Slower Descent From Orbit?

[c0rbin]

I raised a question to my lunch buddies, one of whom is a Mech Engineer by schooling.

The question:

If we return to Earth through the atmosphere slowly, like with a balloon, for example that off-sets an object's boyancy on slightly, could we avoid the burning friction.

My engineer friend said this was an impossibility that the burn was unavoidable.

Why is this?

I am not an engineer of any kind by training. Can anyone help me out here?

[Stimpson J. Cat]

One problem is the orbital speed. Orbital speed is very very fast. As you slow down, you begin to fall. You would have to be able to slow down to essentially zero speed perpendicular to the direction towards the Earth, and simultaneously counter the Earth's gravitational pull. When you are that High up, there is almost no buoyancy, because there is almost no air. Terminal velocity, even for a huge balloon, would be very high. This means you begin falling very fast. Then the density of the air begins growing very rapidly. Too rapidly for wind resistance to keep you at the ever-decreasing terminal velocity.

Dr. Stupid

[CurtC]

And another thing...

Let's say you want to slow down from 17000 mph to zero, then drop straight down with your balloon thingy. What would it take to slow you down? Have you seen those huge rockets they have to launch the shuttle from zero to 17000 mph? It would take rockets just as big to slow it down from 17000 to zero. Then you have the problem of how to get those huge rockets up there in the first place - you'd have to have super-mondo incredibly huge rockets to launch them.

It would be even more mind-bendingly expensive than it is now. So instead we coat the thing with heat-resistant materials and let the atmosphere slow it down for free.

[UserGoogol]

I suppose you might be able to set up the rockets on the spaceship to push against gravity, not enough to move away from the earth, but enough to slow down the plummet a lot.

Could be a waste of time and fuel though.

[Skeptical Greg]

Originally posted by UserGoogol
I suppose you might be able to set up the rockets on the spaceship to push against gravity, not enough to move away from the earth, but enough to slow down the plummet a lot.

Could be a waste of time and fuel though.

curtc just explained why this is impractical.. Give it another read.


Important clue: you have to get those rockets and enough fuel, to do what you suggest, into orbit with the ship.
You are talking about doubling the payload, just to get a slow ride home.

[CSSMariner]

Originally posted by UserGoogol
I suppose you might be able to set up the rockets on the spaceship to push against gravity, not enough to move away from the earth, but enough to slow down the plummet a lot.

Could be a waste of time and fuel though.

One is not "pushing against gravity." One is pushing against inertia that was imparted into the vehicle to get it to remain in free fall around the Earth. The vehicle and occupants are not floating freely, they are falling. The reason they keep falling is the Earth falls away from them at the same rate. Also contrary to popular opinion they do not experience zero gravity, they experience micro-gravity.

Very early in rocketry, there was a mass of opinion amongst those who did not understand reaction engines that rocket engines would not or could not operate in space because of the lack of air to "push" against. I tried, unsuccessfully, to show my uncle that the reaction and push was all accomplished inside the engine and nozzle, and that even a jet engine did not "push" on the air. He would not be convinced until Apollo and the moon landings. I wonder if he would be among the "Moon hoax" crowd of today if he was still alive.

All of the capsules from Mercury to Gemini to Apollo that were orbited had the same problem, and the Apollo problem was even greater as they were returning at 25,000 to 30,000 MPH as opposed to the Earth orbital speed of 17,500 MPH or thereabouts. All of them had what were known as "Ablation" shields in that they ablated, or shed material as atmospheric friction heated the shields.

The problem is the necessity of converting the kinetic energy of a capsule or the shuttle’s mass moving at that speed. It must all be converted into heat. In the case of the shuttle that is the job of the ceramic tile shield material. The sonic boom of reentry also takes a little of the shuttle’s energy, but not enough to have an effect on the speed and reentry heating. It is 20th century technology in a 21st century world; unfortunately, it is the best thing for the job.

