Amateur Rocket Launch Assist (ARLA)

About this Study

This report documents the results of a study into the possibility of using both mechanical and airbreathing propulsion modes to assist in launching rockets. Low pressure launch tubes and ramjets were identified as having the most potential for performing these functions. The near term users of this concept are expected to be amateur rocketeers.

This report includes the concept, system level components, critical materials, and technologies needed to develop this concept into operational systems. The tools and skills already exhibited within the amateur rocket community are expected to be sufficient to increase the capabilities of the sport ten-fold.

This report does not develop any new rocket technologies. Nor does it offer specific designs for system components though it does suggest how these might be developed by the amateur rocketeer.

During concept development for the Small-Pogo project (http://www.ptw.com/~oglenn/pogo/s-pogo/s-pogo.htm) it was realized that there may be other uses for the intermediate demonstrators such as launching amateur rockets to increase their peak altitude. There was also a desire to achieve the highest possible velocity during airbreathing flight while keeping costs and complexity to a low enough level that an amateur rocketeer could build the components in her/his garage. The simplest airbreathing engine identified was the ramjet which can reach speeds well in excess of Mach 5 (ref Thomas, fig 9) and possibly above Mach 12 (ref ARS). The ramjet, however, produces zero thrust at zero velocity and the Isp does not match that of rockets until about 400 mph (ref Casamasa, pg 63). This led to a search for simple ways to get the ramjet started (http://www.ptw.com/~oglenn/pogo/s-pogo/ramstart.htm). The low pressure launch tube was selected because of its low technology and simplicity.

During interviews with ramjet engineers it was noted that to get to Mach 5 and beyond the vehicle should be accelerated to Mach 2 (about 2000 fps) before ramjet takeover. With the constraints of garage-level technology and a Mach 2 minimum starting velocity the study turned to gun launch, or rather tube launch, systems for the initial velocity.

To keep the launch tube system suitable for garage-level technologies the ability to use low gas pressure was desired. Many retail stores sell air compressors that provide 125 psi pressure air. Local welding supply houses provide a variety of gasses in 2,200 psi bottles in sizes from a few standard cubic feet (scf) to 1,100 scf. Residential water pipes, including PVC garden pipes, are typically rated at up to 200 psi and are relatively inexpensive. However, these are generally not recommended for gas pressures for safety reasons. Many steel supply houses maintain stock aluminum and steel pipes in sizes up to 24 inch diameter which are suitable and safe. This left only a suitable gas to be found. A review of various gasses found several that could be used, helium being particularly attractive.

While other combinations of mechanical, airbreathing, and rockets are possible, the Tri-Mode, as discussed in this paper, uses low pressure launch tubes, ramjets, and conventional rockets.

Over the years I've noticed some confusion and misconceptions about what a ramjet is. For example, I was once told by the proprietor of a local hobby shop that Dynajet sells ramjets. I later found out that it's fairly common for people to refer to pulsejets as ramjets. Here are a few words to help distinguish between the various types of ramjets and other types of jets.

• Ramjet: The basic ramjet is an airbreathing engine that derives its air compression from it's motion through the air only. Thrust is strictly from heating of that compressed air (through combustion of fuel) and exhausting it from the back. Another name for ramjets is "blowpipe" because it resembles a pipe with a fuel injector and a flame holder.

  • Subsonic Ramjet: A ramjet designed to operate slower than the speed of sound.

  • Supersonic Ramjet: A ramjet designed to operate faster than the speed of sound.

  • Subsonic Combustion Ramjet: Ramjet which slows the air to subsonic speeds for combustion. Unless a ramjet is specifically designated as a Supersonic Combustion Ramjet, or SCRAMJET, it is a subsonic combustion ramjet. This includes 99.9999% of all ramjets to date.

  • Supersonic Combustion Ramjet (SCRAMJET): Supersonic ramjet which does not slow the air down to subsonic speeds for combustion. The first publicly acknowledged free-flight of a SCRAMJET occurred in 2003. There had been a few laboratory tests prior to that.

  • Liquid Fueled Ramjet: One that uses a liquid fuel, such as kerosene.

  • Solid Fueled Ramjet: One that uses a solid fuel, often magnesium based.

  • Gas-Generator Ramjet: One that has a section to convert liquid or, more typically, a solid fuel to a gaseous state for combustion. This is usually accomplished by burning a fuel-rich rocket propellant. These can also be pressure jets.

