Are there any rocket scientists here?

[rwguinn]

But it's not all heat dissipation is it, anymore than falling under a parachute is heat dissipation. It's also transfer of momentum from one object to another (parachute to the air).

Yes, it is. Even under a parachute. The deault form of energy is heat. Transferring momentum means something gets hotter.It has to do with that entropy thing you hear so much about.

The whole point is to minimize the heat part. As to how long, I don't know how to do those calculations realistically, but what's wrong with one or two orbits? A few hours?

If you're in orbit, you have to slow down.
the only way to minimize the Maximum temperature is to take lots of time.
The heat rejection is the same, regardless of the time you take. The temperature is a function of how fast. In other words, take 15 minutes, temperature =1200 degrees or so. Take a week or so, temp=200 degrees(1200 and 200 merely a WAG on my part--I don't do thermo any more).

Edited for dumb-thumbs and dyslexai

 

[Terry]

Perhaps I should have asked if there are any aerodynamic engineers here?

Well by training, yes. But I dont' work in the biz. If I remember correctly, the Apollo capsule does "fly" to some extent during parts of the trajectory. Incidentally, this article suggests that the heat transfer to the reentry vehicle is minimised by dissipating as much energy as possible in the upper atmosphere.


--Terry.

 

[MRC_Hans]

As mentioned, you have X joules of energy to get rid of. You have to dissipate both the speed energy from you orbital speed and the potential energy from your altitude. Now, by allowing the surface of the vehicle to be come very hot, you can dissipate a lot of energy. Some is conducted to the passing air, the rest is radiated away. For the radiated energy, the dissipation is roughly equivalent to the square of the temperature, so as you lower temperature, you increase the required time exponentially.

Now, the whole problem is to make the vehicle sufficiently heat-resistant, and we're actually quite good at this, so why bother with impossible aerodynamics? Sure, if your heat-shield fails you are, literally, toast, but the same would be the case if your sophisticated aerodynamics were to fail. We are simply in a realm where any system failure is likely to be fatal.

I don't think size matters much. Many of the meteors that enter Earth's atmosphere are sand-grain sized, and they still evaporate.

The whole problem is that as soon as you start lowering your speed in a low orbit, you are heading for the surface at an insane speed, with a fifty kilometer fall ahead of you, and little air to carry you for the first half of the fall, so the only really workable solution is some hot brakes.

Hans

 

[Elind]

Yes, it is. Even under a parachute. The deault form of energy is heat. Transferring momentum means something gets hotter.It has to do with that entropy thing you hear so much about.

Next thing you'll quote E=Mc2

No, it doesn't have to get hotter, it just has more energy which doesn't have to be immediately converted to heat, either in the form of rising temperature or as radiation. Since such transfers are never perfect there is always some loss in the form of temperature or radiated heat, but it can be a small portion of the total.

 

[Elind]

Well by training, yes. But I dont' work in the biz. If I remember correctly, the Apollo capsule does "fly" to some extent during parts of the trajectory. Incidentally, this article suggests that the heat transfer to the reentry vehicle is minimised by dissipating as much energy as possible in the upper atmosphere.


--Terry.

That's a good article, although it doesn't really answer my question. The history of "reentry" starts long before computers could fly and of course the focus was on how to get down fast and just hot enough to handle, rather than slow and cool enough to touch, so to speak.

I'm still looking for a fundamental principle that explains why it's not possible to "fly" high enough and long enough to avoid this issue.

Perhaps something along the lines of "directional controled flight cannot be achieved in less than "x" gas density, and "y" frictional effects cannot be avoided at 17000 mph speeds in "x" density"?

 

[Elind]

As mentioned, you have X joules of energy to get rid of. You have to dissipate both the speed energy from you orbital speed and the potential energy from your altitude. Now, by allowing the surface of the vehicle to be come very hot, you can dissipate a lot of energy. Some is conducted to the passing air, the rest is radiated away. For the radiated energy, the dissipation is roughly equivalent to the square of the temperature, so as you lower temperature, you increase the required time exponentially.

I'm nitpicking perhaps, but as already mentioned the "potential" energy can be discounted as easily coped with by more or less conventional means. Think Spaceship One.

As also discussed elsewhere, not all energy is lost simply by radiation or conduction. Much is lost by transfer of momentum via the shockwave of the vehicle. Perhaps eventually to heat via sound etc etc, but the point is that one doesn't have to assume that the vehicle has to somehow absorb and discard all the kinetic energy as heat.

 

[Terry]

I'm still looking for a fundamental principle that explains why it's not possible to "fly" high enough and long enough to avoid this issue.

If you had infinte fuel, you could come down as slowly as you wanted.


--Terry.

 

[Elind]

If you had infinte fuel, you could come down as slowly as you wanted.


--Terry.

Actually you could come down fastest with less than that.

To decelerate from 17000 mph at 1G takes only 13 minutes to come to zero at, say, 200,000 feet. Then just glide down while enjoying the view.

