for amateur experiment and exploration. In the light of what I have written thus far I would like to list some of the possibilities that are open to the amateur experimenter.
The following list covers a number of ideas of which I have had both direct experience through my own experiments and indirect experience through reading of the experiments of others...
Electrically Powered Gliders
- Electrically Powered Gliders
- Anaerobic Impulse Jets
- Balloon Assisted Gliders
- The Rockoon
- Solid Fuel Rockets
- The Ramrocket or Solid Fuel Ramjet
- Miniature Rockets
My interest in the idea of an electrically powered glider started when I was briefly involved in one of the man powered flight projects in the early 1960s. The project that I was involved in did not get of the ground - literally, but it did set in train a number of fruitful ideas. As you probably already know several man powered flight projects were eventually successful and it was proven that 200 to 300 watts of power output was sufficient to loft both man and a machine when using ground effect; a combined weight of more than 100 kilos. My thought was that if 200 to 300 watts were sufficient for flight close to the ground, with a similar machine 2,000 to 3,000 watts would be sufficient to reach extreme altitudes.
Gregory Mitchell at 41
The world altitude record for a glider is more than 49,000 feet (without a Motor), so the combination of a glider and a motor should make an even greater altitude possible: possibly twice this. In an attempt to prove this thesis I purchased a man powered flying machine that had only been able to fly a short distance, because it was too heavy. The owner wished to build a better model, but he lacked the money. I equipped this with an electric motor.
A high performance plane of this type requires about 500 watts for level flight, more than the original owner could pedal for more that about twenty seconds. I installed a 2.6 kilowatt 240 volt motor that would give 4 kilowatts if overdriven with 360 volts. This would push the glider at about 100 mph at sea level. As this would pull the wings off at low altitude, the motor was run at 120 volts, until the plane had a reached a sufficient altitude, to increase the voltage safely. My man-powered flying machine, when converted to run on primary cells, could reach heights of between 25,000 and 30,000 feet.
Why an electric motor? A 4 Kw electric motor working at 90% efficiency only dissipates about 400 watts of heat and part of this is in the wires, whereas a 4 Kw petrol motor working at 20% efficiency has to dissipate 16 Kw of heat and all of this is in the motor. Although one may solve the problem of running a petrol motor at high altitudes, let us say above 60,000 feet, by using a cylinder of Nitrous Oxide or Nitrogen Dioxide the combination of petrol and oxidizer is heavier than the higher power primary batteries and in the thin air at these altitudes the excess heat from the motor is almost impossible to get rid of, without a very heavy cooling system.
In 1980, I converted a high performance racing glider. It was carriying about 130 liters of water ballast to increase the speed, so I calculated that I could carry at least this weight of batteries. Sixty 6 volt, 7 ampere hour motor cycle batteries weighed 120 kilos, and the motor only weighed about 10 kilos as it had been rewound with aluminum wire which is only one third of the weight off copper, increasing the total weight of the glider by 130 kilos. Lead acid batteries are not very efficient, particularly at very high rates of discharge, so I could only get about 24 minutes of flight and could only reach an altitude of about 6,000 feet, but it proved that an electric glider will work.
Satisfied with the first experiment, I converted the glider to primary cells. I constructed a 120 kilo battery, using AA cells of the Duracell type to make 8, 120 volt battery blocks comprised from arrangements of cells in series and parallel that could be connected in various combinations of series and parallel, which could be put in different configurations with a rotary switch. This gave me the possibilities of 120, 240, 480 and 960 volts for different speeds and power outputs at different altitudes. Fine speed changes were made by changing the voltage supplied to the field winding of the electric motor. By using this method, speed of the airplane could be increased by 20 to 30%. The efficiency of this control system was nearly 100%.
Note. At higher altitude, the motor speed has to be increased to compensate for the thinner air. At an altitude of 40,000 feet has to fly three times as it needs to at sea level. In my first experience in subspace, I reached 44,000 feet, at which altitude I had to use 480 volts for safe level flight.
Note. With 960 volts, the plane theoretically could have reached 90,000 feet, but this was not possible, as I did not have a pressure suit. Furthermore, I would have had to fly at 500 miles per hour just to maintain level flight: this is close to the practical limit for a propeller plane.
