And just to see if it was possible, I've made something like this in KSP, using 6.4x RSS, RealFuels, FAR, Deadly Reentry, Reach Chutes and a bunch of other Mods.
I'm glad to see discussion on the penultimate post since it has been heavily distracting me all the past two weeks!
Ascent and orbital operations were superb, but reentry was a problem. Orbital Module wouldn't disconnect and one mishap during reentry was all thats needed to rip the thing apart... >_>
I don't have KSP, and doubt it would run on my current platform if I had it, so a lot of caveats and qualifications confuse me. I gather that in its default mode, the Kerbal planet and Kerbals themselves are smaller than Earth and everything operates in a sci-fi alternate system where space travel is easier, and probably the technology modules provided, and most add-ons, are not realistic for real Solar System conditions (or in any possible location of our Universe, in some cases.
) But one can purchase or otherwise acquire add-on mods purporting to make it more realistic. Still, it doesn't prove much of anything if something works in KSP--or if it doesn't.
In the real world, how hard would it be to guarantee that, provided one's reentry capsule were reasonably robust, any failure of non-reusable, orbit-only modules to separate as designed would be survivable due to the structural elements attaching them being guaranteed to melt off or burn up before the entry module was subjected to dangerous exposures? How much of the risk is the entry module being sent into a spin or some such, versus being ripped apart, versus being attached to something that is probably ablating rather explosively?
OTL a number of early space missions were threatened by the refusal of disposable parts to come loose on schedule, but generally they came apart in reentry anyway and left the entry vehicle free and sound.
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I want to thank nixonshead in particular for clarifying something, with the illustrations, that has never been explained to me before, and that is the meaning of a "biconic" capsule as the authors intended it.
I've been tripped up by customary OTL spacecraft jargon before, such as the distinction made between "aerocapture" and "aerobraking" that seems counterintuitive to me and therefore impossible to remember, leading me to use circumlocutions to avoid either term. But as far as I can tell strictly speaking, "biconic" is a term for any object that incorporates sections of two different cones in any way. If we were to simplify a sketch of a Soyuz headlight-type capsule by using a polygon to define its vertical cross-section (think late 1970s computer graphics, with objects sketched by a few straight lines) it would be represented as a bi-conic, that is a relatively wide angle cone mated to the truncated base of a narrow-angle cone, with a pentagonal cross section. It would indeed be a biconic object (if you made a real model to match the sketch) but it would presumably reenter just the way a Soyuz capsule does, circle-side down, bearing the heavy TPS. Like a Soyuz capsule or a conical American capsule, and unlike a sphere, it would gave some dynamic lift if the capsule were angled so the slipstream came in a bit off-axis. "Biconic" merely describes that it has conical sides.
As far as I could discern in prior offhand references to "biconic" capsules, they too would be meant to enter with the circular, or perhaps elliptical, cross-section "down" and being the main TPS surface; I gathered some of them might involve skewing the central axis of the cones to the "vertical" defined by the TPS surface being flat, or even skewing the two cones differently, and figured all talk of this improving the hypersonic lift/drag ratio and so forth referred to effects on the afterbody airflow.
It is nixonshead's pictures alone that clarify that whether "biconic" refers to all double-cone bodies in general or is reserved for a peculiar class of them in astronautical jargon,
this biconic differs radically in that instead of entering with the circular section down, it enters "on its side," with one strip of its conical surface being turned to the slipstream, and thus an entire half of its conical surface is exposed more or less directly to the hypersonic blast. "More or less" because the parts near 90 degrees away from the "down" strip would be getting it at an oblique angle, but of course not only the air incident on that limb but also all the air coming in between it and the central strip would have to flow past it, so the blast is different in character from what hits the dead center of a traditional almost-flat circular capsule bottom, similar to what flows around the edges of such heat shields to form the afterbody flow. And unlike conical capsules, the afterbody only curves away from that flow gradually at first; the break is sharper even on a Soyuz-type headlight shape, and sharper still on a NASA style conical capsule; such flows resemble the flow around the spherical Vostok capsules more, at least in the plane perpendicular to the cone axis. (Or other planes, diagonal to that axis, along the flow vector of the incoming hypersonic air--the forms the flow goes around there would be elliptical instead of circular). So clearly the sides need good protection too, and aside from the temperature and density of the flowing air remaining high (perhaps reduced by Bernoulli effects but still hot and dense) after passing the "terminator" as seen by the incoming air, the craft might also roll a bit off that central strip, so the heavy TPS needs to be extended for more than 180 degrees of the cones' circumferences. The entire nose cone down to a certain distance down the axis will need complete coverage, and the rear circle (probably still a section of a sphere as on most "flat-bottomed" capsules) will also need heavy protection, perhaps a lot lighter than if it were the main entry surface but still pretty good, because that surface will also be exposed to direct incoming flow though at a glancing angle.
