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Methods for Converting a Vacuum Rocket Nozzle to Operate at Sea Level




            This sub-page to is meant to describe methods for converting a vacuum rocket engine nozzle (with a large bell shape) into one that can operate efficiently and safely at sea level (with a much smaller bell shape, with today’s typical design).  Three primary alternatives are described…  ‘1) One could put an array of hinged gates (NOT permitting one-way flow of hot rocket exhaust gasses into the lower-pressure ambient environment, for vacuum or low-ambient-pressure applications) into the extended areas of the bell shape (those exceeding what is needed for sea-level operation).  However, the one-way-gas-gates WOULD allow ambient air to flow (at sea level or in other higher-pressure environments) inwards into the rocket nozzle, thereby reducing “back flow” of air (along the insides of the outermost rim of the nozzle, when the nozzle is “over-expanded”), which is associated with inefficiency, chaotic oscillations, and danger.

            ‘2)  One could design the outermost (lower) extensions of the nozzle to be used in a vacuum environment (as in, for example, the Space-X Falcon 9 interim upper stage, formerly being design-tweaked to begin emulating the upper stage of the “BFR” system, AKA, the BF Spaceship), but to fall away (be discarded) immediately after the final de-orbit burn.  This converts the bell shape from a vacuum nozzle into a sea-level nozzle, by drastically reducing the nozzle in size.

            ‘3)  One could formulate the materials and construction of the outermost extensions of the nozzle to be burned away by the heat of atmospheric re-entry.  This might be an especially attractive option if one uses a shuttlecock-style re-entry method with the rocket engine(s) at the “fore” end during re-entry.  Options for precisely this method (rocket engines at the fore end in shuttlecock-style re-entry) have already been described (citations provided here).  Designing these outermost nozzle portions to be sacrificially burned or ablated away will help protect the engines, and facilitate their re-use.

            As with other sub-pages of , the intent here is to “defensively publish” propulsion ideas, to make them available to everyone “for free”, and to prevent “patent trolling” of (mostly) simple, basic ideas.




            To acquaint oneself with the basics, please see for a basic rocket nozzle diagram and associated notes.  Then see for some more basics about vacuum rocket engine (rocket nozzle) shapes v/s sea-level rocket engine (rocket nozzle) shapes.  And THEN move on to see for a dual-bell-shaped nozzle, which will be the starting basis for what is described here.

            To put all of this in context, please see the news at , concerning Space-X “mini BFR” experimental designs.  The designs here described could allow Space X (or Blue Origin, or others) to configure a rocket engine for vacuum operations, efficiently, and then re-configure for sea level operations, as in a retropulsive landing operation to reclaim an interim upper stage.  A large bell shape works well in a vacuum; roughly 10% more efficiently than a smaller bell shape for sea-level operations.  Using the larger vacuum-style nozzle at sea level invites “flow separation” (especially “chaotic flow separation”) and associated problems.

            An older but still-relevant analysis can be found at , and discussions of at-least-somewhat-associated matters can be found at .  Suggested search strings to find more would be all of the following: "vacuum nozzle", "sea level nozzle", “flow separation”, and “over-expanded nozzle”.


Starting Diagrams


            Here are starting diagrams, before dealing with our two options in more detail.  The basic dual-bell shape is derived from what is shown in .



Figure #1


            For a basic preview of idea #1, the nozzle extension (but not the base) is perforated by many-many one-way gas-flow valves or gates.  These gates will be described in more detail later.  They are shown here as being round, but that may not be the optimal shape.  By allowing higher-pressure ambient air to flow inwards towards the middle of the rocket engine nozzle (towards the plume of the flame), in a shorter route as opposed to having to flow past the outer rim and then backwards (as is shown in the drawing at , at the top of the drawing, labelled “recirculation” in the “sea level mode”), the problems of “chaotic flow separation” should be reduced.  This (method shown here) may or may not work sufficiently well, but is documented in the name of completeness.  Basically, this method here attempts to cause to “vanish”, a substantial portion of the nozzle extension, as far as what “appears” to the high-pressure ambient air at sea level.