A solid or sprayed material as had been suggested has problems with uneven heating that is presently compensated by having individual tiles that allow some independent expansion caused by uneven heating. A single or solid heat shield would tend to buckle in areas where the heating was uneven, probably causing separation and failure. Even the SR-71 Blackbird grew about six inches during flight purely from frictional heating, even at 70,000+ feet in altitude. Where the shuttle operates, molecules of air are few and far between so they do not pose a problem with an astronaut doing a space walk.

The shuttle already has "Reaction" engines that peel away a little inertia, speed, and cause the craft to drop lower into the atmosphere where dynamic braking can slow it down. The shuttle is going backwards when they fire the reaction engines, and they must turn it around to be aligned for reentry, a tricky proposition at best. If they are not at the optimum angle for reentry, they can skip on the atmosphere like a stone on water and miss the entry envelope. Too steep, and the heating is too great which will probably cause a reentry burn up.

The problem with a slow reentry is the simple fact that the shuttle is a heavier than air vehicle and cannot possibly float down to the surface. Even a parachute system would not work at the speeds the craft is traveling. A balloon with enough volume to make it possible to float the shuttle would be as big as a city, and where would get the helium or hot air for that matter. Furthermore, it is correct that slowing the shuttle down to a crawl would require about the same amount of fuel as required to get it up there in the first place. Like it or not, we are stuck with the reentry speeds and the absolute necessity of converting all that kinetic energy into heat energy in just the same manner that the brakes on motor vehicles slow them down.

[neutrino_cannon]

Correct me if this is wrong but:

It requires and enourmous amount of energy to get the shuttle into space, and that energy is essentialy conserved while it is in orbit. To get back to earth, you have to take the shuttle out of orbit, which means that all the energy is going to be converted somehow. Furthermore, you have to brake the shuttle, as it is traveling at fantastic speeds, and to get rid of all the speed and energy, some conversion into heat is inevitable.

[CSSMariner]

Originally posted by neutrino_cannon
Correct me if this is wrong but:

some conversion into heat is inevitable.

ALL of the kinetic energy of the space shuttle is converted into heat, be it by the ceramic tiles, aerodynamic drag, sonic boom, drogue parachute, or brakes when it touches the concrete in Florida or desert in the case of a California landing. There is no other energy form to which it may be converted under the circumstances. Energy is never lost, 100% is converted to some other form if 100% is to be lost from anything possessing energy.

The shuttle eventually comes to a full stop, and sits there and sheds heat for a long time, and thereby raises the level of energy input into Florida for a tint, tiny bit. Even the act of braking the shuttle from its landing speed expends some energy into the ground that will in some infinitesimal, but calculable manner perturb the revolutionary motion of the Earth.

The work that was expended to lift the shuttle to whatever average altitude at which it happens to be orbiting is supplied by the conversion of potential chemical energy of the solid rocked boosters and liquid rocket engines combined thrust which is also lost as heat, and the incredibly loud noise of a liftoff, which is in turn lost as heat in the end.

Just as in the case of tire noise from a car, or high lift 4X4 redneck truck, even the energy converted into sound by the tires rolling along the pavement comes from the conversion of chemical energy of the gasoline in the tank, which in turn came from plants and other material that in their turn obtained it from the Sun and stored it to be converted into petroleum.

And that is the reason that creationists who maintain that the second law of thermodynamics, or entropy, makes evolution impossible. The Sun constantly puts energy into our system, so entropy is negated.

[Soapy Sam]

CurtC says it all. The problem is not descending; it's braking. If we could bring the orbiter to a dead stop above a point on the Earth-(geosynchronous orbit) , we could then slide it down a cable. (See Artur C. Clarke's "Fountains of Paradise). Sadly, absent a space elevator, the laws of celestial mechanics are pretty clear about how much fuel it takes to stop in space.

We can descend slowly if we have power to burn. That's how we landed on the moon where the absence of atmosphere made aerobraking impossible. But the fuel needed to make a controlled descent to Earth is immense- and we have to get that fuel (and reactant mass) up there to start with- which takes more fuel and a multi stage launcher.

