{"id":28230583,"date":"2026-10-16T09:58:47","date_gmt":"2026-10-16T07:58:47","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/?p=28230583"},"modified":"2026-10-16T09:59:02","modified_gmt":"2026-10-16T07:59:02","slug":"sr-71-inlet-spike-explained-shock-waves-mach-3-unstart","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/sr-71-inlet-spike-explained-shock-waves-mach-3-unstart\/","title":{"rendered":"SR-71 Inlet Spike Explained: How the Blackbird Tamed Shock Waves at Mach 3"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\n<!-- mfsh:top -->\n\n<style>\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid #0F1720;background:#fff;color:#0F1720;font-family:inherit;font-size:14px;font-weight:700;line-height:1;letter-spacing:0;text-transform:none;cursor:pointer;box-shadow:none;transition:background .15s,color 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They are not decoration. At Mach 3 those spikes, and the doors and ducts behind them, did a job no jet engine of the time could do on its own: they slowed air arriving at more than 2,000 mph to below the speed of sound before it reached the compressor.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A reel doing the rounds on Instagram explains the basic idea correctly. Here is how the system actually worked, using figures from NASA&rsquo;s technical history of the Blackbird.<\/p>\n\n\n\n<div style=\"background:#f5f5f5;border-left:4px solid #1a1a1a;padding:18px 22px;margin:24px 0\"><p style=\"margin:0 0 8px;font-weight:600\">Quick Facts<\/p><ul style=\"margin:0;padding-left:20px\"><li><strong>Engines:<\/strong> Two Pratt &amp; Whitney J58 afterburning turbojets, 34,000 lb thrust each with afterburner at sea level<\/li><li><strong>Inlet type:<\/strong> Mixed-compression, variable-geometry inlet with a movable centrebody spike<\/li><li><strong>Compression ratio:<\/strong> About 40:1 at cruise<\/li><li><strong>Airflow:<\/strong> Each inlet swallowed roughly 100,000 cubic feet of air per second at cruise<\/li><li><strong>Thrust at Mach 3:<\/strong> Inlet 54%, exhaust ejector 29%, turbojet 17% (NASA figures)<\/li><li><strong>Failure mode:<\/strong> Inlet &ldquo;unstart&rdquo;: the shock wave is expelled and thrust collapses on one side<\/li><\/ul><\/div>\n\n\n<h2 style=\"padding-top:22px\">Why the Air Had to Be Slowed<\/h2>\n\n\n<p class=\"wp-block-paragraph\">A jet engine&rsquo;s compressor cannot cope with supersonic air. NASA&rsquo;s history puts it simply: air velocity had to be reduced because no existing engine could run on supersonic flow. The usual way to slow air down at supersonic speed is a shock wave. Air passing through a shock slows abruptly and its pressure and temperature jump. Do it in one big shock and you waste a lot of energy; do it in a series of weaker shocks and you recover much more pressure. That is what the SR-71&rsquo;s inlet was designed to do.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The principle is the same as the shock cone on a <a href=\"https:\/\/migflug.com\/afterburner\/mig-21-nose-cone-inlet-shock-cone-explained\/\">MiG-21&rsquo;s nose<\/a>, but the Blackbird pushed it much further. Its inlet was a mixed-compression design: some compression happened outside the inlet lip on the spike&rsquo;s cone, and the rest inside the duct, ending in a final normal shock that dropped the flow to subsonic speed.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/10\/sr-71-blackbird-left-engine-inlet-spike-1024x682.jpg\" alt=\"Close-up of the movable inlet spike of an SR-71 Blackbird engine nacelle\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The inlet spike of an SR-71. It moved fore and aft to keep the shock waves in the right place. Photo: Clemens Vasters, CC BY 2.0<\/figcaption><\/figure>\n\n\n<h2 style=\"padding-top:22px\">The Moving Spike<\/h2>\n\n\n<p class=\"wp-block-paragraph\">The spike was the key control. According to NASA, the cone translated forward and aft to control the position of the shock wave and the Mach number inside the inlet. At low speed it sat fully forward. As the aircraft accelerated beyond about Mach 1.6, it moved progressively rearward, by roughly 26 inches in total by cruise speed, keeping the oblique shock from the spike&rsquo;s tip positioned on the inlet lip and the inner shock in the right place in the duct.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Two sets of doors worked with it. Forward bypass doors opened and closed automatically, based on pressures measured in the ducts, to hold the shock in position. Aft bypass doors, set by the pilot according to Mach number and the position of the forward doors, controlled the airflow at the face of the engine. A bleed system also removed the slow, low-energy boundary layer that built up along the spike, so that the whole inlet passage carried clean, high-energy air.