Duracells provide at least 100 Watt hour per kilo in this application, about six times as much as lead acid batteries, so with the aid of thermals, and by adjusting the voltage accordingly, I could get between two and three hours flying time. With this combination, it was not possible for me to reach 49,000 feet, but it was possible to reach about 44,000 feet with the aid of thermals. Furthermore, at this altitude, the Wattmeter indicated that the efficiency of the electric motor had increased from 85% at sea-level to 92%. This I believe is the result of reduced air friction on the motor armature at high altitude.
I was limited in my experiments both in the 60s and early 80s by the non availability of powerful batteries: either the Lithium cell had yet to be invented, or it was unavailable, except to the military, so without the assistance of thermals, I could not do much better than 20,000 feet. A more powerful battery than a Duracell could be made by charging a motor cycle battery, emptying it of acid, then refilling it with Fuming Perchloric Acid (HClO4). This would give 150+ Watt hours per kilo or making up primary cells using Magnesium as an anode to give about 200+ Watt hours per kilo. This would give access to a higher altitude. Currently, a new type of primary cell is produced by Tadiran in Israel that has six times the capacity of a Duracell. These batteries are expensive at the moment, but I expect the price will come down.
Note: Recently, I have made a Tadiran Lithium Battery Survival Torch, using four 4.1 volt 5 ampere hour Tadiran lithium AA cells that only weigh 20 grams each. This is both a high power (15 Watt) torch and a power supply for much of my other survival equipment and is only about twice the size of a packet of cigarettes. Although, as yet, these batteries are 15 each, they are well worth it, as this torch is capable of giving a ten second pulse of 50 amps, sufficient to turn over a car starter motor. If the batteries are given a one minute rest between pulses more than 30 such pulses can be made. The capacity to reach 150,000 to 200,000 feet with an electric glider is now possible for the rich amateur.
Note: Open-Circuit breathing apparatus would set an upper limit of about 44,000 feet, above which fearsome things could happen. Above that altitude, some form of pressure suit is required.
On the downside, nobody was willing to be my patron, so I had to bear the full costs of this exercise, but on the plus side it proved that a high altitude could be reached using electrical propulsion with primary cells and the finding that with full power 960 volts, in a shallow dive from about 40,000 feet, as a consequence of the thin air at that altitude, the glider could exceed 300 mph without exceeding the 'not to exceed' speed of the glider.
If a similar glider were constructed using Lithium Cells with a capacity of 400 Watt hours per kilo, a larger propeller for the higher altitude, a motor of 8 Kilowatts plus and the pilot were equipped with a proper pressure suit, it should be possible for the pilot to reach at least 100,000 feet. Recent experiments by NASA indicate that a specially constructed plane, with a very large wing area, perhaps a Biplane or Triplane, could reach 150,000 feet (50 km.) the threshold of subspace.
In 1985, by using a more powerful plane that uses Lithium Sulfur Dioxide batteries which have twice the capacity of Duracells and what is called a partial pressure suit, which I acquired at government surplus, I have gone much higher than 44,000 feet. At 60,000 feet, the sky is black and one can see the Sun, the Moon and the stars at the same time, so for all practical purposes, you are at the edge of space. This is what the U2 pilots called the dead zone. At an altitude of 64,000 feet, without some form of pressure suit your blood would start to boil. At that altitude you would be dead in less than a minute, so don't do this at home!
The diagrams below are similar to my first electric airplane:
About the Jet Stream...
If you climb in a rocket to 44,000 feet, you will, if you are not careful, end up a thousand or more miles east of your home base, with certainty. This is because of the Jet Stream. The Jet Stream is a high speed wind between approximately 40,000 feet and 55,000 feet. It blows at an average speed of 400 mph, so 44,000 feet is not a good place to be. If you were unlucky you could end up in Russia or Armenia. The secret is to get from 40,000 feet to 60,000 or more feet as quickly as possible, if you wish to explore subspace. Likewise, when you return to your home base, it is wise to get down to 35,000 feet, or less, as quick as you can!
