it's hard to explain the engineering marvel that is the sr-71 blackbird a long-range plane capable of flying 26 kilometers above the surface of the planet so high that the pilots could see the curvature of the planet and the inky black of space from their cockpits it flew so fast that the engineers had to develop entirely new materials and designs to mitigate and dissipate the heat generated from aerodynamic friction entirely unique engines were needed to function from zero all the way up to mach 3. 2 dealing with the myriad of problems like cooling fuel efficiency and supersonic shockwaves interfering with airflow a plane so advanced that when it detected a surface-to-air missile its response was simply to change course and speed up even though the missiles had a higher top speed they couldn't achieve the range and high-altitude maneuverability the Blackbird could this allowed the sr-71 to run hundreds of missions through Vietnam North Korea and Iraq without ever losing an aircraft to enemy fire despite multiple attempts the entire plane was built around the propulsion system which alone was a miracle of engineering design for one no turbine driven jet engine can operate with supersonic flow at its in list yes this plane was powered by the pratt & whitney j58 turbojet engine forget this off the shelf these engines could only provide seventeen point six percent of the thrust required for mach 3. 2 flies a speed which the sr-71 could cruise at for extended periods of time how on earth did it manage that in order to achieve those kinds of speeds a ramjet is typically needed a ramjet as you can probably guess from the name relies on RAM pressure to operate RAM pressure is simply the pressure that occurs as a plane rams itself through the air so as the engine moves through the sky it funnels this high-pressure air inside before entering the combustion chamber the supersonic airflow must be first slowed down this basically acts like the compressor stage of a normal jet engine elevating the air pressure before it enters the combustion chamber once the air enters the combustion chamber it is mixed with fuel and ignited it expands and accelerates once again out of the exit nozzle with no moving parts this type of engine is capable of flying at speeds far greater than a typical turbine driven engine but it cannot start from zero this needs forward movement in order to achieve the correct compression of air in the combustion chamber so there are either dropped from a conventional plane have a secondary propulsion system or are a hybrid of a conventional jet engine and a ramjet which is precisely what the sr-71 used the turbojet j58 engine of the sr-71 is nestled inside the nacelle here in front and around the j58 is a complicated system of airflow management these control mechanisms allow the propulsion system to transition from a primarily turbojet engine to a ramjet engine in mid-flight first the inlet spike this is capable of moving forward and back by 0.
66 meters this adjusts the inlet and throat area which controls the airflow entering the engine it also keeps the position of the normal shock wave at its ideal position between the inlet throat and the compressor this is the most efficient position for the shock wave as it minimizes the energy lost due to drag as air flows over the shock wave the inlet spike stays in the forward position until Mach 1. 6 after this point it begins to move backwards by 41 millimetres for every 0. 1 increase in Mach number keeping the shock wave in its ideal position the inlet spike contains perforations which connect to the outside of the nacelle through ducts initially the airflow will come from the outside in to provide additional airflow to the turbojet engines but once the plane hits about Mach 0.
5 this airflow reverses as the plane speeds up the inlet spike develops a significant boundary layer of air a boundary layer is a layer of very slow-moving air that clinks to the surface of objects by bleeding this layer of slow-moving air off the inlet spike it frees up a greater area of the inlet area for high-energy fast-moving air and thus improves efficiency around the engine there is a bypass area which takes air from the inlet and bypasses it around the j58 engine this air was used to cool the j58 which again improved engine efficiency and allowed the plane to fly faster after the air passes the engine it rejoins the airflow just after the engine afterburner adding additional thrust as more oxygen becomes available for combustion and increases the pressure through the ejector nozzle air got into this bypass area in a number of ways there was a shock trap otherwise known as the cowl blade located here which again helped minimize boundary layer growth there were suckin doors located here which only opened from Mach zero to Mach 0. 5 to add additional air to the bypass for engine cooling air from the aft bypass doors located just before the j58 engine also fed into the bypass these together with the forward bypass doors which vented to the atmosphere were used to control the pressure level in the inlet at the optimum level if it was getting too high a pressure sensor would trigger the forward bypass doors to open allowing more air to exit the inlet while the aft bypass doors were controlled by the pilot these doors played a critical role in maintaining the position of the normal shock wave if this was mismanaged the engine would lose control of the normal shock wave and may even spit it out of the intake resulting in a sudden power loss called an on stairs which would cause the plane to violently yaw in the direction of the faulty engine if this happened the forward bypass doors opened fully and the spike would move to the forward position to reduce back pressure and get the shockwave back into its normal position besides this bypass area that took air from the inlet and dumped it into the ejector there were also six bypass ducts that took air from the compressor and dumped it directly into the afterburner these ducts were the primary mechanism that transformed the engine from a turbojet into a ramjet afterburners are great they significantly add to thrust without needing a whole lot of additional waste they basically just inject fuel into the exhaust of the jet engine and ignited with whatever oxygen is left to provide additional expansion and therefore thrust but they are really inefficient however as the speed increases they are the only feasible way to generate thrust and they do gain efficiency thanks to the forward motion providing the compression of air needed to run them instead of the turbine needing to be powered to turn the compressor stage the crazy thing about the sr-71 is that the engineers could have eked out more thrust from this engine to increase the top speed even more ram jets can go up to Mach 5 so why did they stop at three point two would they have run out of fuel fuel efficiency in terms of cost doesn't mean a whole lot to a military plane like this the military doesn't care about cost but the more fuel you carry the