<\/p>\n\n\n<h2 style=\"padding-top:22px\">An Inlet That Made Most of the Thrust<\/h2>\n\n\n<p class=\"wp-block-paragraph\">At cruise, the J58 behaved less like a pure turbojet and more like a ramjet. A large share of the air entering the inlet bypassed the engine core through ducts and went straight to the afterburner, where at Mach 3 fuel burned more efficiently than in the main combustor. Engineers called the powerplant a turbo-ramjet. We look at what that meant for the aircraft&rsquo;s top speed in <a href=\"https:\/\/migflug.com\/afterburner\/sr-71-mach-4-blackbird-j58-limits-nasa-844\/\">SR-71 at Mach 4<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">NASA&rsquo;s numbers show how dramatic the shift was. At Mach 2.2 the inlet provided 13 percent of the thrust, the engine 73 percent and the exhaust ejector 14 percent. At Mach 3 cruise the inlet provided 54 percent, the ejector 29 percent and the turbojet itself only 17 percent. The air was compressed about 40 to 1 by the time it reached the engine.<\/p>\n\n\n<h2 style=\"padding-top:22px\">When It Went Wrong: The Unstart<\/h2>\n\n\n<p class=\"wp-block-paragraph\">The system depended on keeping the shock wave inside the inlet. Large airflow disturbances or a fault in the inlet control system could make the inlet expel the shock instead. That was an unstart. Pressure and airflow to the engine collapsed, thrust on that side vanished, and the aircraft lurched violently in yaw, pitch and roll. Crews were thrown hard against their straps, and their helmets could hit the canopy.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Unstarts were usually recovered by the inlet control system or the crew. But on 25 January 1966 an unstart during a test flight contributed to the loss of an SR-71 at Mach 3.17 over New Mexico, the accident that <a href=\"https:\/\/migflug.com\/afterburner\/bill-weaver-sr-71-blackbird-disintegration-mach-3-survived-1966\/\">Lockheed test pilot Bill Weaver survived<\/a> after the aircraft broke up around him. Later Blackbirds received a digital automatic flight and inlet control system that made unstarts far less frequent.<\/p>\n\n\n<h2 style=\"padding-top:22px\">What the Viral Reel Gets Right<\/h2>\n\n\n<p class=\"wp-block-paragraph\">The reel&rsquo;s short caption is accurate: the J58 could not take supersonic air directly, and the variable-geometry inlet with its movable spike controlled the shock waves, compressed the air and delivered stable flow to the engine. What it leaves out is the most surprising part: at Mach 3, that inlet was producing more of the Blackbird&rsquo;s thrust than the engine behind it.<\/p>\n\n\n\n<blockquote class=\"instagram-media\" data-instgrm-permalink=\"https:\/\/www.instagram.com\/reel\/DcOBPNfP5At\/\" data-instgrm-version=\"14\" style=\"background:#fff;border:0;margin:24px auto;max-width:540px;min-width:280px;width:100%\"><a href=\"https:\/\/www.instagram.com\/reel\/DcOBPNfP5At\/\">View this post on Instagram<\/a><\/blockquote><script async src=\"https:\/\/www.instagram.com\/embed.js\"><\/script><p class=\"wp-block-paragraph\"><em>Sources: Peter W. Merlin, &ldquo;Design and Development of the Blackbird: Challenges and Lessons Learned&rdquo;, NASA Dryden \/ AIAA 2009-1522; Wikipedia, Pratt &amp; Whitney J58 and Lockheed SR-71 Blackbird.<\/em><\/p>\n\n\n<!-- mfsh:bottom -->\n\n<button type=\"button\" class=\"mfsh-trigger mfsh-bottom\" data-mfsh-placement=\"bottom\" aria-haspopup=\"dialog\"><svg><use href=\"#mfsh-i-share\"\/><\/svg>Share this story<\/button>\n\n<!-- \/mfsh:bottom -->\n\n\n<style>.mfq{margin:34px 0 8px}.mfq h2{font:26px\/1.3 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117;padding-top:22px;margin:0 0 6px}.mfq .qa details{border-top:1px solid #5C91FF;margin:0;padding:0}.mfq .qa details:last-of-type{border-bottom:1px solid #e2e7ee}.mfq .qa summary{cursor:pointer;list-style:none;display:flex;justify-content:space-between;align-items:center;gap:16px;padding:22px 26px;font:18px\/1.7 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117}.mfq .qa summary::-webkit-details-marker{display:none}.mfq .qa summary::after{content:\"+\";font:24px Helvetica,sans-serif;color:#3568e0;flex:0 0 auto}.mfq .qa details[open] summary::after{content:\"\\2013\"}.mfq .qa details:hover summary{color:#3568e0}.mfq .qa .a{font:16px\/1.7 \"Gilroy regular\",Helvetica,Arial,sans-serif;color:#454e5e;padding:0 26px 24px}.mfq .qa .a a{color:#3568e0}@media(max-width:680px){.mfq .qa summary{padding:18px 14px;font-size:17px}.mfq .qa .a{padding:0 14px 20px}}<\/style>\n\n\n<section class=\"mfq\"><h2>Frequently Asked Questions<\/h2><div class=\"qa\"><details open><summary>What does the spike on the SR-71 engine do?<\/summary><div class=\"a\">The cone, or spike, in each SR-71 inlet moved forward and aft to control where the shock waves formed and how fast the air was moving inside the inlet. Together with bypass doors, it slowed air arriving at Mach 3 to subsonic speed before it reached the J58 engine, compressing it about 40 to 1 at cruise.<\/div><\/details><details><summary>How far did the SR-71 inlet spike move?