This web page last updated 18 July 2015
A Soft-Landing Method for Europa (or Other Icy World) Using RPGs (Rocket-Propelled Grenades)
This sub-page to www.rocketslinger.com is meant to describe a method of soft-landing on an “icy world” like Europa. Variations and embellishments are also described, but the basics are as follows: The lander spacecraft has a central vertical (in-line with its direction of travel as it lands) structural, central spine that doubles up as a gun barrel. A revolving (or other style of) ammo-dispenser at the base of the gun (top of the lander) serves to feed multiples (8, for example) of RPGs (Rocket-Propelled Grenades) cartridges to the guns. The RPGs are rocket-propelled only just enough to control their X and Y locations with respect to their target locations; the “Z” axis is not covered, where the “Z” axis is parallel to the downward direction of travel. X and Y control on each RPG round means that (in conjunction with “robot-swarm” intelligence on each round, informed by radio location transducers on each RPG round) the small swarm of RPG rounds can embed themselves into the ice, in a reasonable semblance of a circle. By forgoing the “Z” axis of directional control, the RPG rounds are entirely “Z”-propelled by the explosions in the gun, delivering maximum “kick-back” or retro-grade propulsive impact to the lander. Mid-flight-deployed fins snap out on the RPG rounds, so as to prevent them from penetrating into the ice, too deeply.
The RPG rounds embed themselves into the ice in a round pattern, at near-optimal depth. Their on-board intelligence allows them to await an explosion command from the lander. The lander approaches a landing spot in the middle of the round pattern of charges. At the appropriate time, the charges are set off. The shock waves (and dislodged ice, some of which will turn into liquid water and steam) will converge and collide in the center of the circle of explosions, creating an upward geyser. The lander’s bottom is covered by an impact-resistance, energy-absorbing shield. The shield does three things: It protects the lander, and (in conjunction with the impact forces of the central geyser) stretches out in time, the deceleration of the lander (reduces peak “G” forces), and softens out the final landing on the hard icy surface, killing whatever lander-speed is left after the backward “kicks” of the gun, and the geyser impacts.
Alternatives for materials and construction of the gun and the energy-absorptive shield, as well as overall design alternatives, are described here.
Also note that, although this design is intended for landing on ice (which will at least partially liquefy and gasify, making for a softer landing), it might also be able to be adapted to landing on a rocky world, like the Earth’s moon, or Mars. Rocks, sand, and gravel impacting onto the landing shield will be more difficult to deal with, though, than chunks or particles of solid, liquid, and gas phases of water.
The gun barrel needs to be optimized for minimal weight, with costs being far less important (since every pound of mass thrown this far into space, comes at very high costs). The gun barrel needs to withstand only a minimal number of rounds (explosions) as well. Many alternatives are possible; Here is one proposal.
The innermost layer of the gun barrel (the layer that touches the RPG rounds and the exploding gasses) might, for minimal mass, be constructed of a bare-minimal-tolerable, plus safety margin, thickness of “glassy metals”, AKA “amorphous metals”. Such glassy metals are well known and documented by now, so no links are provided here. Similar high-tech materials could involve laser-sintered quasi-crystals; see http://www.sciencedaily.com/releases/2014/10/141030100331.htm
“3-D printing incorporates quasicrystals for stronger manufacturing products”.
Alternatively, the designers could go with more prosaic materials, such as a thin layer of tool steel or stainless steel. Think of this layer as the hard, thin layer on teeth… The enamel layer.
The next layer, moving outward, may or may not be needed. It may be needed to provide thermal insulation from the innermost to the outermost layers of the gun barrel (to protect the outermost layer from high temperatures), and/or, it may be needed to for additional mechanical strength (explosive-forces tolerance), and/or, to accommodate forces generated by different materials expanding and compressing at different rates, due to temperature changes. It should be less dense than “glassy metals”, and need not be as hard as glassy metals. In the teeth analogy, it is “dentin” compared to the “enamel” of glassy metals. It could be arranged in concentric layers, abutting one another (with air gaps, or more properly, vacuum gaps, between segments), to provide for differential thermal expansion and contraction. (Most likely high-tech) suitable materials for this layer should be high-temperature tolerant, but lightweight. Here are examples:
“High strength cellular aluminium foam for the automotive industry”, and
“A metal composite that will (literally) float your boat”.