Figure #2


            Method #2 causes the nozzle extension (all of what is shown in BLUE above) to fall away (be discarded) after a final de-orbit burn, and so it hardly deserves a preview drawing here.  Method #3 burns or ablates it during re-entry instead, so is also not shown here.


Perforated Nozzle Extension Method Details


            Powered, active (gas-flow-control) gates are possible, but would probably be too heavy and complicated.  Also, how would one power them?  They are here judged to be too troublesome to be discussed any further.

            What is envisioned here are passive hinges on the upstream side of curved metal gates designed to fit into voids on the walls of the nozzle extension.  The voids have “casings” (just like the receptacle for a door in a house), where the metal of the nozzle extension overlaps the metal edges of the gas-flow gates, so that the gates are allowed to swing freely inwards, to allow higher-pressure ambient air (as in, at sea level) to flow in.  The gates are NOT allowed to swing past the walls of the nozzle extension, and so, higher-pressure hot rocket exhaust is NOT (generally) allowed to flow into a vacuum.

            A circular shape has been shown in Figure number 2 above, for simplicity, for these gates.  The shape shown below may be more optimal, for impeding the flow of the hot gasses less, as the gates are forced (by pressure differentials and gas flow) into proper position for vacuum-mode operation.



Figure #3


            Just to eliminate any possible confusion, the solid lines indicate an edge is visible, and a dotted line indicates that the edge is not visible.  We repeat from above, then, and show the hinged gate from inside the rocket nozzle as well as from the outside of the rocket nozzle.



Figure #4


            The gory details about the hinges are not shown here…  A conventional house-door-type hinge (with interlocking finger-loops and a central bearing-pin) would do the job.  Some comments here about the hinges though:  Yes, they will add mass and complexity to the design.  They will also impede gas flow (impede exhaust flow) at least by some amount.  Furthermore, they will require deviation from a simple bell shape, at the hinge locations, since one cannot create a functional hinge with a curved, solid bearing-pin (bolt).  The bearing pin needs to be straight, so the “bell shape” will need to be flattened out in these limited areas.

            Once again, whether this design will accomplish the task (eliminating or drastically reducing “chaotic flow separation”) or not, is really unknown, experimentally…  And I haven’t simulated it, either.  As is, if it does work, is it worth the prices paid?  That’s unknown as well.  But at least the idea is now documented…

            One last set of comments:  If, during the launch process, anyone should worry about these latches and hinges “rattling around” a lot, under shock and vibration forces, possibly making (shock and vibe) matters worse, then here is a potential solution to that:  Add glue seams between these latches (at their edges) and the overlapping seal-flanges of the nozzle extension wall.  Formulate the glue to degrade under heat.  When the rocket is used in orbit (plus also during the heat of re-entry), the glue will heat and degrade.  Thus, the joints are firm during launch, but free so swivel and swing during retropulsive uses later, at sea level.

            Some small magnets could also be placed as described above (at the glue-seam locations)…  Either complementary magnets embedded into both surfaces, or, if at least one of the surfaces is made of ferrous metal, then one could get away with only one set of magnets.  A good analogy would be to the magnets embedded into a pet door for cats and dogs.  The magnets would have to be appropriately sized…  They would add “hysteresis” to the gate-opening function, thereby cutting down on “chatter”.  Magnets could be used with or without glue being used as well.

            This concludes the “idea number 1”, concerning perforating the nozzle extension with one-way-flow gas valves.  Now we move on to idea #2, which is to totally discard the nozzle extension after the final de-orbiting burn.


Details for Discarding the Entire Nozzle Extension


            The basic design described here is analogous to an old-style wooden barrel (the same type as has been commonly used to age whiskey in).  Instead of many-many wooden staves, divide the extension nozzle into 3, 4, or 5 segments…  Let us pick 4 segments for the drawings here.  Instead of heavy steel bands (as in a wooden barrel), let us use rope or cable.  Rope is more light-weight than steel (or other metal) cable, but will be less heat-resistant…  So here, we will describe options for using either rope or cable, to do the job done by steel dual bands in a wooden barrel.  That job, of course, is to compress the “staves” together to form a cylinder or barrel…  In our case, the barrel is the nozzle extension.