 

[rwguinn]

Actually you could come down fastest with less than that.

To decelerate from 17000 mph at 1G takes only 13 minutes to come to zero at, say, 200,000 feet. Then just glide down while enjoying the view.

All right, one more time.
Your "parachute" or glider has to work at very, very low air density. That means it is very, very big. Yes, you can then transfer energy to the air in terms of momentum exchange.
Unfortunately, when you get into higher density air, this big humongus outrageously large "wing" is going to keep you up for a very, very long time. We won't even mention the weight of the thing, and the fuel it takes to get it into orbit in the first place.
Conversely, a smallet whatever requires a lot faster shedding of energy, and/or a higher KE when you enter atmospher of any density, and the corresponding potential for burning up/self destruction through unreasonable and un-survivable loading.

 

[Elind]

All right, one more time.
Your "parachute" or glider has to work at very, very low air density. That means it is very, very big. Yes, you can then transfer energy to the air in terms of momentum exchange.
Unfortunately, when you get into higher density air, this big humongus outrageously large "wing" is going to keep you up for a very, very long time. We won't even mention the weight of the thing, and the fuel it takes to get it into orbit in the first place.
Conversely, a smallet whatever requires a lot faster shedding of energy, and/or a higher KE when you enter atmospher of any density, and the corresponding potential for burning up/self destruction through unreasonable and un-survivable loading.

I understand. That is a logical train of thought, but I'm just not convinced that it is entirely accurate, as stated.

First, "humongus" is not exactly very specific, and I'm just not convinced that it is a requirement either, at 17000 miles per hour. Second, momentum transfer happens over an airplane wing too.

If a relatively streamlined Shuttle encounters what, about 3 Gs on a fast hot reentry, why couldn't something be designed to fly shallower and higher at say only a 1G loading or less? It doesn't necessarily have to be parachute/drogue either, that was just a thought thrown in here.

It could be possible, but not cost effective for reasons you suggest, or
as I suggested before, I'm thinking there is not enough atmosphere there to allow such control, for aerodynamic reasons that I don't know enough about. After all a lot of smart people have thought about this already, but on the other hand they didn't come up with Burt Rutan's ideas earlier either.

BTW, when I said decelerate in 13 minutes, that was a response to having unlimited fuel available, not some other method. Perhaps a really humongus chute could do that too, but likely not by staying high at the same time. On the other hand however, the real requirement is not to get rid of all the 17K speed, just perhaps half of it and the rest becomes much easier.

 

[bjb]

Going back to the original question, the concept of a 'skipping' vehicle is not new. The Nazis had the idea back in WWII, where an atmosphere skipping rocket plane would make its way to the US, drop an atomic bomb, then continue skipping around the world until it got back to the Fatherland. More recently, the X-43 tested technology that would make it possible for a hypersonic vehicle to skip in and out of the atmosphere, gathering oxygen while in the atmosphere to generate thrust to leave the atmosphere again. Once outside of the atmosphere, the vehicle would coast along until it fell back in.

In these scenarios, the vehicle reaches a speed of several Mach, but not the Mach 25 of an orbital vehicle of the space shuttle orbiter. The heat dissipation requirements are much easier to satisfy in such a design. Still, thermal control (heat shielding) would be very important. Even jets like the SR-71 and XB-70 had deisgn issues with aerothermal heating, and those only went at Mach 3 or so. The X-43 relied on liquid cooling of its surface, plus, it only flew at high mach for a few minutes. If you want to build a vehicle that doesn't need a heat shield, the maximum speed has to remain below mach 2 or so, and that means an orbital vehicle (mach 25) is out of the question. Sorry, Burt Rutan!

You guys might want to look information on the DC-X, which was designed to launch and land vertically, just like the rockets in those cheesy sci-fi movies from the 50's.

 

[rwguinn]

I think I'm going to call "Troll" on this one.
Elind can't seem to see that you can't "Stop" an object in orbit at a location over the earth: every bit you slow down drops the orbit; He/she/it also can't see that the shedding of hundreds of mJ of energy is essential to de-orbiting; he/she/it has a very limited mis knowledge of basic physics.
He/she/it needs to do some reading.
I'm done.

 

[Elind]

I think I'm going to call "Troll" on this one.
Elind can't seem to see that you can't "Stop" an object in orbit at a location over the earth: every bit you slow down drops the orbit; He/she/it also can't see that the shedding of hundreds of mJ of energy is essential to de-orbiting; he/she/it has a very limited mis knowledge of basic physics.
He/she/it needs to do some reading.
I'm done.

I'm going to call "rude dork" on this one.

Of course you can "stop" an object in orbit, dickhead, if you have enough fuel to do so, or if you can dissipate enough energy in some other manner, which is precisely what this previously polite discussion was about.

Why don't you move over to the politics section, where I can less gently tell you what I think of your kind?

Or perhaps you'd like to sober up first, and repost?

