[Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

I searched for the answer to these questions, but the results I got were more confusing than enlightening. How much armor dDR is required for a ship capable of atmospheric re-entry/aerobraking? Can it all be up front, or does there need to be minimum armor on center and rear hull sections? I'm assuming a streamlined hull here; an unstreamlined one would need more. (How much?)

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

My rule of thumb, from back in the 3e days, is DR 100 (i.e. dDR 10) on either the front or the back.

To be fair, the longer you're prepared to take over re-entry or the more dV you're willing to spend, the less heat shielding you need. (An extreme form of this is Vertical Landing, see the box on Spaceships p. 40; that basically brakes to a stop at orbital altitude, then descends slowly.) If you don't mind taking many hours, and you have enough manoeuvre capacity to stay under control, you can decelerate over multiple passes getting a little lower and a little slower each time, and use very little heat shielding.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Realistically, being re-entry capable is a specialized system that is only incidentally related to armor; no material armor is capable of soaking the total heat involved in a re-entry on an earth-sized planet, so you need specialized construction to dissipate as much heat intot he atmosphere (instead of you) as possible.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

That's a bit much for a small ship with medium advanced (TL9) armor.
Ah, but aerobraking is all about using atmospheric friction to provide the ΔV to transition from escape to orbital velocity. There should be an equation to calculate how long it takes. Maybe N=k*ΔV/A where N is the number of passes required, ΔV is velocity that needs to be shed to attain orbit, A is the ship's armor in dDR and k is a constant. Not sure how to convert N into time units. (Atmosphere density is probably a factor too.)

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

By spaceships RAW one of:
-One streamlined armor module in front or middle regardless of the DR value.
-Soft landing system.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Look at the Life Pod/Drop Pods. I think they have DR1oo but 80 of that is ablative with DR 20 (dDR2 in Spaceships) remaining after ablation.

There was an old Ve2 rule requiring DR 20 for hypersonic flight.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

In reality, "orbital velocity" and "escape velocity" are situationally dependent -- the formulas in the books are generalizations. For any particular case, the only basic requirement is to change from an open, hyperbolic orbit (e > 1) to a closed, elliptical orbit (e < 1). That, in turn, depends on how close to parabolic (e = 1) your original approach orbit is. (All this relative to the body you intend to orbit.) The math is messy, since it's a variation on Kepler's problem with deceleration based on atmospheric density. You would normally solve it through numeric methods, rather than analytically.

Once you are in closed orbit, you may be able to use subsequent passes through the atmosphere to lower your apoapsis still further and circularize your orbit. But that's a different maneuver.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

You can approximate close enough as no interesting difference without advanced math. Your velocity at closest approach starts out as sqrt ( approach velocity ^2 + escape velocity ^ 2), and must drop below escape velocity for capture to occur.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

That tells you the delta-V required. It doesn't solve the problem of how long you are in the atmosphere, what rate of deceleration you experience, and thus how much heat damage you can expect. The deceleration is a function of atmospheric density, which is (in turn) a function of altitude, which varies non-linearly as you approach and decelerate.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

In some ways that's even easier. You need to dissipate energy equal to your approach energy, and you have the amount of time you are in atmosphere to do so. This is enough energy that, if absorbed, it will reliably kill you, so you need to figure out how fast you can dissipate heat -- which is a tricky problem but not for reasons of orbital mechanics.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Well, the drag force a craft would experience is F_d = 1/2*roh*C_d*A*v^2, where roh = atmospheric density, A = area of the ship's cross-section normal to its velocity vector, C_d = the ship's coefficient of drag, and v = velocity. If you know how much ΔV this will take, you can always set [Ship's Mass] * ΔV = F_d*t and solve for time that way. It's worth noting that roh is a function of altitude, and v is dependent on the drag force at any given time, so this might get kind of nasty. You'll also have to decide on values for A and C_d pretty arbitrarily.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

In practice, any spaceship that isn't huge can achieve however much drag you really want just by going deeper in the atmosphere, but it may not survive the process.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

That is the implication of roh varying with altitude.

It is also worth noting that the portion of the ship facing forward during aerobraking should generally be blunt to keep the shockwaveoff of the craft's frame. Otherwise, the craft will be subjected to substantially higher stresses and heat than otherwise.

