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Some rockets, mainly (only?) ICBMs have Aerospike nosecones

Why are they preferred in lieu of traditional nose cones?

Illustrations:

enter image description here An aerospike nosecone on a trident ICBM

Video including the spike in flight deployment of a M51 ICBM:

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Aerospike nosecones have a couple of big benefits which, to fully appreciate, I'd need to give a little background on.

At the risk of dating myself, "Sherman, set the WABAC machine to the 1950s":

The United States had a problem. It was trying to develop a missile with a range of 1,000 miles based on the work of Wernher von Braun and a number of other German scientists after the war. It was widely recognized that the most efficient way to get a missile to travel with that kind of range was to launch it out of the Earth's atmosphere up into space and have it re-enter the atmosphere at its desired impact point. However, whenever they tried to re-enter anything from an altitude above about 100 miles, it would melt from the friction of air moving over it. Conventional wisdom told everyone that the answer was to make the nosecone more streamlined - that is, make it longer, sharper, like a bullet. But no matter how sharp or streamlined they tried to make it, the nosecone kept melting.

But then, H. Julian "Harvey" Allen came along. He was one of the greatest engineers of our time and fit pretty much every stereotype about engineers. He was odd, enthusiastic, random in his thinking, and brilliant. And Harvey had an idea:

If the nosecones kept melting because of the extreme friction, then maybe the answer was to make it less streamlined. He floated the idea of making the nosecone look like a "Civil War cannonball." He and his colleagues ran the numbers and published a paper in 1953 that stated "Not only should pointed bodies be avoided, but the rounded nosecone should have as large a radius as possible." By having a large, round nosecone, your body develops an extremely turbulent boundary layer on its leading edge. This boundary layer actually ends up with a fairly stable temperature and protects your body from the excessive heat of re-entry.

Wind tunnel tests of different nosecone geometries

By 1955, the Air Force had adopted Harvey's idea and developed the Atlas ICBM.

Ok, so let's fast-forward to the 1970s and the introduction of the aerospike. As a body approaches and exceeds Mach 1, it experiences a tremendous amount of heat and turbulence. It's really difficult to maneuver and control something in an environment that intense. Well, we knew from back in the 1950s that missiles needed round nosecones to reduce friction and aerodynamic heating, but we also knew that we needed pointy, streamlined bodies to reduce drag and conserve fuel when leaving the atmosphere. Adding an aerospike to a big, round nosecone is a great solution that gave you the best of both worlds! The aerospike pierces the air in front of the body and displaces the shockwave, reducing drag and friction/pressure changes around the body itself.

Aerospike Wind Tunnel Test

There are further benefits to an aerospike which might one day allow for supersonic travel over the continental U.S. (something which is currently banned due to - mostly - noise). By having a long aerospike, you can multiple small shockwaves instead of one very large shockwave. It's the keynote feature of the "Quiet Supersonic Jet". That's less applicable to your question about ICBMs - it's just more of a fun tidbit that I wanted to toss in there since I did some work on the QSJ a few years ago.

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Steve is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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    $\begingroup$ The round nosecone that the aerospike sticks out of is not the reentry body, so your sentence "Adding an aerospike to a big, round nosecone is a great solution to that problem." doesn't make much sense. The reentry vehicles are rounded cones hidden under that nosecone. i0.wp.com/www.llquakers.org.uk/hemel/web/wp-content/uploads/… What we see as the nosecone at launch is jettisoned before entry and the reentry vehicles are deployed. The rounded nosecone with aerospike is specifically an ascent design and has nothing to do with entry. $\endgroup$ – Organic Marble Apr 15 at 16:10
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    $\begingroup$ @OrganicMarble You're absolutely right, I worded that terribly! Made some edits and added some pictures. Thanks for the feedback! $\endgroup$ – Steve Apr 15 at 16:32
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    $\begingroup$ Interesting! How quiet can you make a supersonic jet with those? (Maybe this should be a question on Aviation?) $\endgroup$ – Eth Apr 15 at 17:44
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    $\begingroup$ @Eth they can get pretty quiet. The latest projections I heard were around 60-70 dB or so. The Concorde's sonic boom was in the 110 dB range so this would be somewhere between 15-30 times quieter. $\endgroup$ – Steve Apr 15 at 18:43
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    $\begingroup$ Note that it's not generally "friction" that causes the majority of the heating, but rather compression of the air. But for the purposes of your answer, keeping the boundary layer as far from the skin as possible helps with both sources. $\endgroup$ – Perkins Apr 15 at 23:13
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You'll notice it is used on submarine launched ICBMs. Blunt noses are very efficient for the missile to get out of water (and are shorter than long, profiled ones, which helps with submarine diameter), but aren't great in atmosphere; hence the deployable aerospike to make them efficient in both media.

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    $\begingroup$ The Trident, specifically, had to fit in the Poseiden-sized launch tubes which it was replacing, and carried an extra stage over the Poseiden. The deployable aerospike was the only available solution given the space constraints - there are other nose cone designs which would have been suitable but there was no room to fit them into the Trident design. $\endgroup$ – J... Apr 15 at 14:48

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