The final, outermost layer of the gun barrel might best be constructed out of lightweight, highly stretch-resistant material. This material, optimally, may not need to be high-temperature tolerant. There are already at least two well-established applications for woven carbon fibers to be used in similar applications. For example, for wrapping concrete structural cylinders, see http://www.concreteconstruction.net/concrete-strength/wrapping-it-up.aspx
“Wrapping it Up (return) Strengthening concrete members with carbon fiber fabric and epoxy composites”, and for commercially available gun barrels, see http://www.brownells.com/items/carbon-fiber-gun-barrel.aspx (for just one randomly grabbed sample). If the designers want to spend the extra time, trouble, and money to go higher-tech for even better numbers for strength and light weight (mass) for this outermost layer, see http://www.sciencedaily.com/releases/2015/05/150515001353.htm
“First large-scale graphene fabrication”… Graphene sheets could be rolled up into cord-like or string-like shapes, which could then, in turn, be woven together.
A cross-section of the gun barrel, then, would look like the below illustration:
The Shield / Shock Absorber
The shield / shock absorber will shield the bottom of the lander from the impact forces of chunks of ice that impact the lander, from the bottom of the lander, as the lander settles down upon the “geyser” of materials that are thrown upwards as the circle of slightly-buried (circle-arranged) RPGs explode. If any reader is confused, or needs more details, more drawings are easy enough to generate… Explosions patterns, centrally converging shock waves and ice-water-steam being thrown towards the central “geyser” and converging, then being deflected upwards… All that is easy enough to illustrate… Just email me at RocketSlinger@SBCGlobal.net , please… Another way to summarize the scheme is, we are “cheating” on carrying reaction mass, here, if you will. To go only slightly off-color, here, propulsion often means “throwing mass out of your ass”, to get a Newtonian “action proportionate to reaction”… Why not borrow the mass of your target, to react against? That’s what we’re doing here…
Here is a parenthetical note for you fans of the history of space travel ideas: Do you doubt my variation upon “mass out your ass”, whereby we make a slight variation, and don’t actually throw “mass out your ass”, and instead, use the impact velocity of mass being thrown into your ass, if you will? Well, how about “going nuclear with the idea”, and going to “Project Orion”, where we absorb the energy (and mass) of nuclear explosions being thrown into shock absorbers on our aft end? See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) . This proposed-but-never-built idea is VERY similar to the exploding-grenades idea proposed here, in that explosions-driven matter and energy is directed at shock absorbers on the aft end of a spacecraft.
Anyway, one possible approach is to build a “layer cake” shield out of semi-rigid plates (made out of deformable, impact-absorbing metals, for example) and energy-absorbing “crunch zones” of lightweight, solidified foams. An everyday example of energy-absorptive, crunchable foam, that most readers will be familiar with, is insulating spray foam. See
https://en.wikipedia.org/wiki/Spray_foams_(insulation) for the basics. Also see http://www.sprayfoamkit.com/blog/2011/10/temperature-and-spray-foam-insulation/ , where we learn that “SFI has the best adhesion and most ideal curing process when it is applied in 60-80F degrees (15-26C).”
At launch-from-Earth time, not only do we want to reduce the weight of the launched assembly; we also want to reduce the volume. Extra volume means extra air resistance during the launch process, at lower altitudes. So we will want to wait till we are in low Earth orbit, or higher, before filling “crunch-zone voids” in our shields. There may be trade-offs involved here… For higher cure temperatures, we will want fill the shield-voids with spray foam, ASAP after leaving Earth’s atmosphere (where we are still closest to the sun’s heat). The foam, however, may possibly degrade over time, if it is cured a long time before it is crunched (before impact on Europa). If needed, “resistance wires” (steel-nickel-chromium heating wires) could be added. The lander would pre-heat one segment (and spray can) at a time, before filling each segment (void) with spray foam to be cured during travel.