            In all design variations described here, the outer rim of the (solid, not segmented) base nozzle will be made “ribbed” to meet (couple to) a complementary formation of the inner rim of the segmented nozzle extension.  This calls for a simple drawing to illustrate this idea.



Figure #5


            The ribbed arrangement shown above (in conjunction with dual constriction bands as described earlier) should be strong enough for mechanical integrity.  The mating surfaces might best be roughened up to increase bonding friction.  Acid etching might be used to accomplish this.  As mentioned under method #1, we could also use a glue formulated to degrade under heat.  However, whatever-all methods are used to strengthen the bonds, we do NOT want to over-do it…  We want the bonds to fail, and the “barrel staves” (segments of the nozzle extensions) to fall away before, or during, re-entry into the atmosphere.

            The next drawing is simply for showing the approximate locations of the constriction bands.


Figure #6


            The next drawing simply shows an aft view (or a view of the outer rim of the nozzle extension) so as to illustrate a tongue-and-groove method of coupling the staves together.  The tongue-and-groove joints should run the entire lengths of the sides of these “barrel staves”.


Figure #7


            If the constriction bands are made of metallic cables, then such cables should be able to be manufactured to be heat-resistant.  Thus, there is no need to thermally insulate them.  They can be strung through metal loops that are integrated into the structures of the “barrel staves”, on the outsides of the staves.  The cables are tightened, then spliced end-on-end with cables clamps or any other suitable method.  All of this scarcely deserves a drawing.

            However, weight (mass) always comes at a premium on spacecraft, so using ropes rather than cables would save some mass.  Ropes are NOT going to be as heat-resistant as metal-based cables are going to be.  So what we could do to accommodate the use of ropes would be to periodically fasten a metallic “Y” shaped post, protruding outwards from the staves.  The crotches of the “Y” posts would contain ceramic thermal insulators, and the ropes would be tightened into the bottoms of these “Y” posts.  The drawing below shows only one of the 4 “barrel staves”, and the sample “Y” post has been rotated 90 degrees before being shown in the amplified close-up view.  These methods could be used to protect cables as well as ropes, from heat, of course.


Figure #8


            The constriction ropes or cables (of the constriction bands) will need to be cut after the final de-orbiting burn (to be briefly discussed slightly further below).  Both of them will need to be cut if the outer-rim constriction band is located significantly forwards (upstream in the gas flow), away from the outermost lip or rim.  However, if we chose to use heat-resistant cables there, we can locate this cable very close to the outermost lip, using metal loops closely integrated into the staves (omitting the “Y” posts) to secure the cable, then we can NOT BOTHER to cut this cable at staves-jettisoning time!  Only the innermost constriction band will need to be cut with this method, since the outermost constriction band’s tension will be enough to hold everything in place, till the innermost constriction band is cut, AND this same outer-cable tension will help to pull the staves away after the inner band is cut.

The price of using this particular method is that we have to cut 4 “V” notches at the outermost tips of the inter-stave joints (between the stave-tips).  This must be done to allow room for the 4 staves to fall away (allow for the outer-most stave-tips to collapse together).  These small notches may detract VERY slightly from total thrust in the vacuum mode of operation.  See the drawing below:


Figure #9


            The V-shaped cuts as shown above are shown larger than they really need to be.  If they are cut too large, the cable may be too exposed to the exhaust plume, possibly threatening to melt the cable, and certainly impeding the flow of the hot gasses.  Proper sizing of the notches, and optimal placement of the cables, should minimize such problems.

            The need for the V-cuts or triangle-cuts should be obvious for the mechanically inclined reader.  If not, I can explain and-or diagram some more…  Email me at

            One more possible (optional) refinement here would be to add metal (heat-resistant) springs in between the outer rim of the base nozzle and the nozzle extension.  This would involve moving the upstream constriction band further upstream as compared to what is shown in Figure #6 and Figure #9 as well…  Move the upstream constriction band to where the “ribbed regions” of the base nozzle and the nozzle extension overlap and grab onto one another.  Figure #5 shows these ribbed rings region at the overlap, and this is configuration is repeated in Figure #10 below for reference.  In the refinement described here, this arrangement would continue to be accurate for the majority of this overlap ring, except that we would now flatten out the ribs periodically, to make room for the “Y” posts as are shown in Figure #8.  These “Y” posts are now hollowed out (in their bases) to create cylindrical voids, in which mechanical springs reside.