  • Ducted Rocket-Ramjet: This uses a single combustion chamber for both rocket thrust and ramjet thrust. Typically the vehicle is accelerated to ramjet speed by a solid rocket motor. When the rocket fuel is expended an air duct is opened in the front, the exhaust nozzle is expanded, and the rocket combustion chamber becomes the ramjet combustion chamber.

• Pulsejet: Pulsejets have intermittent thrust and typically use valves in their inlets to prevent exhaust gasses from going forward. Thrust is derived from the fuel/air mixture inside the jet combusting and exiting from the back.

  • Valveless Pulsejet: This is a variant on the pulsejet. It has an intermittent thrust but, instead of using valves, it uses the configuration of the combustion chamber to ensure that more thrust is aft than forward.

  • Pulse Detonation Wave Engine: Where a standard pulsejet deflegrates (flame front moves at subsonic speed) its fuel/air mixture the PDWE detonates (flame front moves at supersonic speed) its fuel/air mixture. A detonation wave is more efficient at converting pressure to gas velocity than deflegrating fronts, but only slightly.

• Pressure Jet: This type of jet uses high speed gasses to entrain air and ensure that the exhaust goes out the back. A simple type of pressure jet pipes propane through the combustion chamber to superheat it. The propane is then released through a nozzle located near the inlet and facing aft. The superheated propane enters the airstream at supersonic speed and draws fresh air with it for combustion.

  • Gluhareff: A variant on this is the Gluhareff which improves performance by sonic tuning.

  • Injector Ramjet: The injector ramjet is a pressure jet which may use a rocket motor for the injection gasses.

• Motor Jet: This is a jet engine that uses a non-turbine engine to drive a blower to produce a jet of air inside a duct. An example of this is the Dynamax ducted fan for model airplanes.

• Turbo Jet/Turbo Fan: This is a turbine-driven jet.

• Variants: Of course, there are variants on each of these as well as innumerable combinations.

A review of the available literature showed several partially related topics.

Two 1994 postings to sci.space.tech were found in my files on 12 Dec 98 that seem to be relevant.

• The first was posted by Tim Miller on September 30 with the thread title "Maglev launcher."
  • "Dean Ing developed this concept in a novel called either _The_Big_Lifters_ or _The_Heavy_Lifters_, in which our heroes who were the design team for the maglev train, developed a scheme for boosting payloads into LEO for *dollars* a pound."
  • "His treatment consisted of a payload module, rocket module, ramjet module, and maglev rail. The maglev boosts the module assemblys to low ramjet speeds; the ramjet (scramjet?) runs the assembly to sub-orbital altitudes, and the rocket takes you the rest of the way."
  
  
• The second was from Dani Eder on October 19 with the thread title "Re: Microwave launch assist (was Maglev launcher)."
  • "It is much more efficient to put a combustion chamber at the back end of a big pipe, into which you insert the vehicle you want to accelerate. This concept is called 'Rocket-Fed Gas-Gun'.
  • "The exhaust gases from the combustion chamber keep the barrel filled behind the vehicle, so the pressure, and thus acceleration, is fairly level. This is in contrast to a conventional gun, where the pressure is at a peak initialy, and drops thereafter."
  
  
• As I stated, these were in my files, though predating my own interest in such things. I will have to review both these in more detail to see how far the ideas were developed.

Dani Eder also suggested a two stage launch system which used a high temperature, high pressure helium gas gun (ref eder, ). This system was expected to achieve 2 km/sec velocities while recovering much of the gas. The second stage was a rocket. This is similar to the Tri-Mode but without the ramjet stage. The major differences in the first stage are gas pressures, tube length, and exit velocity. By using the ramjet stage it is expected that the launch tube would be significantly less expensive, the sonic boom (Mach 2 vs Mach 4) would be much less, and the rocket stage would be significantly smaller.

Gordon L. Dugger (ref Jamotte) analyzed expected performance characteristics of subsonic combustion ramjets to supersonic combustion ramjets in the Mach 5-10 range using kerosene fuel. He concluded that subsonic combustion ramjets should be possible to at least Mach 10. In the Mach 6-8 flight regime the efficiencies should be comparable. Above Mach 8 the supersonic combustion ramjet should be more efficient.

Arthur Thomas suggested using ramjets for spacelift in 1983. "The insertion of a ramjet stage as part of an accelerator for a ballistic missile (or surface-launched spacecraft) will increase the payload or reduce the launch weight." Extensive studies of this concept in the 1960's showed that single stage-to-orbit vehicles with high payload-weight ratios could be achieved by incorporating a ramjet stage." (Thomas, Exploding)

A review of the World Wide Web (WWW) found many sites describing the construction and use of "potato guns." Some of these use PVC garden plumbing and compressed air. These were all quite interesting but the highest velocity claimed was 450 mph. The largest being the Punkin Chunkin guns, one of which lobs 8-10 lbm pumpkins over 4,000 ft.

