From the Wikipedia, E-D nozzle works similarly to aerospike nozzle which compensates the exhaust gas in the atmosphere to increase specific impulse.

Reference Simplified Diagram

  • $\begingroup$ Good question. Sutton doesn't say much on E-D nozzles. Seems like research isn't going toward compensating nozzles because it's more cost-effective to just pile on low-cost, low-efficiency thrust (e.g. SRBs) for the first stage of a 2STO. $\endgroup$ – Russell Borogove Jan 21 '17 at 12:45

From Hill & Peterson, "Mechanics and Thermodynamics of Propulsion", 3rd printing, November 1970.

I summarize from the relevant portion of Chapter 13, Chemical Rockets: Expansion in Nozzles (pp 413 & 414):


  • ...the presence of a free jet boundary to the expansion process lends a kind of self-adjustment to back-pressure variations.
  • ...[they are] substantially shorter than comparable internal-flow nozzles.


  • The main difficulties associated with these configurations arise in the process of adequately supporting and cooling the central plug.
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    $\begingroup$ I really wonder if it would be viable to move such an engine to roughly halfway the height of the rocket and have one cryopropellant tank go through the middle of the nozzle, pumping the propellant through the plug and radial structural supports "upwards". That way cooling would be solved and the overall rocket length would be reduced. Although the idea of exhaust "licking" the tank surface is quite scary. $\endgroup$ – SF. Jan 22 '17 at 10:08
  • $\begingroup$ @SF. That is what they did in radial aerospike cooling, I have attempted to do so, but I highly doubt how the structural integrity of the plug (spike) when the outside is around 1000 K and inside is 200 K, and also both plug and aerospike are brittle structures and needed to absorb enormous amount of pressure (100 atm) at least, I believe it is not the best solution for the issue. $\endgroup$ – Raze Jan 22 '17 at 23:24

Reaction Engines Ltd undertook two studies to investigate the E/D nozzle, STERN and STRICT and published some results. They planned to use the principle in the air-breathing part of their SABRE4 engine to increase payload and/or reduce the takeoff run of the Skylon D1 HTOL SSTO spaceplane concept design.

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    $\begingroup$ Yes, but what were the sorts of results they published? $\endgroup$ – Nathan Tuggy Feb 15 '19 at 4:01
  • $\begingroup$ not as good as they hoped, but still useful. Sorry, this is a bit of a placeholder answer, offered because it may prove a little useful in its current form. I don't know when I'm going to get time to flesh it out. $\endgroup$ – JCRM Feb 15 '19 at 19:05

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