A large void entirely filled with foam will be heavier and stiffer than needed. So what is envisioned here is a centrally located, strangely shaped bag (made of canvas, plastic film, or graphene, say) that will be collapsed (empty) during initial launch. It is like a round pancake, with a hole in the middle, to accommodate the gun barrel and RPGs travel path (like a flattened doughnut, including a hole in the middle). Protruding from the top and bottom of the flattened doughnut, there are “fingers” added, for the express purpose of absorbing impact energy. At launch time, these strange-shaped, deflated (empty) bags are layer-caked with alternating energy-absorbing but rigid metal plates (also in the shape of flattened doughnuts). The over-all (simplified) design, prior to inflation of the shock-absorbing bags, might look something like this cross-sectional view:
Before we forget, now, let’s mention what the metallic layers of the “layer cake” of the shock absorber might be made of: A good candidate is as follows:
“Citrus fruit inspires a new energy-absorbing metal structure”.
The next drawing will elaborate on what shapes of the bags should be, when filled with the impact-energy-absorbing, sprayed foam, so as to minimize weight while also maximizing “crunch space” (which translates to minimized max-gee-impact forces, by spreading deceleration over time). The same color scheme is retained from the previous drawing. Once again, the “to-be-crunched” layer of spray foam takes the shape of the containing bag, which is a flattened doughnut (torus) shape, with “fingers” protruding from top and bottom. The “fingers” could be straight and simple… But then they would buckle and break once, which is not optimal. “Optimal” is up for discussion, obviously, but this writer believes an optimal shape would be a helix or screw-shape… It bends all along its body, then breaks, with space left between these helical “finger-bags” for broken pieces to fit into… Stretching out the “crunch time” and minimizing the maximum “peak G forces” upon impact. So… On, to the drawing:
The above only shows the helical “fingers” (growing out of the top and the bottom of the doughnut-shaped bag) in a single line, to reduce visual clutter. In reality, they should be spaced, at moderate density, across the entire surfaces of the doughnut. Also to reduce visual clutter, control (and probably power to the “resistance wires” as well) wires to the inflating spray cans are not shown. “Control wires” would, for example, open solenoids to spray-foam-fill the bags after the spacecraft is out of the Earth’s atmosphere.
Now, what happens if the (inflated, deployed) shield / shock absorber needs to be so tall, that a “topple hazard” sets in, whereby the entire lander could topple over to the side, and be damaged? To cover this, the designer might consider sacrificing a bit of “payload area”, so as to array a bottom circumference of outward angled helical foam bags… Perhaps a thick array of various lengths, all around this bottom, outer perimeter. Here is another top-level cross-sectional drawing, which will hopefully be enough to illustrate the general idea:
The above drawing, for simplicity, shows the anti-topple bags as inflated, and the main stack as not being inflated; hopefully, this is enough to be generally illustrative. If not, once again, please email me at RocketSlinger@SBCGlobal.net . Also note: A probably-more-sensible arrangement, better than what is shown, would be for the entire bottom of the payload area to be surrounded by a large inner-tube-like toroid shape (filled with solid foam), and for the anti-topple “fingers” to protrude out from there. This would help to prevent one finger from becoming entangled with another, as they all inflate, and would mean that (as the entire assembly came into danger of toppling severely), the solid-foam toroid shape would provide much more “crunch resistance” than the fingers alone. After all these anti-topple provisions are made, the lander is still not guaranteed to make its final landing, anywhere near totally flatly. Other provisions will have to be made to accommodate this. Yet another idea is to cover even more of the surface of the “payload area” with spray-foam-filled bags… Such bags being detachable, post-landing, if needed.
This concludes the most-essential, core ideas to be presented here. The remaining items below are non-essential alternative ideas, variations, and explanations.