Laatly as a follow-up on drag coefficients, a reasonable number would probably fall between around 0.4 ish for an Apollo-style capsule down to as little as 0.04 for an extremely thin, perfectly smooth airfoil-shape. The latter is pretty unlikely to be achieved, though, if for no other reason, no craft will be that smooth. I could believe in the 0.1 ti 0.2 range readily enough, though.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Coefficient of drag varies with mach number, I wouldn't take the subsonic numbers as indicative of performance in a mach 40 aerobrake.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Drag for aerobraking is most efficient from blunt rather than streamlined surfaces. A flat plate perpendicular to the airflow has a Cd ~ 2.0. SMAD III (p. 145) recommends 2.2 for a generic satellite.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

What you reference in SMAD III in section 6.2.3 is perterbations due to atmospheric drag that cause orbital decay in satelites. Furthermore, it includes drag from extended solar arrays and does not even address aerobreaking. All of that said, it is pretty obvious that increasing any of Cd, A, or rho (although you can't affect rho with spacecraft design in any meaningful way) will increase the drag force felt by a reentering craft, but intentiinal atmospheric injection with extended solar panels seems like a good way to lose your solar panels. So again, the broad blunt plate for aerobreaking is primarily there to keep shockwaves off the airframe, allowing for lighter construction. The side benefit is slowing down faster, but deceleration will happen no matter what.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Atmospheric drag == aerobraking. The figure was for a generic satellite, which may or may not include extended solar panels -- the main difference would be the wetted area and the resulting area-to-mass ratio.

Edit to add: You should look at the history of the Magellan spacecraft.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

I never said atmospheric drag wasnt aerobreaking. I said that section addresses a satellite's ability to stay in orbit despite pertebations from atmospheric drag. And the figures given are explained in the text to include large flat surfaces like solar panels, because satellites in orbit have to contend with drag resulting fron such structures.

I have yet to say air drag is not the chief force in aerobreaking. In fact, I specifically called it out as providing a method for finding the time to land, as the OP asked. I simply contend that the figures you are using are misleading. I am also pointing out that the predominant reason to have a blunt nose is to avoid excessive stresses from the bow shock. If you want a faster descent, you need more weight in DR, probably, to account for this.

On a side note, it is possible to determine how much energy is dissipated as a function of time, which can then be used to find an energy pressure, through which you miiiiiight be able to sort out an actual DR, or at least a self-consistent one. Odds are, most GMs can just set a number and move on, but I know Dalton likes detail in these matters.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

The figure that I proposed is pretty routine for these types of calculations. The SMAD III reference is standard and was simply easy for me to locate:

"The drag coefficient for satellites in the upper atmosphere is often approximately 2.2 (using a flat plate model). Spheres have Cd ~ 2.0 - 2.1." Vallado, Fundamentals of Astrodynamics and Aerodynamics, 3d ed., p. 549.

"...Cd is the coefficient of drag ~ 2.2, ..." Squibb, Boden, and Larson (eds), Cost Efficient Space Mission Operations, 2d ed., p. 359.

Your figures (0.4-0.04) are just way too low. The Apollo Cd values I've seen quoted are around 1.4-1.6. The "smooth airfoil" number is for a sub-critical angle of attack, which is specifically the way to minimize drag, not maximize it for braking effect. Once you exceed the critical angle, a wing surface acts more like a flat plate (though at a lower angle to the flow at that point).

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Again, you're talking about whether or not SkyLab will stay in orbit (or how long it will remain there), not safely dropping a can down to Earth. And again, you continue to argue over minutiae while distracting from the general methodology that would actually help the original poster.

Completely without context.

Last I checked, airfoils don't behave as airfoils if you don't present them as airfoils. That's pretty obvious, so this pretty troll-y. You'll also note, if you bother to read my previous post, that using an airfoil-shaped craft would provide the fastest descent. You don't go fast if you maximize drag.

Based on your replies, I'm going to assume you are just looking to provoke an argument, so I'm going to just stop responding to you from this point out. If anyone else has any interest in continuing what has devolved into a rather esoteric discussion on aerodynamics, I'm game, but otherwise, I hope I've helped Dalton with his question.

That's a good point, as would be noting that no spacecraft will ever be perfectly smooth. Even a few micrometers of roughness noticeably increases drag.

Really in the end, so much of that calculation would require the GM to just invent numbers that it's hardly even worth working out - especially for anyone using the Spaceships books, since those intentionally introduce broad generalizations to preserve people's sanity.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

Looks like my question generated a little ... ahem ... friction and the atmosphere has become a bit heated. That wasn't my intention. Some of the math provided is very interesting (I'll save it for future reference) but a bit complex for gaming. I think I'll stick with minimums of 10dDR on the leading edge and 2dDR for other hull sections as a general rule-of-thumb. (Had to tweak my cargo lighter design by reducing the control room size to make room for more armor. |:-/ )

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

No idea why the math would be too much for gaming. It's not like it's rocket sci...oh, um...

I'd probably just go with a Soft Landing Module for simplicity's, because wouldn't even straightforward atmospheric shielding need to be replaced each time anyway?

