From RocketSlinger@SBCGlobal.net (email
me there please)… This is a sub-site to main site at www.rocketslinger.com …
This
web page last updated 06 Nov 2021
Methods for Converting a Vacuum Rocket Nozzle to
Operate at Sea Level
Abstract
This
sub-page to www.rocketslinger.com 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 www.rocketslinger.com , 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.
Introduction
To acquaint oneself with the
basics, please see https://www.mpoweruk.com/rockets.htm
for a basic rocket nozzle diagram and associated notes. Then see https://www.quora.com/Why-are-the-vacuum-versions-of-a-rocket-engines-nozzles-more-expanded-than-the-regular
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 https://www.researchgate.net/figure/Principle-of-a-dual-bell-nozzle_fig1_305632663
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 https://phys.org/news/2018-11-spacex-mini-bfr-falcon.html
, 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 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840011402.pdf
, and discussions of at-least-somewhat-associated matters can be found at https://forum.nasaspaceflight.com/index.php?topic=39970.0
. 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 https://www.researchgate.net/figure/Principle-of-a-dual-bell-nozzle_fig1_305632663
.
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 https://www.researchgate.net/figure/Principle-of-a-dual-bell-nozzle_fig1_305632663
, 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 RocketSlinger@SBCGlobal.net …
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 https://lapplimited.lappgroup.com/products/high-temperature-cable.html, 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 https://www.nasa.gov/centers/ames/thermal-protection-materials/tps-materials-development/low-density-ablators.html)
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 https://www.researchgate.net/publication/354612024_Methods_of_Decelerating_a_Spacecraft_Through_Atmospheric_Re-entry_Using_a_Shuttlecock-like_Design,
https://www.researchgate.net/publication/355039141_Harvesting_and_Managing_Energy_While_Re-entering_an_Atmosphere_Using_a_Shuttlecock_Design,
and https://www.researchgate.net/publication/355211954_Designing_and_Deploying_a_Shuttlecock-Style_Re-entry_Vehicle_to_Use_Lifting-Body_Principles. These papers (the same texts and drawings)
are also available (in the same listed order) at http://www.rocketslinger.com/BadMinton/
… http://www.rocketslinger.com/Bifrost/
… and http://www.rocketslinger.com/Lift_Body/.
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 RocketSlinger@SBCGlobal.net
…
Stay
tuned… Talk to me! RocketSlinger@SBCGlobal.net
References
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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