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

[Humabout]

Originally Posted by DaltonS

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.)

Dalton “because 'how long it takes' can be critical” Spence

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.

[Anthony]

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.

[Humabout]

Originally Posted by Anthony

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.

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.

[Anthony]

Originally Posted by Humabout

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.

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

[thrash]

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.

[Humabout]

Originally Posted by thrash

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.

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.

[thrash]

Originally Posted by Humabout

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.

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.

[Humabout]

Originally Posted by thrash

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.

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.

[thrash]

Originally Posted by Humabout

I simply contend that the figures you are using are misleading.

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).

[Humabout]

Originally Posted by thrash

"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.

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.

Originally Posted by thrash

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

Completely without context.

Originally Posted by thrash

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).

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.

Originally Posted by Anthony

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

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.