<\/summary><div class=\"a\">The SR-71&rsquo;s inlet spikes stayed fully forward at low speed and began moving rearward as the aircraft accelerated beyond about Mach 1.6, travelling roughly 26 inches in total by cruise speed, to keep the shock waves correctly positioned on the inlet lip and inside the duct.<\/div><\/details><details><summary>How much thrust did the SR-71 inlet produce?<\/summary><div class=\"a\">According to NASA, at Mach 3 cruise the SR-71&rsquo;s inlet produced 54 percent of the total thrust, the exhaust ejector 29 percent and the J58 turbojet only 17 percent. At Mach 2.2 the inlet produced just 13 percent.<\/div><\/details><details><summary>What is an inlet unstart on the SR-71?<\/summary><div class=\"a\">An inlet unstart happened when an airflow disturbance or inlet control fault caused an SR-71 inlet to expel its shock wave. Pressure and airflow to that engine collapsed, thrust was lost on one side, and the aircraft yawed, pitched and rolled violently. An unstart contributed to the loss of an SR-71 at Mach 3.17 in January 1966.<\/div><\/details><details><summary>Was the SR-71 engine a ramjet?<\/summary><div class=\"a\">Partly. The Pratt &amp; Whitney J58 was an afterburning turbojet, but at high Mach numbers much of the air bypassed the engine core and went directly to the afterburner, so engineers described the SR-71&rsquo;s powerplant as a turbo-ramjet.<\/div><\/details><details><summary>Why can jet engines not use supersonic air?<\/summary><div class=\"a\">Jet engine compressors are designed for subsonic airflow; supersonic air entering the compressor would cause shock waves, stalls and severe losses. Supersonic aircraft therefore use inlets that slow the air through shock waves before it reaches the engine, from a simple shock cone on the MiG-21 to the SR-71&rsquo;s variable spike and bypass doors.<\/div><\/details><\/div><\/section>\n\n\n\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What does the spike on the SR-71 engine do?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The cone, or spike, in each SR-71 inlet moved forward and aft to control where the shock waves formed and how fast the air was moving inside the inlet. 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Supersonic aircraft therefore use inlets that slow the air through shock waves before it reaches the engine, from a simple shock cone on the MiG-21 to the SR-71\u2019s variable spike and bypass doors.\"}}]}<\/script><div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\">\n<p style=\"margin:0 0 8px;font-weight:600;color:#333\">Related Posts<\/p>\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/sr-71-mach-4-blackbird-j58-limits-nasa-844\/\">SR-71 at Mach 4: What It Would Take to Make the Blackbird Faster<\/a><\/p>\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/mig-21-nose-cone-inlet-shock-cone-explained\/\">MiG-21 Nose Cone Explained: How the Shock Cone Feeds a Mach 2 Jet<\/a><\/p>\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/bill-weaver-sr-71-blackbird-disintegration-mach-3-survived-1966\/\">The Pilot Torn From His Blackbird at Mach 3<\/a><\/p>\n<\/div>\n\n\n","protected":false},"excerpt":{"rendered":"<p>At Mach 3 the SR-71&rsquo;s engines could not swallow supersonic air. A moving spike and a set of doors held a shock wave inside each inlet, and at cruise the inlet produced more thrust than the engine itself. How it worked, and what happened when it failed.<\/p>\n","protected":false},"author":24,"featured_media":28230275,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665],"tags":[],"class_list":["post-28230583","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aviation-world"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SR-71 Inlet Spike Explained: Shock Waves at Mach 3 | MiGFlug<\/title>\n<meta name=\"description\" content=\"How the SR-71\u2019s moving inlet spike and bypass doors slowed Mach 3 air for the J58 engines, why the inlet made most of the thrust at cruise, and what an unstart was.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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Air Force's Plant 42 in Palmdale, California, shortly before delivery to the Ames-Dryden Flight Research Facility (later, Dryden Flight Research Center) at Edwards, California. NASA operated two of these unique aircraft, an SR-71A, for high-speed, high altitude research, and this SR- 71B pilot trainer for most of the decade of the 1990s. The \\\"B\\\" model is special because of its raised rear cockpit, which provided a second pilot position so a trainer and an experienced pilot could both see what was going on during flights. The SR-71 was designed and built by the Lockheed Skunk Works, now the Lockheed Martin Skunk Works. Studies have shown that less than 20 percent of the total thrust used to fly at Mach 3 is produced by the basic engine itself. The balance of the total thrust is produced by the unique design of the engine inlet and \\\"moveable spike\\\" system at the front of the engine nacelles, and by the ejector nozzles at the exhaust which burn air compressed in the engine bypass system. 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