Jet streams are caused by a combination of a planet's rotation on its axis and atmospheric heating by solar radiation. The jet stream is located near the tropopause, the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height). The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions. The strongest jet streams are the polar jets, at around 7-12 km (23,000-39,000 feet) above sea level, and the higher and somewhat weaker subtropical jets at around 10-16 km (33,000-52,000 feet). The northern hemisphere and the southern hemisphere each have both a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.
Anaerobic Impulse Jets
One of the objections to a battery powered airplane is the cost. Those days it worked out at about eight hundred pounds per go, so today it would be about £4,000 at least, if you were able to get a large quantity of AA cells from a wholesaler, at a decent price. The following, is a cheaper solution based on the Impulse Jet used on the Doodle Bug flying bomb, during World War Two. Although I have lacked the courage to try this type of motor with a manned glider, I have built a successful model, and I believe a scaled up version would work.
Sometime about the end of 1970 an American Magazine 'Modern Mechanix' advertised plans, so that model airplane enthusiasts could build their own Impuse Jet. (Note: these are illegal in the UK). An impulse jet is simple to construct, and it consists of a long tube with a louver in front to admit the air, fuel injectors and a spark plug.
The design I constructed had a thrust of 40lb and it worked on acetone, and nitrous oxide when operated at high altitude. It was about three feet long and, four inches in diameter and it was powerful enough to push a bicycle at more than 60 miles an hour.
A basic impulse jet using petrol will flame-out at about 50,000 feet. However, because of its simplicity, this type of jet will work on a wide range of inflammable liquids and gasses. Certain liquids such as Hydrazine, Nitro methane and Ethelyne Oxide and gasses such as Acetylene require little or no Oxygen once they have been ignited. For example: if an impulse jet is fueled with Acetylene it will act as a jet motor until it reaches an altitude of circa 45,000 to 50,000 feet, then the pressure in the combustion chamber will keep the louvers at the front of the engine closed and it will act like a rocket.
The best gas for this purpose is Acetylene. It liquefies at minus 82 degrees C , the critical pressure is 600 pounds per square inch, so a separate fuel pressurization system is not needed and it gives a specific impulse of 420 when burning in air and a specific impulse of about 250 as a mono-propellant.
Experiments with large models (50 kilos +) suggest this type of motor will work at 100,000 feet on a full scale manned craft and that speeds of 500 miles per hour are realistic.
Balloon Assisted Gliders
Electric propulsion is expensive and to use up a set of batteries before you have reached 30,000 feet, because there are no thermals that day can be very frustrating; a balloon provides a method of reaching altitudes of between 50 & 80,000 feet (depending on the type of balloon) before you throw the electric switch or turn on the acetylene etc.
There are several types of balloons with different costs, dangers and levels of performance as outlined below...
Hot Air Balloons: These are the cheapest and the safest, but they have the lowest level of performance. Hot air only provides about 20 pounds of lift per thousand cubic feet, so the effective ceiling is about 36,000 feet. Furthermore a hot air balloon requires oxygen from the air, and continuous attention so it is not suitable for a unmanned vehicle.
Hydrogen Balloons: Hydrogen has the greatest lift of all the gasses, circa 80 pounds lift per thousand cubic feet. The maximum practical altitude for a manned vehicle would be 100,000 feet or so and the maximum for an unmanned model would be about 130,000 feet. Higher altitudes are possible, but this requires millions of cubic feet of hydrogen or helium. Note: Hydrogen balloons have reached 100,000 feet manned and 130,000 feet unmanned but these balloons have capacities of millions of cubic feet. There are disadvantages, however. Explosion and the fact that hydrogen is stored in cylinders, so a balloon large enough to lift a glider would require a large lorry load of cylinders.
Helium Balloons: Helium lifts about 70 pounds per thousand cubic feet so it is comparable with hydrogen. It has the advantage that it is non-inflammable, but it costs at least ten times as much as hydrogen and like wise, you will need a good lorry load of cylinders.
Ammonia Balloons: Ammonia gas has just over half the weight of air, so it will lift about 35 pounds per thousand cubic feet. It is not very inflammable, but it is toxic. Ammonia is stored in liquid form, rather than as compressed, so sufficient to fill a balloon could be fitted into a small van. Although not as powerful as Hydrogen a manned Ammonia balloon could easily reach 60,000 feet. I have experimented with this type of balloon in the 1960s, so I know from experience.