If you want fiddly bits for in setting crunch, demand that each such module be specialized for an atmospheric density and gravity range.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

I wouldn't think that's necessarily the case. If you've got minimally ablative reentry insulation and (like most fictional spaceships) are operating on generous enough margins to overspend mass here and there, I don't see anything stopping you from having a reentry shield that is good for multiple uses. Unlimited use without repair might be impossible.
You could probably stretch that considerably depending on how much control of the landing profile you have.

Or rather, the reentry shield part can be used in a very wide range of conditions with good trajectory selection. The part that softens your actual landing is a bit more specific.

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

TL:DR version: Aerobraking is complex, and probably too complex for the abstract system in Spaceships to tackle beyond saying "buy a softlanding system" and assuming that system includes the design features needed to survive aerobraking from orbit to surface.

In reality, DR doesn't represent a vessel's ability to survive aerobraking and (re)entry. The Space Shuttle's Thermal Protection System (TPS) was actually very delicate. The tiles were made of foamed silica, and you could scratch them with your fingernail. The nose cone and wing leading edges where carbon-carbon, and that part was damaged catastrophically by a piece of foam once. The top and sides of the Shuttle were covered with fabric.

It's more about moving the keeping the hot air away from your body to reduce heating, and being able to deal with the heating you do experience. How much heating takes place can vary A LOT depending on circumstance.

From my experience with KSP, aerobraking from a low energy transfer orbit (e just about 1) to a capture orbit (e just below 1) doesn't take a lot of delta-v, and even today's space probes can do that just be grazing the top of the atmosphere. Then they can keep grazing to circularize, doing a little each pass.

The big deal is slowing from orbit (or faster) to hit the surface. From LEO, that means getting rid of 8 km/s of velocity. Not a mean feat, but there's more to surviving that just lots of DR. (Apollo had to dump 11 km/s, and only got one pass. Galileo hit Jupiter's atmo at 47.8 km/s and lived (for almost an hour), but it went through 300 gees deceleration). And most of it is beyond the scope of an abstract system, beyond giving it a softlanding system, and saying that that system includes all the design considerations and equipment needed to survive the deorbit.

Now Lithobraking...that's another matter. ;-)

Re: [Space, Spaceships] Armor needed for Aerobraking/Re-entry
 

As far as aerobraking being hard, though, what matters isn't really the delta-V involved, it's the total energy you're dissipating. A vehicle aerobraking is dissipating energy at a rate of force * velocity.

Let's say a spaceship's approach velocity towards Earth started at 3 km/s, which is a kinetic energy of 4.5MJ/kg. At the top of atmosphere, it has gained another 62.5 MJ/kg from potential energy, so its total energy is 67 MJ/kg for a velocity of 11.57 km/s. To be captured, its energy needs to drop below escape energy (62.5 MJ/kg), so we need to dissipate 4.5 MJ/kg and about 400 meters per second.

Now, the big trick here is that atmospheric density varies substantially with altitude, and our altitude during this pass will vary a lot. You can reasonably approximate this pass as if we were falling 'up', so at 10s away from apogee you have gained 500 meters; as the scale height of the atmosphere is about 8 km, we can figure our total time in atmosphere as if it were about 80 seconds (it's actually longer than that but a lot of it is in very thin atmosphere), during which time we need to shed 400 m/s and 4.5 MJ/kg. 400m/s in 80s isn't hard (it means a peak deceleration of about half a G), the challenge is the 4.5MJ/kg. If we assume we have a 10 ton probe that is a 4 meter diameter sphere (33 cubic meter volume), it has a surface area of 50 m^2, so we're looking at 45000 MJ/50 m^2 = 900 MJ/m^2 (it's actually uneven heating, max is about 4x that).

It takes about 7.5 MJ to ablate a kilogram of steel, so if we just ablated away it would be 120 kg/m^2 (about 15mm), or a total of 6 tons of steel gone. Since we're only a 10 ton craft and we still have another 625 GJ of orbital energy we want to get rid of, that's not really a viable option. Thus, we have to figure out how to resist it without ablation.

Our peak energy flow rate is 900/80 = 11.25MW/m^2. An idealized blackbody sheds heat at a rate of 5.67e-8 W*m^-2*K^-4, which we can solve for temperature, T(K) = (1.125e+7/5.67e-8)^0.25, or 3750K (and as noted, it's not going to be evenly spread, so it actually reaches 5300K on the leading edge). That's higher than any realistic material can withstand, but it's not dramatically higher; it we can find some trick that causes heat to be radiated outwards without ever striking the craft, we should be okay.

At that point, the surface of the craft isn't ablating -- and we no longer need armor at all, we just need a framework that can hold our shielding in place without buckling, plus insulation. This roughly describes the way the space shuttle's tiles work -- while they would likely stop attacks for a little while, they aren't really armor as such.