The review of the WWW also found a commercial air cannon called the LoCAT. This device launches explosive charges at 1,000 fps into snow banks near ski areas for avalanche control. This device is the length of a cannon and so the gas pressures must be much higher than anticipated for the Tri-Mode.

Uses and Benefits of the Tri-Mode

The Tri-Mode concept could allow amateur rockets to achieve altitudes never before possible. Most amateur rockets are currently achieving about 1,100 fps and 10,000 ft altitude. The more powerful ones are achieving 2,500 fps and 25,000 ft from a ground launch. The first two stages of the Tri-Mode would raise the rocket to 5,000 fps and 100,000 ft altitude at ramjet burnout. A rocket that added 2,500 fps would coast to an altitude of 973,000 ft or 184 miles altitude.

While ramjets have been operated, albeit inefficiently, as low as 250 mph (ref German) and at speeds well in excess of Mach 5, theoretically there is no limit to their velocity.

The Tri-mode offers four distinct advantages over building bigger amateur rockets to get to higher speeds and altitudes.

The Tri-Mode should have lower costs than all-rocket systems because of:

• Launch Tube Stage
  • The launch tube can be made from low-cost, easily available, robust components.
  • The launch tube hardware is completely reusable, only the low-cost gas is used up.
• Ramjet Stage
  • The ramjet is much simpler to build than a rocket.
• Rocket Stage
  • The launch tube alone can achieve the performance of some mid-sized rockets. The launch tube with the ramjet can outperform almost all amateur rockets built to date.
  • Lower technologies. The cost of technologies increases exponentially with performance. The Tri-Mode allows the use of three low technology propulsion systems.

For the Tri-Mode three teams could be working in parallel to develop low technology systems. This should be much faster than one team trying to develop a single high technology system. Because each mode of the Tri-Mode is independent of the others there should be few interface issues.

The performance of the Tri-Mode should be the same as much larger single, or multiple, stage rockets.

As the performance (i.e. altitude and velocity) increases the technology difficulty and expense increase exponentially. This is because ever higher percentages of the rockets must be in the fuel rather than the dry mass (everything but the fuel). In fact, this is true in general for all technologies. The Tri-Mode uses three different technologies, each at a fairly low level, making the overall effort simpler.

• Launch Tube Stage
  • The tube launch hardware can be built very robustly since it does not leave the ground.
  • There's no need to search for the lightest, strongest metals around.
  • Safe, inert gasses can be used as opposed to hazardous fuel/oxidizer combinations.
• Ramjet Stage
  • The ramjet uses a safer, much smaller amount of fuel than rockets.
  • The ramjet's Isp is typically 1,200 - 1,800 seconds as compared to a rocket's 350 seconds. This means that the ramjet stage can be about 1/10th the size of an equivelant rocket with equivelant savings.
  • The ramjet doesn't fire until well above the ground and the rocket stage doesn't fire until miles above the ground (assuming the ramjet is used as a second stage).

Mass of a rocket consumed during flight

The Tri-Mode is much more efficient than all rockets. The tube launch design does not lend itself easily to an Isp calculation but it's equivelant is probably in the thousands of seconds. The ramjet's calculated Isp is as high as 2,400 seconds (more typically 1,200 - 1,800) as compared to a rocket's 350 seconds.

Isp Comparison between rockets and ramjets (from multiple sources)

By using the tube launch first stage the launch vehicle is given a large velocity vector (up to 2,000 fps) in the desired direction. This eliminates many of the take-off control problems.

What's In This Report

• Safety and Legality Warnings

  This is a MUST READ before beginning any construction. This warns of significant, if not severe, safety and legal issues that need to be managed, not just considered.

• Launch Tubes
  • - Launch Tube Basics

    This section discusses some of the gas dynamics and kinematics of launch tubes.

  • - Example Launch Tube Designs

    This section describes two different launch tube designs and the necessary calculations for each.

• Ramjets
• - Ramjet Basics, Part 1
• - Ramjet Basics, Part 2

  These sections describe how a ramjet works, the essential components, and the critical design features to make them useful.

• - Review of the Bomarc Ramjet

  This section uses the Bomarc ramjet engine as a real world example of how a ramjet is constructed and how to apply the basic principles.

• - Garage Level Ramjet Construction

  This section describes how the amateur rocketeer might go about designing and building a ramjet. Two designs are offered, subsonic to low supersonic and high supersonic.