So it is not entirely clear to me just how this is supposed to be vastly superior to the traditional circle-down approach; it seems clear that a rather higher portion of the whole surface must be high-heat bearing, which must penalize the weight somewhat.
Perhaps, especially on elongated ones where the length--the "height" of the two conical surfaces--is greater than the diameter of the "base" disk, we get the benefit of a larger net area and thus less intense heating of any particular unit area section--this won't save overall weight since the area to be covered is greater, but for materials of given capabilities it might improve the safety margin attainable, which is clearly a good thing. But the version NASA was considering in this ATL is not elongated like that and so the directly exposed cross-section is very similar to the circle-base area--the heat load is spread out over a greater area which is good for radiative heat dissipation I guess, but correspondingly heavier.
I gather the major advantage is supposed to be in lift options available.
I have not been able to find any clear and comprehensive discussion of the general comparisons of such "sideways" biconics compared to traditional circle-side-down capsules; I did stumble across a
Google book search page (
Basics of Aerothermodynamics
By Ernst Heinrich Hirschel) that showed an attempt to compute a proposed biconic's hypersonic lift/drag polars (including pitching moment) but the plot of data, while suggestive, cut off around 60 degrees. I assumed the practically sinusoidal lift curve, which peaked around 40 degrees, would continue past the plot shown and hit zero around 80 degrees. When I think about it though I'm not sure there would be zero before 180 degrees though perhaps a non-zero minimum-or it might indeed cross the axis and be negative before rising back to zero at that angle. I think it clearly must be zero when the flow is parallel to the axis since the body has radial symmetry. Tipping it as far as 90, or even 80, degrees, would expose the tail-end circle to direct flow and presumably change the overall polar considerably at that point.
These plots suggest though that the planned entry flow would be around 40 degrees, right near the peak lift, where the pitching moment also goes to zero. Even if a range of negative lift exists at angles above 80 degrees I think the design would avoid that range.
A traditional circular-base-first capsule would have zero lift when the axis is aligned with the slipstream; by tilting the capsule in any direction "lift" forces transverse to the slipstream would be created. I don't know if the pitching polar would tend to increase the pitch (positive feedback) or reduce it (negative feedback) but I know that capsules in general are always designed with the internal mass distribution such that the center of mass is shifted "down" toward the heat shield, giving a pendulum moment that tends to center it--and also on Apollo and Gemini and I would guess Soyuz, shifted off-axis toward one side to bias it to a certain pitch that yields a standard lift.
Presumably on a side-entry biconic, the center of mass is well off the axis toward one side, toward the center of TPS strip in fact, to guarantee that no matter how the nose is pitched the side of the circular cross-section that is heavily shielded will roll toward the stream; the weight of the TPS itself might do much to guarantee this. And lengthwise, along the axis, the CM would be adjusted to combine with the pitching moment to be stable at the angle yielding the desired lift coefficient.
So it bothers me that the zero-pitch-moment angle appears to coincide with maximum lift; it would seem this biconic (which is quite different from the one in the post, being longer in proportion by far) is designed to enter at maximum lift, and there is no option for raising it should the craft be entering with insufficient lift. One could lower it by pitching in either direction; going to higher angles would also raise the drag and the net combined force vector would rise (but gently) and presumably the heating as well. Going to lower angles would lower both forces but trying to avoid excessive heat or acceleration at any one moment that way would put the axis more nearly aligned with the flow, meaning that the upper side of the conic would be more exposed.
If they wanted high negative lift--say the craft were entering the atmosphere too shallowly and was in danger of bouncing off into an undesired orbit--they could roll the thing around the axis of the slipstream so the nose is down instead of up; a circle-side-down capsule would do that by tipping across the neutral axis to tip the other way.
I don't know if the sideways biconic's lift/drag characteristics as I've described them here would actually be more desirable than the simpler sketch of a disk-side-down capsule; it seems we trade off a higher attainable lift coefficient for greater difficulty in varying it. Intuitively it does seem to me it would be easier to set up the mass balances inside the biconic to set the pendulum moments where we would probably want them than to get the right balance on a traditional capsule; this issue is one that discouraged me from suggesting a capsule solution to the downmass problem. The question is, can we do without the ability to have emergency increases in lift coefficient (because the natural rest angle of the entering biconic selects for maximum already)--do we then have adequate control?
