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Issue No 52, 4 December 2023
By: Anthony O. Ives
'Rocket Scientist' is often a term used to describe someone who is very clever even though its usually in a sarcastic way. However, the term is probably more related to rocket guidance than rocket motors. Rocket vehicles are unstable and require a guidance system to keep them from flying all over the place much like a busted balloon. The term could also relate to the compliciated mathematics that is needed to calculate speeds and trajectories of spacecraft in orbits and interplanetary spaceflight. The rocket motor itself is quite simple a gas turbine engine [1] is by far more complex.
A rocket motor in its most basic form consists of canister usually a cylinder which has an opening which a nozzle is attached to. The solid fuel is stored inside the canister when it is ignited the fuel burns to produce high pressure which then escapes through the nozzle and produces the forward motion. Rocket motors generally have no speed control hence the fuel usually burns at rate that gives a required speed once the rocket motor is ignited the object it propels will just keep going until the fuel burns out. The diagram below gives a simple illustration of a rocket/missle system:
The diagram contains some additional items such the front and back stabilising and control surfaces these surface will control the direction of the missle or rocket. The payload is usually contained in the nose which include things like warload (essentially a explosive device of some sort) and the guidance electronics. If you have read previous articles [2] you know my views on weapon systems, they can be fascinating but they can only be used to do one thing kill and destroy so I try to stay well away from them. The reason I referred to them here is because its the only place you will find a rocket motor on a helicopter obviously military helicopters.
Attack helicopters are helicopters that specifically designed to carry combat missions to destroy targets. Other military helicopters may carry weapons but they are there mainly for defence their primary mission may be to drop off or pick up troops, search and rescue, transport equipment, etc. However, attack helicopters have the primary mission of destroying targets, typical US examples include the Boeing AH-64 Apache, Bell AH-1 Cobra (which was derived from the Bell UH-1 Huey as a pure attack helicopter). Attack helicopters generally can carry three types of rocket powered weapons, air to air missles which are used to take down other aircraft, typical US examples are the Raytheon (AIM)FIM-92 Stinger, the Cobra actually uses the AIM-9 Sidewinder also made Raytheon. Both the sidewinder and the stinger are heat seaking or infra-red guided meaning the are usually guided to their targets by the heat the target is emitting from its engines. More commonly attack helicopters are used to destroy ground targets using the second weapon system which is air-ground guided missle, a typical US example is the AGM-114 Hellfire made by a number of manufacturers such Lockheed Martin, Boeing and Northrop Grumman. The name 'Hellfire' nasty as it sounds was actually colloquialized from the original meaning which was helicopter fire and forget missle. Fire and forget doesn't sound too nice either but refers to fact the missle can guide itself to the target as some earlier missle systems required the launch aircraft to maintain its position over the target. The final version of weapon system that uses rocket motors is unguided missiles, these missiles just travel in a straight line so have to be aimed similar to gun, they are probably very basic in design compared to other missile systems generally a lot of military helicopters will carry them not just attack versions.
Rocket motors are used in nearly all missile type weapon systems including those installed on fixed wing aircraft. Some experimental helicopters have used rocket motors on the blades tips to turn the rotor. Experimental helicopters that have used rotor tip jets [3,4] and their advantages/disadvantages is something I will discuss in another article. The Roton rotary rocket concept [5] was an very unusual helicopter that was designed as Single Stage To Orbit (SSIO) spacecraft, intially it may have been going to use rocket motors at the blade tips. However, later designs of the Roton rotary rocket just used the rotor for landing, conventional rocket motors where to be used to get the vehicle into orbit but the project finances dried so the Roton rotary rocket was never completed. Roton rotary rocket was considered novel as a rotor was used for landing instead of a parachute or wings.
In the civil world rocket motors are primary used in spacecraft. There are number ways to get a spacecraft into orbit one would be using escape velocity. Escape velocity is the velocity needed to escape the earth gravitation forces it would require a huge acceleration to reach escape velocity and huge G-forces this is why really accelerating something to escape velocity is impractical. The escape velocity has often been source of amusement in cartoons, etc where a cannon is used to propel someone into space. So therefore the multi-stage rocket was devised as the most practial method and hence most successful way to get into orbit. The multi-stage rocket builds momentum over time where each stage is optmised for the region of the atmostphere it is operating in then jettisoned once it burns out reducing the weight that has to be carried into orbit.