Minor Details & Options FYI…
Some of the below will be rather obvious to some of my readers, if I have any readers! If you are a real rocket scientist, some of it will be totally obvious… If not, some ideas may not be so obvious. Are you, Dear Reader, just an amateur dabbler like me, wanting to kick the ideas around? In either case, do you need or want more? Email me please… RocketSlinger@SBCGlobal.net … More illustrations are easy enough to provide…
Europa, of course, orbits Jupiter. As the assembly of main spacecraft, plus lander, enters the Jupiter (“Jovian”) system, a fairly large amount of (expensive) reaction mass will have to be spent to put the assembly into Jupiter orbit. We have to kill our “coming here from Earth” velocity, that is. Then, some more will have to be spent to put the assembly into Europa orbit, as I understand. One variable that I do NOT understand, is how high, or how low, of an orbit, around Europa, is desired, for this assembly of the main orbiter, plus lander? I could help “crunch the numbers” here if desired, but would need an answer to that question, for starters…
Anyway, it is assumed here that the assembly would be placed into Europa orbit, for quite a few orbits, while data is collected and analyzed. Only after careful analysis, THEN the mission management would select an optimal target for the lander. That means that the combination of reaction mass going out the lander’s gun, and the “crunchable” shield / cushion, would be good ONLY for killing accumulated descent velocity… That is, the assembly first orbits Europa. Lander target is selected, and lander detaches. Lander can NOT use the “gun”, now, for killing Europa-orbital velocity, except if additional ammo “rounds” are reserved for this purpose, and for this purpose alone… Having these “rounds” serve a double purpose, as being also the RPG rounds to be pattern-buried in the ice, to create our landing-softening central “geyser”, is simply not practical. Orbital-velocity-killing rounds must be shot in a direction radically incompatible with what would be needed for RPG rounds that double up to kill descent velocity. Need details? Email me…
So anyway, we kill the lander’s Europa-orbiting speed via emissions of compressed gasses, or conventional rockets, or gun rounds carried for that express purpose, and for that purpose alone. Carrying the extra gun rounds to do this, is actually fairly sensible… If we decide to go with the RPGs scheme for starters, then we need to carry the gun barrel anyway, so why not? Gun technology is VERY reliable, and the specific impulse (propulsion proportional to reaction mass carried, or, speed of the ejected mass) is pretty respectable… Certainly better than that of compressed gasses, for example. This (extra gun rounds carried for orbit-killing) is a viable alternative, then. So kill your orbital velocity first, then Europa’s gravity will slowly speed you up, bringing you in for a vertical collision into the surface (ice). This vertical “fall speed”, and this, alone, is what you can help “kill”, with the RPG rounds fired straight downwards.
All the above assumes, though, that we want to assume Europa orbit first, before selecting a landing target. If this assumption is discarded… If we can pre-select our landing target before entering Europa orbit… Or even, at a logical extreme, before entering Jupiter orbit… Then the lander could detach from the orbiter, earlier in time, and yet more total reaction mass could be saved… The lander could approach the surface of Europa with even greater speed, above and beyond just that which is created by the gravitational attraction of Europa. I am no rocket scientist, but this sounds risky to me! In the interests of being complete, though, this possibility… Having the RPGs-powered landing scheme cover more than just the gravitationally-induced “fall” of the lander, and the price to be paid for it… Is covered, here.
If considerations of reliability and specific impulse (and others?) permit, it might even be desired to use the lander’s gun to kill velocity of the entire assembly of orbiter plus lander, as the assembly enters Jupiter orbit, and/or, as the assembly enters Europa orbit. I cannot think of why this couldn’t be done. This option would preserve the probably-much-treasured option of studying Europa’s surface, up close and (almost) in person, before selecting a landing site, while also using the gun for all that it is good for. If one objection to the gun is lack of precise control, then a solution might be: Use the gun to kill 95% +/- 2% or so of speed needing killed, and some other method, for the final fine-tuning.
Major Option… Use of “Shaped Charges”
Dear Reader… We are “brainstorming” here, including all ideas. Join me at will… RocketSlinger@SBCGlobal.net , once again… I don’t honestly know which ideas are most practical. I personally consider only ONE remaining idea to be very practical (but list more “for grins” further below). The one (optional) idea that I consider to be most practical is as follows: So far, we have discussed a circular pattern of RPGs (Rocket Propelled Grenades) that bury themselves into the surface ice, without mentioning an idea that would require a bit more complexity in the control circuitry onboard these RPG rounds: This is OPTIONAL, now, but… To increase the efficiency with which the exploding RPG rounds “blast” ice towards the central “geyser” (created when all the icy streams collide in the center), we could use “shaped charges”. This would require the RPG rounds to land in a manner in which their ORIENTATION (as well as their location) is controlled.
“Shaped charges” simply means that the explosive is “shaped” such that one part of it explodes, then another, then another, and so on. The timing is usually a matter of milliseconds (thousandths of a second) from one area of the explosive to another, as I understand it… Also, air-gaps or voids or fillers or metal plates may be involved. See https://en.wikipedia.org/wiki/Shaped_charge for details. I am no expert here at all, and am lazy; have not studied up on it! Suffice it to say, I think that if the orientation of each RPG round was controlled, the explosions could be “shaped” to throw more ice in the desired direction, and less in the un-desired directions. You experts out there, please chime it…
Minor Option… Last “Cartridge” is a Sample Collector
Now we will delve into what I consider to be less practical “wild and crazy” ideas, but here they are… Some will not be compatible with other ideas.