I did also note, in searching for OTL examples of biconic-sideways real or proposed, that it seems normal on such proposals to include some kind of flap on the trailing edge (where the conical sides meet the circular surface, on the bottom, slipstream side). Presumably then in addition to shifting masses internally (a costly method I'd think) or using attitude jets (suitable for quick realignments but not to sustaining an angle offset from the natural product of aerodynamic pitching moments and pendulum moment, unless we have lots of reaction control fuel) the flap can also be moved, which will change the characteristics, most importantly here the pitch moment, to adjust the angle it wants to hold. Obviously such a flap would be exposed to very high heating and must endure high temperatures while still being maneuverable, but OTL we are familiar with this problem on the STS Orbiter.
One rarely if ever sees this suggested for traditional disk-side-down capsules, although I believe one of the early competitive company bids for Apollo included a conical capsule much like the one Faget and von Braun wanted in the first place, but with flaps included.
On a disk-down capsule, even if one side of the capsule were designated as the preferred "leading" edge, I'd think more than one flap would be needed for adequate control. On the sideways biconic I suppose just one can do in a pinch, though the proposals I've seen mention it being split presumably to provide some yaw control as well as pitch. On the biconic though I suspect yaw headings would tend to be stabilized by the flow and thus bursts of thruster fire would be adequate, being brief; it is maintaining various pitch angles I'd worry about with that method since anything off the fixed pendulum/aerodynamic rest angle would require constant thrust to hold.
Now that I've come to finally understand what is meant by "biconic" in the sense shown here, I can see that other proposals I've seen before are also in this category, such as Kliper--particularly the wingless versions, though I'd describe winged Kliper as a mere modification of the basic biconic theme, just as Spiral's spaceplane was essentially a lifting body with wing/fins added for control and low-speed lift. It is much clearer to me why the space travelers (ESA also toyed with the idea, so I'm not just saying "cosmonauts) had acceleration couches with their backs to the pointed tip--at angles like 40 degrees off a zero defined by that tip, the net forces would be "backward" like that, with "down" toward the nose, though the high lift force would tend to shift it toward the TPS side.
One idea I had, especially for a wide, relatively short length version such as NASA considers ITTL, was for the crew compartment to be a rotating drum, with its axis at 90 degrees to both cone axis and the plane in which the circular circumference of the conical surfaces would be centered against the slipstream. The drum could be rotated so as to put the crews' backs downward across the rocket axis during launch, and then relocated some 120 degrees away to have their backs facing the net reaction during nominal entry, and able to shift back and forth as the craft undergoes necessary pitch changes--I haven't quite worked it out in my head but it seems to me this would introduce a moving mass that would shift the net center of mass forward or aft to support holding lower or higher pitch angles; if so the drum's internal mass distribution could be designed to accentuate this shift, putting heavy equipment within it under the crew couch backs or anyway alongside them. If I'm mistaken and the shifted mass destabilizes things, that same stuff can be placed opposite the crew to balance the drum, though it becomes less desirable then and its obvious liabilities tell against it more strongly.
One such liability would be that the drum would have to be rotated a certain way to give access to any parts of the spacecraft not included in it; it would be bad if it jammed in the wrong position. Also we'd want it oriented right at least for average reentry if not freely rolling to provide optimal g-force mitigation; that should be the standard orientation. The biconic proposals I've seen suggest putting a hatch on the "top" side of the conic, that is opposite te TPS center; with a drum it would have to be on the drum axis which is to say, on the side, where it might take on water if the thing lands in water.
I suppose such a drum would be too extravagant, but without it I suspect the crew will have an odd time of it, if not actually unpleasant, reentering with widely shifting net "downward" vectors--I suppose the easier answer would be to mount the couches along that "drum" axis individually so they can swing separately like so many hammocks. Now they are sideways to the flight direction instead of backwards to it, which might create more confusion for pilots trying to control it.
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I've also had the chance to reflect on how this version of lifting-body, sideways biconics might apply to other problems, such as achieving reusable second stages. The consensus of this TL seems to be that when reusable first stages will be achieved, it will be by means of vertical-landing rockets. My chief difficulty with that idea is that although it is typically the upper stages that contribute most of the necessary transverse velocity to actually achieve orbit, the first stages have historically still contributed a significant amount of it even so; to change launch profiles to reduce that downrange boost throws a heavier burden on the upper stages. But to retain the traditional modest but significant downrange velocity contribution of the first stage makes return to base on rocket thrust problematic because that downrange velocity must be arrested quickly and reversed. Reversing it is not such a major part of the task because the stage will also impart a lot of upward velocity, which buys time to make the distance back to the launch point, and I guess it is not necessary to cover that whole distance before falling back to the first-stage burn cutoff altitude, since it will cover some more of it in the atmosphere on the way all the way down. So that extra velocity is relatively low, but cutting a downrange speed of say 2 km/sec will require significant fuel masses, which can be regarded as a multiple of the empty mass fraction and thus an increase of "empty" mass fraction for the launch phase, which drives up the mass relative to the upper stack considerably.