These multi-stage early spacecraft and most spacecraft at the time of writing are completely ballistic systems. Ballistic vehicles return to earth via gravity and rely on the atmostphere to slow then and just before touch down depoly a parachute so they land at a survivable speed. The trouble of multi-stage rocket which return from orbit via ballistic means is that they are one use only hence why space program can be so costly. The Boeing X-20 Dynasoar [6] is probably one the earliest attempts to build a spacecraft that at least was partially reusable. Dynasoar refers to dynamic soar as the aircraft glides back down to earth. X-20 was a USAF (US Air Force) sponsered project so its described as hypersonic strategic weapon system. The concept was to be launched a conventional multi-stage rocket into orbit then obviously launch is weapons once in orbit then return to earth and land as a conventional fixed wing aircraft. So therefore the vehicle could reused the only thing that was one use was the stages that got it into orbit.
X-20 might sound familar its very similar to how the Space Shuttle operated, X-20 really never left the drawing board but most of the work they did on it lead to Space Shuttle program. There has been attempts to Single Stage To Orbit (SSIO) spacecraft, this would mean the whole spacecraft is reuseable, the Roton rotary rocket [5] which was discussed earlier in the article was one such concept. The X-33 [7] was an unmanned concept that was Single Stage To Orbit (SSIO), both it and Roton rotary rocket were primarily designed to launch satelites into orbit. However, at the time of writing these concepts are more expensive that multi-stage systems hence have suffered cancelations due to cost overruns. The Single Stage to Orbit (SSIO) concept could also lead to development of spaceplane which could reduce travel between certain continents but with the technical and practical challenges only time will tell if its ever possible.
One of the interesting technical challenges that spacecraft need to overcome is overheating during re-entry, this is due to the high speeds experienced being in the excess of a mach number of 20. The equation of temperature ratio for isentropic flow will demonstrate that even for a very low ambient temperature where the flow impacts a surface the energy is transferred into high temperature heat. See below:
\[\frac{T_0}{T} = \left(1 + \frac{\gamma-1}{2} M^2 \right)\]
\[\frac{T_0}{T} = \left(1 + \frac{1.4-1}{2} (20)^2 \right)\]
\[\frac{T_0}{T} = 81\]
\[T_0 = 81 \times (-200 + 273.15) = 5925 K = 5652 C\]
The above calculation shows that even for a temperature of -200 C at Mach number of 20 temperatures can be as high as 5652 C when the flow impacts the surface. This is due to the kinetic energy of the flow changing to heat when the flow stagnates near the spacecraft surface. There is number of ways to protect the surface some spacecraft use ceramic type matericals which are highly heat resistance, others use transpiration cooling. Transpiration cooling injects fluid into area the flow contacts the surface. This is very similar to that experienced in engine turbine blades [1] and the method employed to protect the surface can also be similar. Anti-icing systems can also use similar systems but the difference being you want to heat the surface up to protect it form ice build up. In some future articles I will discuss the various systems however more in relation to engine turbine blades and anti-icing systems rather spacecraft. I will also explain the isentropic flow equations in more detail in a later article.
All sections of modern aviation have come to use GPS (Global Positioning Systems) or some other equivalent which are satelite systems and hence require spacecraft for launching, maintenance, etc. Satelite systems are also important for a wide range of other things such as weather observation and prediction. So while rockets and spacecraft seem a bit out scope for helicopters they are probably something you want to be a aware of.
Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:
Mathematics
Helicopters
Woodworking and Boatbuilding
If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.
References:
[1] http://www.eiteog.com/EiteogBLOG/No49EiteogBlogGasTurbine.html
[2] http://www.eiteog.com/EiteogBLOG/No17EiteogBlogMissions.html
[3] German Jet Engine And Gas Turbine Development, 1930-1945, Antony L. Kay, 2002, Airlift Publishing Ltd
[4] The Development of Jet and Turbine Aero Engines Paperback, 3rd Edition, Bill Gunston, 2006, Patrick Stephens Ltd
[5]'A New Spin on the Reusable Launch Vehicle', Bill Norton, Air International, Vol 58, No 2, February 2000, Key Publishing Ltd
[6] Dyna-Soar: Hypersonic Strategic Weapons System, Robert Godwin, 2003, Apogee Books
[7] The X-Planes X-1 to X-45, Jay Miller, 2nd Edition, 2001, Midland Publishing
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