 

[Lisa Simpson]

Please do not personalize the debate.

Replying to this modbox in thread will be off topic  Posted By: Lisa Simpson

 

[rwguinn]

I'm going to call "rude dork" on this one.

Of course you can "stop" an object in orbit,(rule8 deletion by me), if you have enough fuel to do so, or if you can dissipate enough energy in some other manner, which is precisely what this previously polite discussion was about.

Why don't you move over to the politics section, where I can less gently tell you what I think of your kind?

Or perhaps you'd like to sober up first, and repost?

One last time:
If you try to stop it, it starts coming DOWN FROM ORBIT. Right then. Unless you use a force to keep it up.
Firing the retro rockets on a spacecraft slows it down so the orbit will decay. Over time. slowly. not all at once. as in, this will take a bit of time.
Delta V= A*T.
How you planning on holding it up there?

ETA-the "Me" in the quote is Me, this user. One does not need vulgarity and/or personal insult.

 

[Roboramma]

Wow.

I was enjoying this thread and maybe even starting to learn something. rwguinn, if you think Elind wasn't listening to you, why not just stop posting?
I was trying to understand this topic, but my understanding of physics is as limited as anyone's in this thread.

Your posts were informative, but somewhat vague, and it wasn't that easy to see that what you were saying was true. I could just assume that because you are more informed than me that you are probably right, but I want to do more than that, I'd like to understand.
I can understand the frustration that you've made basically the same response a few times, and Elind still doesn't see what you're saying, but neither do I.

Point being, I don't think Elind is a troll. Just doesn't see that what you're saying is necessarily true.
And I do understand that if you slow something down that's in orbit it will start to fall, and accelarete as it does so. But the idea was that the lift from the wings could balance that out so that the acceleration was as low as possible. Or just low enough that the heat sheilding wouldn't need to be so strong.
Anyway, I'm certainly not going to say you're wrong about this, just that it's hard for those of us who don't have degrees in phyics to understand. If you don't have patience to explain, I can understand that, but there's no reason to insult someone for not getting it, or for continuing to ask questions until they do get it.

Elind, on the other hand, your response was just completley uncalled for.

 

[Elind]

Elind, on the other hand, your response was just completley uncalled for.

You are right. I should have expressed contempt for an insult more calmly. For that I apologize to other readers, while stating that I consider no insult worse than being labelled a troll.

Perhaps some don't really know what it means, and never will.

 

[rwguinn]

Wow.

I was enjoying this thread and maybe even starting to learn something. rwguinn, if you think Elind wasn't listening to you, why not just stop posting?
I was trying to understand this topic, but my understanding of physics is as limited as anyone's in this thread.

Your posts were informative, but somewhat vague, and it wasn't that easy to see that what you were saying was true. I could just assume that because you are more informed than me that you are probably right, but I want to do more than that, I'd like to understand.
I can understand the frustration that you've made basically the same response a few times, and Elind still doesn't see what you're saying, but neither do I.

Point being, I don't think Elind is a troll. Just doesn't see that what you're saying is necessarily true.
And I do understand that if you slow something down that's in orbit it will start to fall, and accelarete as it does so. But the idea was that the lift from the wings could balance that out so that the acceleration was as low as possible. Or just low enough that the heat sheilding wouldn't need to be so strong.
Anyway, I'm certainly not going to say you're wrong about this, just that it's hard for those of us who don't have degrees in phyics to understand. If you don't have patience to explain, I can understand that, but there's no reason to insult someone for not getting it, or for continuing to ask questions until they do get it.

Elind, on the other hand, your response was just completley uncalled for.

Keep that thought. Just because someone "sounds" better informed than yourself does not make them an expert, or even mean that they know what they are talking about.
I deal with energy and momentum on a daily basis. The understanding has become second nature. That does not mean I can communicate it to someone without the basic knowledge to build upon.
It is a lot like trying to explain how a Pentium IV chip does what it does. IF the listener's electronic knowledge consists of "If I stick stuff in a light socket, I get sparks and it gets dark", you aren't going to get very far. That is why I recommended some reading and studying. The knowledge that energy cannot just disappear, the relationship between orbital velocity and orbital diameter, the very fact that heat is the "default value" for energy--all these are necessary to understand the greater issue.
The math is not hard. Grasping the concept is difficult, but if you remember that unlike people, nature does not bow to wishful thinking, you will get the idea.

 

[MRC_Hans]

Mmmm, I recon this thread de-orbited quite fast.

Hans

 

[Iamme]

It would waste fuel to do a retro rocket descent. So they skip on the atmosphere as you said, like a flat rock on a pond. They bleed off the acceleration by doing that very skillfully.
Notice that on the Moon, they couldn't do that due to no atmosphere.
And wings would not be a good idea. The forces involved would probably rip them off, given our present level of material technology.

Jeff, ol buddy ol pal. I thought maybe you died.

I used to be the force between ripping wings off........























...of flies.