The last “cartridge” at the
base of the gun barrel could serve as a sample collector… It could be an empty cartridge. The down-ward-pointing “business end” of the
gun barrel could be funnel-shaped (blunderbuss style), so as to collect ice,
water, and steam, as the lander travels through the central “geyser”, and a
solenoid could slam shut at the entry to the empty cartridge, at the
near-precise moment of final landing, to capture and contain the sample that
was taken. The sample would be
contaminated by the remnants of the explosive charges, is one disadvantage. The near-certain need for a funnel is
another… The funnel would need to go,
where the shield / shock absorber needs to go.
Minor Option… Scrounging for Reaction Mass
After each individual round is fired, the casing that held the explosives to propel the bullet, or RPG, is wasted mass. At a bare minimum, the casing should be as lightweight as is possible, and the empty casing should be ejected from the spacecraft, after the round is fired. Carrying an empty case would mean just that much more useless mass that needs to be decelerated, that is. These 2 options mentioned so far, should be sufficient. At some extreme of the imagination, though… This sounds too “Rube Goldbergish” to me… We could add mechanisms to grind up the remnants of the empty casings, and / or eject them from the spacecraft, with some significant velocity. Cramming the ground-up remnants into the gun barrel, before the next round is fired, is one option.
After the gun is fired the last time, the cartridges dispenser, and/or the gun barrel itself (in fragments if possible) could be expelled “at significant speed” also. Seeing as to how we want to accumulate as much “fall speed” as possible, though, as we descend to the surface, before killing said “fall speed”, there will not be much time available for such schemes to work. In the name of completeness, though, the idea is hereby mentioned…
Minor Option… Gun Barrel Doubles up as “Pogo Stick”
Instead of using a spray-foam-based (or other) shield and shock absorber, the gun could be mounted on large (mechanical or other, most likely mechanical) springs. As the rounds are fired, the entire gun recoils and recovers on these springs… This reduces the sharp peak G forces (on the payload) as each round is fired. If the payload is mounted at the base of the gun, the gun barrel is long, and the springs are strong enough, we could get some “pogo stick” action as the lander hits, bounces, hits again, bounces, etc., till the bouncing is done (energy dissipates as heating and other inefficiencies in the springs, for example).
This would obviously require active “balancing” in the payload, to keep it vertical, as the bounces subside. It would also require the lander to be tolerant of “falling on its side” after the last bounce of the pogo stick. The active balancing could be done via: ‘A) Rockets, the release of compressed gasses, or other conventional means, or ‘B) the spinning of internal, fairly large gyroscopes (wheels), which, of course, requires additional mass, which is of no other use… Unless their spin energy is later tapped as an energy supply? Or they are made of, or contain, chemicals that can be dissolved or otherwise processed to be combined in, say, a fuel cell, for energy supplies? Just blabbering at the keyboard here…
Europa’s surface seems likely to be made of fairly pure ice, but also some ice that is highly contaminated with salt… See http://www.nasa.gov/topics/solarsystem/features/europa20130305.html , for example. I am no expert here, but it seems to me that more-pure ice is going to be hard, and salt-contaminated ice will be softer. If the lander is going to have little mobility (not be a rover), then the limited mobility afforded by the “pogo-sticking” timeframe (if intelligently guided on-site and in real time) may be of value. The initial impact of the lander could be on more-solid ice, with subsequent “bounces” directed towards ever-softer, ever-saltier areas. Otherwise, it may be possible that the initial (higher-speed) landing would be in a too-salty, too-soft area, and the whole lander could be buried. So the pogo-stick approach may actually be a reasonably sharp idea.
