Space X to build reusable launch vehicle

[Skipjack]

The kinetic energy to get rid of is huge, more than 45,000 MJ. If completely turned into heat it would elevate the temperature of a block of 20 t of aluminum more than 2,500 K, and with steel is even worse.

Hmmm, SS1 and SS2 were/will be in a free fall from ~120km and barely experience any heating of the surface at all. Normally the heating of the sourface of a reentering vehicle is from slowing down from orbital speeds, not from the fall through the atmosphere after it has slowed down to terminal velocity.
The F9 first stage does not require a heatshield now, when it hits the atmosphere at probably still close to Mach5. Why would it need one once it is slowed down to terminal velocity.
There are concepts for people base jumping from 100+ km with just a spacesuit on. Again, they wont burn up.
Something here just does not add up.

[ladajo]

A small falling body such as a person has much less inertia than a large falling body and is thus more receptive to energy transfers. In short, the braking effect of increasing atmospheric density will have more affect on a small body than a large one. Thus the change in terminal velocity as atmosphere density increases is more acceptable to the lower inertia object. I see it as a question of the scale of momentum compared to applied forces and inertia.

Drop a brick, drop a wadded up ball of paper. Add a crosswind or fan blowing up from underneath, see which deviates more from its path.
You may want to drop the brick second.

[Skipjack]

A small falling body such as a person has much less inertia than a large falling body and is thus more receptive to energy transfers. In short, the braking effect of increasing atmospheric density will have more affect on a small body than a large one. Thus the change in terminal velocity as atmosphere density increases is more acceptable to the lower inertia object. I see it as a question of the scale of momentum compared to applied forces and inertia.

Drop a brick, drop a wadded up ball of paper. Add a crosswind or fan blowing up from underneath, see which deviates more from its path.
You may want to drop the brick second.

I dont get this at all. Are you saying that the terminal velocity for a brick is lower than for a ball of paper?
Because I would assume the oposite. An almost empty rocket stage certainly has more in common with a ball of paper than with a brick:low density, high bouyancy high surface area to weight ration.
This means that the drag will slow the object down higher up in the atmosphere than it would do with a smaller and denser object.
Or what am I missing?

[ladajo]

I meant that the brick will drop faster and resist change more than the ball of paper.

This is part of the reason you can do high altitude free falls with people.

In the case of the rocket, obviously there is more to terminal velocity than weight. However, the other point to remember is that falling objects have inertia. And a higher inertia means higher resistance to change. A falling rocket body is going to have much higher inertia than a falling person. That is my point, and the part you were not grasping above.

[Skipjack]

However, the other point to remember is that falling objects have inertia.

So have resting bodies.

And a higher inertia means higher resistance to change. A falling rocket body is going to have much higher inertia than a falling person. That is my point, and the part you were not grasping above

I still dont get your point. Decelerating a body of a certain mass to a standstill requires the same energy it took to accelerate it to the speed it is at. In case of the rocket stage, you have the advantage that its mass will be much lower that it was at take off and the air resistance is your friend and not your enemy. All benefactors that work for you, when you are trying to do a vertical landing.

[charliem]

The kinetic energy to get rid of is huge, more than 45,000 MJ. If completely turned into heat it would elevate the temperature of a block of 20 t of aluminum more than 2,500 K, and with steel is even worse.

Hmmm, SS1 and SS2 were/will be in a free fall from ~120km and barely experience any heating of the surface at all. Normally the heating of the sourface of a reentering vehicle is from slowing down from orbital speeds, not from the fall through the atmosphere after it has slowed down to terminal velocity.

I don't know about SS2, but comparing F9 and SS1 is not straightforward.

SS1 falls from a maximum high of 112 km. F9 first stage falls from ~280 km (after staging it keeps a lot of upward inertia).

SS1 dry weight is just 1.2 t, versus 10-20 t for a F9-1st.

The drag coefficients and L/D ratios are also very different. As a result SS1 has some glide capability even at high altitudes, while F9 has none.

There are concepts for people base jumping from 100+ km with just a spacesuit on. Again, they wont burn up.

One of the things that make space diving so hard is that you can't use a "simple" space/pressurized suit, you need to add some serious heat resistance or the jumper gets cooked by the air friction.

The F9 first stage does not require a heatshield now, when it hits the atmosphere at probably still close to Mach5. Why would it need one once it is slowed down to terminal velocity.

I'm not saying that the first stage is going to need a heat shield, nor the opposite, just that the conditions it encounters while reentering are not all that well understood ... as SpaceX found when they failed to recover anything but the Dragon capsule from its two flights to date.

[ladajo]

However, the other point to remember is that falling objects have inertia.

So have resting bodies.

No kidding, really?

And a higher inertia means higher resistance to change. A falling rocket body is going to have much higher inertia than a falling person. That is my point, and the part you were not grasping above

I still dont get your point. Decelerating a body of a certain mass to a standstill requires the same energy it took to accelerate it to the speed it is at. In case of the rocket stage, you have the advantage that its mass will be much lower that it was at take off and the air resistance is your friend and not your enemy. All benefactors that work for you, when you are trying to do a vertical landing.

Yes, you are correct. And this is the point. Its inertia will resist change, so it will take more time to get the affect sought after for a given force with more inertia. You cited the falling person verses the falling rocket. This is what I am talking about. Also, the wind resistance is going to take longer to slow it, given the higher inertia than a smaller object like a person. Now that can be offset by the surface area, and resistance effect but that is beyond what I am trying to point out.