OK then, assuming we can pay whatever price it takes (distributed between the first and upper stages) and solve the problem of returning the first stage this way or some other--how to get back the upper stages? Say it only takes one other stage; the goal is to get to orbital velocity, so that, or nearly, is where that stage winds up. Clearly it could orbit around the Earth and come in toward the launch site, much as any returning space capsule or the like eventually will. But what then? Unlike a rocket-returning first stage that might need essentially no TPS or even a winged or other flyback form that will need only a fraction, going at a low fraction of orbital speed as it will, it will face the same stringent requirements a return capsule does, and to be reusable an ablative coating is probably not practical since it would have to be reapplied for a second mission.
Do biconic lifting bodies coming in sideways offer solutions that help broaden our options usefully?
Given that the engine will be a big part of the mass of any returning upper stage, and must be located on the tail end of a compact body (conceivably there could be two sets on the wing tips of a winged body, straddling center of mass a la Skylon, though thrust structure might be problematic and so would be thermally protecting those exposed pods) I guess the biconic-sideways solution would require, not only extensive TPS covering over half the upper skin, but an elaborate scheme to detach the engines and slide them up a tunnel in the tankage to rest halfway along the length of the stage or more, on the TPS side. The stage would not have its mass along the axis of the cones even at launch, with the biases imposed by TPS (unless we can afford to cover the whole surface uniformly) and an offset tunnel, and the shift would become more pronounced as the burn proceeded, so the engine would have to be correcting for a shifting center of mass constantly.
It seems maybe a traditional disk-down body, with the engine or engines mounted symmetrically and either drawn into protective bays with heavy TPS hatches closing to protect them, or the engine nozzles being designed to take the full heat of reentry (after all they have to survive the heat of thrusting) and the craft entering bottom-side down and then landing as the first stage does on rocket thrust and deployed legs might be more practical after all. Even if the engines still have to be moved, stowed for reentry, they don't have to move far and not through the volume where we want fuel tanks to go. The problems I see here are, first of all, the bottom disk is not all that large compared to the total empty stage mass (especially if we need reserve propellant for landing) so it would be intensely heated, whereas a simple cylinder with straight vertical sides would probably suffer pretty severe heating too. I'd be confident it would maintain a generally tail-first attitude due to the weight of the engines, but it would probably wobble somewhat meaning one side another of the tankage cylinder gets exposed to hypersonic heating.
Perhaps with good enough TPS and the reflection that the total empty mass is not tremendous we can get away with this?
A tapered cone like say the Mercury or Gemini capsules seems like a better solution though. At a 15 degree angle of mold line to axis, similar to those capsules, I guess in my head that such a cone about 16 meters high and with an 8 or more meter diameter base could hold enough volume to match the TTL Saturn Multibody standard upper stage's capacity; with a base area of over 50 square meters, could its dry mass still be kept down in the close ballpark of 10 tons, with enough margin for adequate TPS on the main shield and any gadgetry necessary to protect a single engine in the J-2S class, or a cluster of say six of one-sixth that engine's thrust? Bearing in mind the two American crew capsules of the early and mid 60s needed substantial TPS on their upper bodies as well, a high-temperature metal shingle? My guess is the pressure and thus heat flux on the surfaces will be a tenth or less that on those two capsule's shields, but on the other hand they used ablative main shields.
Anyway, with a 4 or more meter radius, the bottom of such a stage would be enormous compared to any real-world rocket structure diameter (except maybe the STS fuel tank or Energia core tankage); since Saturn Multibody standard first stage units have the same diameter as the standard upper stage I guess a new version of the tankage to merely match the 8 meter diameter bottom would have a quarter the height or less; separated from the hydrogen-oxygen upper stage it would look rather like a hockey puck if not a pancake!
Well, it might look silly, but such a form might be just fine for rocket-landing return, if it can be stabilized to keep the bottom side down.
The upshot is rockets that look more like flying Gemini capsules, with the first stage as the Transstage and upper one as the capsule itself, and the payload as the nose cylinder. The air drag on the way up might be pretty significant. Then again I believe NASA or anyway someone seriously proposed these sorts of monsters back in the 1970s, and what I'm describing is probably much like many of Bono's proposals for fully reusable single-stage orbiters in the million-pound payload range--except that they aren't single stage of course. But fully reusable!
The biconics on the side don't seem to come into it unless someone has severe objects to the simple tail-down entry solution and can face the issues of moving the mass of the engines to a suitable location inside. It would seem that the old-fashioned winged spaceplane is a better contender than biconics for this role, if it is possible to mount the main engines on the wingtips and for them, the wings, and the spindle-shaped fuel tank in the middle to have adequate TPS--which seems not so crazy considering their extended area when coming in belly-on.