Coal Gas Balloon: Coal Gas or Methane (CH4) has about the same lifting power as ammonia and for most people it is literally on tap for about 6 pounds per thousand cubic feet. Yes! it is inflammable, but it is much safer than using hydrogen or traveling by rocket.
Note: When using balloons to lift a glider, at least three balloons should be used and the glider should be at least 50 meters below the balloons to place it beyond the radius of explosion from over-pressure of the balloons.
A small, self-guided glider, designed to fly at very high altitudes. The glider is carried up to altitudes of up to 85,000 feet above sea level, and then released to fly back to the launch point. During the flight, it transmits down navigation data, some sense data, and low-res digital photos. The combination of weather balloon, GPS, and sailplane technology makes it possible. Above 60,000 feet, the sky is black, the horizon is curved, and incredible speeds are obtainable in the frigid, near-vacuum environment.
The Rockoon is a cost effective method of sending a rocket projectile to high altitudes. It comprises a high altitude balloon, which takes the rocket up to 80 or 100,000 feet before the rocket motor is ignited. This method was much used by NASA during the 1950s.
At an altitude of 100,000 feet a rocket is above 99% percent of the atmosphere; in the absence of significant air resistance a rocket will reach four to ten times the altitude it would reach were it fired at sea-level. In short, all things else being equal, a small rocket will perform as well as a large one. A small rocket weighing a kilo or so that would be hard put to reach 500 miles an hour at sea level will reach more than a mile per second at 100,000 feet.
I have conducted some experiments with the Rockoon using two types of commercially available small rockets. The first type of rocket was the Le Prieur Signal rocket that can be purchased from yachting supplies for about five pounds. Fired at sea level this type of rocket will reach a top speed of about 400 mph and carry a 25 gram flare to an altitude of just over 1,000 meters. The second type of rocket is the Boxer Line carrying rocket. Without the line, it will touch 600 mph. and reach an altitude of 1,700 meters. If fired at 60,000. feet the signal rocket will continue to climb ballistically at least 10,000 meters above its launch point and the larger Boxer rocket more than 15,000 meters.
Both these types of rocket have a good thrust to weight ratio, in excess of 30 to one, so a multi-stage rocket composed of signal rockets at five pounds per round and Boxer rockets at thirty pounds per round could be constructed and velocities of more than two kilometers per second could be reached easily, if the rocket in the rocket balloon combination, was not ignited, until the balloon reached 50,000 feet.
I was unable to take this series of experiments to its final conclusion due to the over active interest that the Police showed towards my experiments - the only official body that has ever shown an interest - otherwise I would have gone the whole hog and put up something rather large with many stages to see how far it would go.
Solid Fuel Rockets
Although liquid fuel rockets provide the highest specific impulse Hydrogen/Oxygen having a specific impulse of about 500 liquid fuel rockets have not been very lucky for me. The would be experimenter would fin that he has two problems if he wishes to construct a small (less than 50 kilos) liquid fuel rocket. Generally, high impulse liquid fuels have a low density, for example Hydrogen Oxygen has a density of 0.324, so the first problem is to get sufficient fuel to the combustion chamber, such that the thrust of the rocket is greater than its weight, and the second problem is to get a good ratio between the weight of the fuel and the dry weight of the rocket.
In my experience with high energy fuels, I used liquid C02 at 50 Bar to pressurize the fuels, as a turbine was too complex and I was only able to put about 50 pounds of fuel in a rocket with a gross weight of 50 kilos - a ratio of less than one pound of fuel to one pound of rocket, when Acetylene was the fuel. The net effect was a rocket that fizzed on the ground until the fuel was half expended, it rose to about fifty feet then exploded.
Although a small liquid rocket can be built if one uses high density oxidizers such as Fuming Perchloric acid and high density fuels such as Carbon Disulphide. With such fuels the specific impulse is low - less than 200 and the fuel to dry weight ratio is only about two to one. In contrast, the best solid fuels have a specific impulse in excess of 300 and with careful construction a fuel to dry weight ratio can be better than six to one. In short, a good home made Solid Fuel Rocket will always out perform a homemade Liquid Fuel Rocket unless it weighs a couple of tons or so.