To get a feel for the problem,I suggest trying one of the many moon-lander programs around.
Watch what happens to your fuel consumption when you make a slow, controlled descent.

[c0rbin]

Thanks for all your replies.

I knew you all could help :)

[Agammamon]

In the case of the elevator you can recover almost all of the energy through regenerative braking on the way down, though there will be inevitable losses, even if you don't use it to lift the orbiter up. All you have to do is get your object into a stationary orbit and you will by definition be at the same orbital speed as the endpoint of the cable.
'course the problem here is that the shuttle can't reach that high.

[Agammamon]

Another way to conserve energy is to send up the same mass as you are sending down. Basically using a counterweight. Then the only energy you have to input is to overcome friction.

[Skeptical Greg]

Originally posted by CSSMariner ...snip

Even the SR-71 Blackbird grew about six inches during flight purely from frictional heating, even at 70,000+ feet in altitude


This reminds me of a story I heard about the SR-71 leaking fuel until expansion caused everthing to seal up tight.. Any truth to that..



Oh, the memories.. Got to see those bad boys flying out of Okinawa in 68.. Awsome..

[Andonyx]

Originally posted by Diogenes


This reminds me of a story I heard about the SR-71 leaking fuel until expansion caused everthing to seal up tight.. Any truth to that..



Oh, the memories.. Got to see those bad boys flying out of Okinawa in 68.. Awsome..

In the popsci year end special, This year it was US military equipment, they mention the blackbird is shaky, and rattly, and completely unstable until a certain speed, then it kind of pulls itself together.

They did not mention fuel leakage, but I have to assume fuel leakage on any jet powered aircraft is extremely dangerous, and would not be tolerated.

[Skeptical Greg]

Originally posted by Andonyx


In the popsci year end special, This year it was US military equipment, they mention the blackbird is shaky, and rattly, and completely unstable until a certain speed, then it kind of pulls itself together.

They did not mention fuel leakage, but I have to assume fuel leakage on any jet powered aircraft is extremely dangerous, and would not be tolerated.


Good points. I sort of thought the leaky fuel thing, might be a bit of embellishment.
However, I thought most jet fuel was a lot like kerosene, and not all that volatile..

Here I go a'Google'ing.....:)

added: Nothing for sure about leaking yet, but I did find..


" The SR-71's engines run on JP7 fuel, which is a special low-volatility fuel. "

[CurtC]

From an interview with an SR-71 pilot (http://www.combatsim.com/archive/htm/htm_arc4/sr71_int2.htm): The SR-71 does leak its JIP-7 fuel quite profusely while on the ground. Its proven over the years to be an impossible task to seal up all six fuel tanks because of the heating and cooling cycles the tanks experience inflight. This leaking has nothing to do with us refueling after takeoff The SR-71 takes off with a reduced fuel load to improve tire wear and reduced tire heating, and provides better takeoff performance in case of an engine failure. We generally used a 45,000, 55,000, or a 65,000 pound fuel load and refueled soon after takeoff to full tanks (80,000 pounds).

[CSSMariner]

I have stood near a SR-71 being prepared for flight, and the floor is always covered with fuel that constantly drips out of its skin almost everywhere. The wing area and almost the entire fuselage is fuel tank, and fuel plays an important role in cooling the aircraft in flight. Due to the extreme heating, there was no sealer material that could be used. The heating from friction eventually seals the tanks, almost. The stream one sees behind the unbelievable bird on climb out from take off is fuel. The engines have more power than the Queen Mary ocean liner sitting in Long Beach, CA.

There was not one instance of one of the birds being shot down although the Russkies and Chinese tried, desperately tried, many times. One is sitting on the deck of USS ENTERPRISE in NY and on the apron at Offit AFB, SAC HQ. in Omaha, and they are made almost entirely of titanium that was purchased from the Russians by a covert CIA-owned coompany.

[CurtC]

The one in New York Harbor is the only SR-71 I've ever been near. It's the USS Intrepid though, not the Enterprise. I was surprised at how small it was. I sure would have liked to see them land that thing on the carrier! ;)

[CSSMariner]

Oops, I forgot that it was the USS INTREPID having never been aboard her.