There are concepts for people base jumping from 100+ km with just a spacesuit on. Again, they wont burn up.
Something here just does not add up.

The rocket may not reach terminal before it hits dirt. The person is certain to. The terminal point for a person is much slower than a larger mass, and will be well below heating points. Drop a person, they will never reach Mach 5, if they do, you will have the remains of a roasted person. Case in point, shuttle over Texas.

[charliem]

An almost empty rocket stage certainly has more in common with a ball of paper than with a brick:low density, high bouyancy high surface area to weight ration.
This means that the drag will slow the object down higher up in the atmosphere than it would do with a smaller and denser object.
Or what am I missing?

It's not so much a question of density (weight per unit of volume) as of weight per unit of area, the characteristic area for the drag.

If you take a brick and you take a ball of paper with the same weight and SECTION of that brick, and let them both fall, terminal speeds should not be that different.

[Skipjack]

One of the things that make space diving so hard is that you can't use a "simple" space/pressurized suit, you need to add some serious heat resistance or the jumper gets cooked by the air friction.

Kittinger jumped from a balloon at 30km and did quite well without any heat shield.
Currently Red Bull Stratos is attempting at getting Felix Baumgartner to reach mach 1 after jumping from a balloon at almost 40km. The guy is wearing a space suit, but does not have any termal protection from what I understand. I doubt that coming in from higher than that will make that much of a difference, but I may be wrong. Dont forget that the atmosphere slowly gets thicker and so a very bouyant body will experience a gradual breaking.

Drop a person, they will never reach Mach 5

Neither will the F9 first stage.
The shuttle is getting so hot because it is using the atmosphere to break from orbital speeds. The F9 first stage never had a speed higher than Mach 5.5 (?) and is using its engines for breaking long before it hits the atmosphere. So all it will have to deal with is decelerating against earths gravity.

It's not so much a question of density (weight per unit of volume) as of weight per unit of area, the characteristic area for the drag.

True, but I mentioned the other factors as well. They kinda condition each other.

[Skipjack]

One of the things that make space diving so hard is that you can't use a "simple" space/pressurized suit, you need to add some serious heat resistance or the jumper gets cooked by the air friction.

Kittinger jumped from a balloon at 30km and did quite well without any heat shield.
Currently Red Bull Stratos is attempting at getting Felix Baumgartner to reach mach 1 after jumping from a balloon at almost 40km. The guy is wearing a space suit, but does not have any termal protection from what I understand. I doubt that coming in from higher than that will make that much of a difference, but I may be wrong. Dont forget that the atmosphere slowly gets thicker and so a very bouyant body will experience a gradual breaking.

Drop a person, they will never reach Mach 5

Neither will the F9 first stage.
The shuttle is getting so hot because it is using the atmosphere to break from orbital speeds. The F9 first stage never had a speed higher than Mach 5.5 (?) and is using its engines for breaking long before it hits the atmosphere. So all it will have to deal with is decelerating against earths gravity.

It's not so much a question of density (weight per unit of volume) as of weight per unit of area, the characteristic area for the drag.

True, but I mentioned the other factors as well. They kinda condition each other.

Either way, even if you completely disregard the atmosphere and drag, you have to fight the acceleration of earths gravity for the entire way down. That is only 10m/s2 with a very light first stage. The entire second stage is missing as is most of the fuel in the first stage. I think things are still very much in favor of the F9 first stage. But eiter way, you dont have to believe me, believe the guys at SpaceX. I am 100% sure that they know what they are doing.

[hanelyp]

The SR-71 sustains mach 3.5 with an attached shock wave on the leading edge. A Falcon9 first stage hits the atmosphere a fair margin faster, but for a fairly brief heating surge. And with attitude control it would have a blunt face forward for a detached shock wave and reduced heating.

[Skipjack]

A Falcon9 first stage hits the atmosphere a fair margin faster

I havent calculated it, but I am not so sure about that.

[93143]

The SR-71 sustains mach 3.5 with an attached shock wave on the leading edge. A Falcon9 first stage hits the atmosphere a fair margin faster, but for a fairly brief heating surge.

The SR-71 is also titanium. The Falcon 9 first stage is an aluminum alloy, which melts at about 1000 K less and can only soak 2/3 as much heat adiabatically.

Also remember that the stagnation temperature rise goes roughly as the square of the speed...

[charliem]

Drop a person, they will never reach Mach 5

Kittinger jumped from a balloon at 30km and did quite well without any heat shield.

Currently Red Bull Stratos is attempting at getting Felix Baumgartner to reach mach 1 after jumping from a balloon at almost 40km. The guy is wearing a space suit, but does not have any termal protection from what I understand. I doubt that coming in from higher than that will make that much of a difference, but I may be wrong. Dont forget that the atmosphere slowly gets thicker and so a very bouyant body will experience a gradual breaking.

I'm afraid that's not exact. The atmosphere does not get SLOWLY thicker, but exponentially. At sea level has a density of ~1.2 kg/m3, at 25 km is 0.04 kg/m3, and at 50 km it drops to 0.001 kg/m3, so about 30-40 times less every 25 km.

Given that drag force is proportional to air density and to speed squared, and that the gravity force is almost the same, that means that for the same body terminal speed at 90 km is > (30^2)^0,5 = 30 times than at 30 km. So, if at 30 km is only mach 0.5, at 90 km that'd be > mach 15.

Let it fall from high enough and a human body could potentially reach earth escape velocity before touching the atmosphere, 11.2 km/s, mach 33.

[Skipjack]

at 90 km that'd be > mach 15.

you are off by an order of magnitude here.