Various other variations and combinations of the above-described ideas are possible. Among them might be, discarding the idea of using grenades (with explosives inside the projectiles) and going purely with kinetic energy only. It is perhaps possible that such purely-kinetic rounds could be fired accurately enough, with differential speeds on all of them according to how soon or how late they are fired (as controlled by how much or how little of an explosive charge is used for sending off each round), that such purely-kinetic rounds could still provide a circular pattern of near-simultaneous impacts, to create a landing-softening central “geyser”. I suspect that this is impractical, but I am no expert. Perhaps one way to make the idea be more practical, would be to have the gun-and-ammo part of the lander separate from the payload part of the lander, well ahead of time, during the descent of the two separated parts, so that the timing relationship of the payload v/s “geyser”-creating bullet impacts can be adjusted appropriately. This idea, though, obviously subtracts out, the use of the retro-“kick” of the gun, to slow down the payload part (unless we use a long, rope-like tether, so that the two parts are not totally separated?). This brings us to the final set of ideas to be presented here (for now).
Using Copper Bullets and Spectroscopy
Instead of using grenades, then, let us consider using copper bullets, and spectroscopy, so that the orbiter can observe multiple copper-bullet impacts on the surface of Europa. Copper is used, where copper is not expected to occur in significant amounts in Europa’s ice, and so that copper’s signature in the spectroscopic readings can be discounted (ignored). This method has already been used with success… See https://en.wikipedia.org/wiki/Deep_Impact_(spacecraft) , for example. The Japanese space agency has a somewhat similar mission in progress; see http://www.newstatesman.com/future-proof/2014/09/japan-readies-space-probe-mission-chase-asteroid-and-shoot-it-cannon .
What this might look like, for a Europa mission, might be for the lander to detach, and make a small retro-blast (via gun or more conventional propulsion) to lower its orbit (and tighten it, or lessen the time for one orbit), so that its orbit is lower and faster than that of the main orbiter. One or more orbits pass by, till the timing and positioning is just right, so that the lander can begin its “strafing run” on Europa, with the main orbiter optimally positioned above the string of impacts created by the copper bullets. The orbiter uses spectroscopy and photography to gather as much data as is possible, about the composition of the icy and salty-icy surfaces on Europa, as is released by the bullet impacts. Good precision targeting of the rounds may or may not be practical, but hitting a variety of different target-types should be easy enough, even with a bit of randomness being present.
The orientation of the lander could be controlled by large internal gyroscopes, and / or conventional orientation-controlling rockets or compressed-gas ejectors (thrusters). The very first rounds are shot at or above the approaching horizon or Europa, to optimally kill orbital velocity. These first rounds will, then, obviously not be useful for gathering data about their impacts. But the lander will, over time, fire more rounds… As it gradually transitions from an orbit to an almost-straight-down drop. That is, with proper control of the lander’s, and its gun’s, orientation, the aft end of the lander (and “business end” of the gun, one and the same) will slowly transition from being aimed at the approaching horizon, to being aimed nearly-straight-down at Europa’s surface. This will create a line of many bullet impacts that can be analyzed. The very last round should be fired at a point in time that is fairly close to the final soft landing, so that minimal “fall speed” can accumulate between the last velocity-killing blast of the gun, and the final landing.
The payload part of the lander should detach from the gun after the last round is fired, and the majority of the “falling speed” has been killed by the gun’s retro-kicks. The payload part of the lander can then make the final landing via more conventional propulsion (rockets or compressed gasses).
In this scenario, the ideas about a “central geyser” and a crunch-able landing shield / cushion are almost definitely best discarded. The base of the gun (ammo dispenser) might best be coupled to the lander, with provisions (electromechanical devices or small explosive bolts) for the two to be separated. The final lander, of course, is positioned above the gun… The gunpoint is always positioned at the leading edge of travel, for the entire approach to landing. The two separate, and the final lander part “flies away” just barely far enough from the gun, to guard against any kind of collision upon final landing. The lander could then have “legs” that are cushioned by mechanical springs, gas pistons, or other methods of giving the payload an optimally soft landing. The final lander might bear a slight resemblance to the Apollo moon landers, then.
This last, above-described option, unlike earlier options, would not easily allow the lander to “steal mass” from the ice at Europa’s surface, to soften the landing. This is a relative disadvantage. However, the provision for purely-kinetic bullet impacts to provide data to the orbiter, is a definite advantage. More illustrations can be provided upon request…
This concludes my ideas as of this time. Please send feedback, suggestions, additions, etc. …
Stay tuned… Talk to me! RocketSlinger@SBCGlobal.net
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