The SR-71 could never land or take off from the deck of any carrier. There are no provisions for catapult launching, or a tail-hook for the arresting gear on the 71. It is strictly a land-based aircraft, and is on the carrier, having been placed there by a crane from a barge just so the public can see it. In fact the Navy never operated a 71, only the USAF and CIA.

I met one pilot, whose name is long lost to my memory, but they were attired in a full pressure suit very similar to an astronaut's garb because of the extreme altitude at which it flew. Otherwise, had there been a cockpit depressurization, the pilot's blood would have boiled immediately and he would have died an excruciating death long before the bird was down to a safe altitude.

Some interesting notes; it was originally proposed as an assault interceptor named the A-12 and a few of them flew as well. The SR-71 designation was supposed to be RS-71 and LBJ got it backwards when he announced it to the world, and was not actually supposed to do so. He called it a SR-71, the press picked it up and so it remained. It was designed and built long before AutoCAD and computers, and the most advanced aid they had was a slide rule. I still have my Mahogany center, Dietzgen Deci-Trig Log Log "Slip-stick" from 1960 on my desk. Some youngsters ask what it is and I tell them it is an old calculator for which the batteries are no longer available. Some actually look all over it trying to find the battery compartment.

Those were Kelly Johnson's boys, and they worked magic with the U-2 and RS-71/SR-71 Blackbird. One little side note about the U-2, the min/max flight envelope is ten knots, or 11 MPH, between the airspeed at which it will stall and go into a spin, and its maximum airspeed before its wings come apart from dynamic loading. Brave and talented pilots those, and they are still flying while the incredibly expensive to operate 71 sits on the ground except for I think one that NASA is using for research, or at least were for a while if not still.

[arcticpenguin]

I saw a two-seat version of the SR-71 at Hill AFB in Salt Lake City.

If you want to get some nice anecdotes on the SR-71 that are not covered in episodes of Wings, I recommend Skunk Works (http://www.amazon.com/exec/obidos/ASIN/0316743003/qid=1045276604/sr=2-1/ref=sr_2_1/102-1894123-0128915) by Ben Rich. He was Kelly Johnson's successor at the Lockheed Skunk Works, and worked on the U-2 and SR-71. He is also perhaps the man most responsible for turning the concept of stealth into a real aircraft.

[CSSMariner]

GREAT Read! I bought it as soon as it hit the shelf. I spent almost 7 years as an ASA spook in Korea, Europe, the Middle East and N. Africa, and have looked at some of the incredible pictures from Dr. Land's equally incredible cameras that were used on the U-2 and SR-71. I am about half finished with my book "We're Just Spooks," that I hope to finish in Texas when I get away from the noise and mess in S. Florida.

[CSSMariner]

Originally posted by CurtC
The one in New York Harbor is the only SR-71 I've ever been near. It's the USS Intrepid though, not the Enterprise. I was surprised at how small it was. I sure would have liked to see them land that thing on the carrier! ;)

I just realized that I missed the winkin' smiley. I would have liked to see one make a low pass over a carrier and show the Navy pilots some real speed. Have you ever seen the shock wave from a Navy jet passing a carrier faster than Mach 1? Well I haven't either, but I did see a video of it.

Have a look at http://www.dfrc.nasa.gov/gallery/photo/SR-71/ for some really nice flicks of the bird.

[Soapy Sam]

Anyone wanting to see an SR71 in the UK- Take a trip to the Imperial War Museum at Duxford. This was a fighter base in the Battle of Britain, and a B17 US base later in WWII. The new US museum features a B52 as well as the Blackbird and many others.
Costs about 7 quid ($12) to get in but any old aircraft buff will spend the whole day. In Summer there are frequent flying displays.


Edited to add:- An "official "answer to the original q about shuttle landing speed, and many other faqs related to Columbia's loss can be found at:-
http://www.io.com/~o_m/columbia_loss_faq_x.html