            This arrangement would add a little more mass, to possibly also include more mass in the rope or cable, to overcome the added force of the springs.  The purpose served here is as follows:  To add additional insurance that the staves will fall away, after the rope(s) or cable(s) is/are cut (after the final de-orbiting burn).  The springs will help enforce the separation process.



Figure #10


            As far as is concerned, HOW do we cut one or both of the ropes or cables, after the final de-orbiting burn, I would suggest “explosive bolts” or other “pyrotechnic fasteners” in-line with the ropes or cables.  If the free-floating post-explosion triggering-wires-ends are considered a hazard to the aft end of the spacecraft, then find or devise a method of cutting these wires (post-explosion) inside the spacecraft.  More exotically, consider a heat-trigged explosive charge inside the pyrotechnic fastener, and trigger it without making electrical or mechanical contact, by using infrared lasers fired from the aft end of the spacecraft.  Needless to say, the explosive charges need to be thermally insulated, away from the heat of the rocket exhaust plume!

            I have no special expertise or ideas concerning pyrotechnic fasteners, so I’ll discuss this no further.  If one is interested in sourcing high-temperature-tolerant cables, see, which provides cables tolerating up to 1,565 C = 2,849 F.


Details for Burning or Ablating the Entire Nozzle Extension


            In this variation of the design, no “barrel staves” or cable (or rope) constrictions would be needed.  The extended nozzle would be constructed out of materials that burn, ablate, or fall away during the heat of re-entry (the base nozzle for sea-level operations would remain the same, or highly similar to designs today).  The ablated (or burned) away (sacrificial) material(s) could be most anything that is high-temperature tolerant, lightweight, and strong and durable enough, that would be serviceable for orbital operations engine burns, yet still burn or ablate away during re-entry.  PICA ablative materials (see are just one example.  Different materials might be layered together, as well.

            Sacrificing the extended nozzle to protect the rest of the rocket engine during re-entry might be especially effective for a shuttlecock-style re-entry vehicle which re-enters the Earth’s atmosphere engines-end first.  For details about such a design, see,, and  These papers (the same texts and drawings) are also available (in the same listed order) at … and

            Major considerations or cautions here in this particular design approach are as follows:  One would need to select materials and methods to prevent burned, ablated, or falling-away materials (from the being-sacrificed outer nozzle) from flying backwards and hitting (causing FOD, Foreign Object Damage, to) any aft-located parts of the descending vehicle.  In the nomenclature of the above papers, these (FOD-susceptible vehicle parts) might especially be “rollers”, “spoilers”, “petals”, or “Bifrost hinges”.

            The above-described method (with probably-highly-justifiably-assumed inherent variability concerning the degree of degradation of the sacrificial outer nozzle, come landing-on-the-Earth time) would likely NOT be amenable to recovering the entire vehicle with a retropulsive landing.  The “toppling hazard” would be too great (unless we add landing legs, which adds extra mass and complexity).  Also, partially degraded outer nozzles would likely cause chaotic or poorly controlled thrust.  The following would be more attractive and plausible methods:  Parachutes and helicopter-snatching, water landing, and catch-tower (Space X Starship-booster style).

I have no more relevant ideas here at this time, so I will sign off for now.  This concludes my ideas as of this time.  Once again, comments or questions are welcomed at


Stay tuned…  Talk to me!




Stauffer, Titus. (2021). Methods of Decelerating a Spacecraft Through Atmospheric Re-entry Using a Shuttlecock-like Design.


Stauffer, Titus. (2021). Harvesting and Managing Energy While Re-entering an Atmosphere Using a Shuttlecock Design.


Stauffer, Titus. (2021). Designing and Deploying a Shuttlecock-Style Re-entry Vehicle to Use Lifting-Body Principles.



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