In the closing phases of World War Two the Rheinnebotte Metal Company developed a solid fuel rocket called the Rheinebotte for use on the Eastern front. The Rheinebotte was a four stage solid fuel rocket using black gunpowder (specific impulse 90). It weighed just over a ton and it would reach 4,000 miles per hour (faster than the V2) and it would hurl an 88 kilo warhead 280 miles. In addition, a five stage prototype was built. Although this version was never fired in anger, it would hurl a 40 kilogram warhead over 400 miles, and without a warhead, it would attain 5,800 miles per hour, traveling 600 miles down range.
The four stage version was developed from scratch in 16 weeks, several hundred were used against the Russians with effect and it cost about £4000 a go. In contrast, the V2 took eight years to develop and cost 50 times as much as a Rheinebotte.
Filled with a modern propellant (Ammonium Perchlorate and Aluminium specific impulse 300 in vacuum), a four stage Rheinebotte would reach 15,000 miles per hour and go from here to Australia and the five stage version would reach orbital velocity.
Small Solid fuel Rockets are easy to build, because there are no moving parts. Fuels, such as Sodium Chlorate (weedkiller) and Sugar will realize a specific impulse of 150+ and with the addition of Aluminium, or better still Magnesium, specific impulse can be raised to more than 250. Thrust to weight ratio is between 15 and 20 to 1, so up to eight stages are possible before the rocket gets too heavy to leave the ground and, at the present cost of Sodium Chlorate Weedkiller and Sugar, the net costs of construction are less than 75p per pound. A well constructed 50 pound rocket would contain 43 pounds of fuel. A ratio such that the rocket, in the absence of air resistance could reach twice its exhaust velocity (circa 3,000 miles per hour, sufficient to loft the rocket to 80 miles).
Very high ratios of fuel to empty weight, ratios of more than twelve to one are attainable, by using a propellant based on Silver Biflouride (density 4.58. H20 = 1) or Silver Perchlorate, mixed with Alkali Metal Cyanides and Azides. A gunpowder type rocket fuel typically has a density of about two, whereas a silver based solid rocket fuel has a density of about four, so a much better ratio of fuel weight to dry weight is possible.
Note: This mixture is expensive, poisonous and very explosive so its use should be restricted to upper stages, then the consequences of a mishap are far away.
An amateur, multistage rocket being launched
The Ramrocket or Solid Fuel Ramjet
The Solid Fuel Ramjet represents another low cost low tech solution to problems of amateur spaceflight. This technique is frequently used with Air-to-Air Missiles operating within the atmosphere to give an extended burn time.
A Ram Rocket is not difficult to make and specific impulses in excess of 900 can be realized, as a Ram Rocket does not need to carry its own oxygen.
A basic Ram Rocket is essentially an open ended rocket. Air comes in at the front and causes the combustion of the solid fuel. A possible fuel for this type of application would be wax with 33% Magnesium Powder and 10% Potassium Nitrate to ease ignition. A Ram Rocket has to be moving forward at 400 miles per hour plus, before it is lit otherwise it will belch forth at both ends. It will not, however, work much above 70,000 feet, as it gets its oxygen from the air. It will serve best as a sustainer or second stage of a multi-stage rocket.
Both the above problems can be solved with more sophisticated fuel mixtures and starting procedures too complex to deal with here and burn times in excess of ten minutes are possible, so the Rocket Ramjet is a possible for the propulsion of a high altitude glider.
Many miniature rockets that are easily available are capable of quite a high speed if they could be fired under vacuum conditions, for example from a high altitude balloon at 50 to 100,000 feet.
A Le Prier Signal Rocket would reach about 1,500 mph; a Boxer, described above, would exceed 2,000 mph; the small model rockets sold in sportshops, would reach 800 mph; and the larger type of firework rocket, say 400grams and up, would reach more than 650 mph. These estimates are conservative and are based on firing captive rockets, measuring the thrust and duration of thrust, then calculating the velocity that would obtain in the absence of air resistance.
In real world applications as upper stages in a multistage rocket system the Vacuum Specific Impulse would be 15 to 20% higher than that which would obtain at sea level.
Shop bought rockets are a low cost solution to the construction of upper stages and they are large enough, that with a modem some sort of signal could be returned to earth.
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