heavier and bigger the plane gets increasing the fuel it uses there is a break-even point and the planes range will be limited but the engineers did manage to fill the plane up with an astounding amount of fuel with some clever engineering the plane was strictly a surveillance plane so no internal volume was used for weapons freeing up space for fuel you have probably heard that the sr-71 leaks fuel on the runway because there were gaps in the fuselage but that's a simple fact that ignores much of the engineering that caused it the sr-71 used something called a total west wing fuel tank system which meant that the fuel was not contained within a separate fuel bladder this was a weight saving measure separate metal fuel tanks would add too much weight and lighter plastic ones would melt from the intense heat generated from the aerodynamic friction so the fuel was contained by the skin of the plane itself the engineers applied sealant to every gap the field could possibly come out of but because the titanium skin of the plane expanded and contracted with every flight it gradually deteriorated over time allowing the fuel to leak out because of this the sr-71 had to regularly go into maintenance and have sealant reapplied but it usually came back still leaking just not quite as much the number of man hours required to reduce it to zero was simply too great to fit it between flights so they just had an allowable fuel leak limit which looked like this this plane like a rocket was mostly fuel it's dry waste depending on sensor payload was between 25 and 27 tons it's wet waste was between 61 and 63 tons making it by weight 59% fuel to feed those hungry engines even then without the ability to refuel in the air this plane would have had terrible range for what was supposed to be a long-range spy plane range varied greatly for example the engines became significantly less efficient when the outside temperature was higher a fully loaded sr-71 could expect to burn nearly 13 metric tons of fuel accelerating from Mach 1. 25 at 30,000 feet to mach 3 at 70,000 feet if the outside temperature was 10 degrees Celsius above standard that is 36% of its fuel capacity if it was 10 degrees below standard the fuel burn nearly half to 7.
2 tons and of course the range was severely affected by their speed and use of the afterburner but on average the sr-71 had a range of about 5,200 kilometers a enough for a one-way trip from New York to London not terribly useful the US was not going to be landing at their target to hand over a top-secret plane to the enemy however with aerial refuelling the plane could stay in the air more or less indefinitely provided there was no mechanical issues really the range was entirely determined by the pilots the longest operational sortie occurred in 1987 when the u. s. flew the sr-71 from Okinawa to observe developments in the iran-iraq war this mission lasted 11 point 2 hours and likely required at least five aerial refuelling x' along the way so if it wasn't the fuel or engines that limited the sr-71 stop speed what did at Mach 3 point to the nose of the sr-71 reached 300 degrees Celsius while the engine nacelles could reach 306 at the front and 649 at the back this is what truly limited the top speed of the sr-71 without careful material selection and design the plane would simply overheat and fail even the fuel needed to be specially formulated to get around these overheating issues this was a specially formulated fuel called jp-7 which has very low volatility and a high flash point this was partially needed because the fuel leaked on the runway and they needed a fuel that wouldn't ignite or easily evaporate and make the ground crew ill but mostly they needed a fuel that wouldn't vaporize in the tanks and cause fuel feed and pressurization problems the jp-7 fuel was so stable that it actually doubled as a coolant for the entire plane the fuel was pumped around the airframe to cool critical components like the engine oil hydraulic systems and control electronics when the fuel got too hot it was simply sent to the engines for combustion the fuel was so stable that the plane actually needed to carry shots of triethyl boring a fuel that spontaneously ignites in the presence of oxygen to start the combustion cycle and afterburners the plane usually only carried about 16 shots of this so the pilots needed to manage them carefully particularly when slowing down for refueling and managing on starts one huge question I had about the sr-71 was why it was painted black airliners are all white to reflect heat and prevent the plane from overheating if that applies to an airliner why not the sr-71 the sr-71 s predecessors were unpainted which saved waste and the areas exposed to highest temperatures were painted black why was this surely black would absorb more heat the Concorde was once painted blue for a Pepsi ad campaign and had to lower its speed as it absorbed too much heat from the Sun however the concorde did not fly nearly as high or as fast as the sr-71 and as the sr-71 Rose the energy it absorbed from the Sun dwindled in comparison to the heat it gained from aerodynamic friction for this we have to refer to something called kirchoff's rule of radiation which tells us that a good heat absorber like a black object is also an equally effective heat and Mitter so the black paint helped the sr-71 radiate heat away from the plane as it allowed the plane to radiate more heat than a gained from the radiation from the Sun these efforts helped to keep the plane cool but the structure of the plane still needed to be incredibly heat stable aluminium is typically the material aircraft engineers turn to it was used for the Concorde but as we saw it too had its speed limited by heat to a much lower Mach 2 aluminium is cheap has a great strength or a ratio and is easily machinable titanium the material that made up 93% of the sr-71 has only one of these properties its strength to weight ratio otherwise known as specific strength is fantastic but titanium is incredibly expensive despite it being the seventh most common metal in Earth's crust the refinement process is incredibly long and requires expensive consumables it's also not easily machinable as it readily reacts with air when welding or forging becoming brittle for these reasons titanium is rarely used in structural parts in aviation however the real benefit of titanium is its ability to resist heat the reasons for this are complex that we will explore in depth in future however the gist is that titanium alloys have incredibly strong bonding within its crystal lattice that resists heat from breaking them apart titanium alloys can resist temperatures up to 600 degrees Celsius before their atoms begin to diffuse and slide over each other significantly allowing it to retain much of its strength even at 300 degrees it has also very low thermal expansion so that expansion and contraction we mentioned earlier is minimized reducing the thermal stresses in the aircraft but titanium has its limits and for the sr-71 this was about 3.
