Alpo question for the rocket scientists here

[Black Angus McPherson]

Spacecraft or meteors entering the Earths atmosphere always (?) result in a large fireball which is why spacecraft have heat shields to protect them.

 

What would a spacecraft have to do to adjust its speed and/or angle of attack to avoid the need for heat shields?  If the re-entry fireball can't be avoided why not?

 

I'm building a rocket in my back yard and I need to know.  Heat shields are expensive.

 

Please keep the answers simple.  I WILL NOT understand the maths.  

 

Angus

[Doc Shapiro]

It's much more expensive to blast off enough fuel to have a slow re-entry than it is to have the heat shield, free fall, and use a chute.

[Big Sage, SASS #49891 Life]

You couldn't carry enough fuel to do it.

[Texas Joker]

If you are using conventional fuel, some form of advanced propulsion may allow it.  The sr71 created enough friction once up to speed to glow.  

 

Reverse thrust and big s turns might slow you down enough to avoid a big poof

[Chantry]

If I understand things correctly, with the current technology, any attempt to re-enter the atmosphere at a shallow enough angle to avoid needing a heat shield would cause the object to "bounce" off the atmosphere.

[Charlie T Waite]

If you keep velocity below mach 5 during a short re-entry, certainly below mach 3 if speeds are sustained, then atmospheric heating will be modest.

The shuttle got away with (basically) a Styrofoam heat shield on its external tank that was only modest scorched during its ascent through the atmosphere. For the brief heat pulse of slowing from about mach 5 to subsonic speeds then you could get away with some titanium and insulation in the hot areas and aluminum elsewhere. If the entry speed is below mach 3 and you slow quickly to subsonic speeds, then an aluminum structure would be fine.  The drawback to slow entry is that you’d need fuel quantities similar to launching to slow down below “hot” entry velocities.

Neither the returning Space-X Falcon-9 boosters, nor the Applied Composites “SpaceShip One” needed heat shields.

In the case of SpaceShip One, it only just brushed the edge of space - and didn’t have to slow down from orbital speeds. It simply glided back to Earth without a heat shield or retro-rockets.

In the case of Falcon-9, it uses active thrust from it’s main engine to keep the re-entry speed low enough.

So, yeah - it’s certainly possible under the right circumstances.

But consider the Space Shuttle. It was re-entering from low Earth orbit - higher than SpaceShip One - but not that high. But it was re-entering at orbital speeds - and it needed those heat tiles to avoid burning up because of that.

Coming in slowly sounds like a great idea - but it requires the space craft to carry a LOT of spare fuel - and that’s a costly thing because the spare fuel ends up being (in effect) part of the payload that has to be lifted up into space in the first place.

This is why SpaceX sometimes cannot re-use a booster when the load it’s putting into orbit is too heavy. In that case, it has no spare capacity for fuel, so it cannot re-enter with thrust - and it simply burns up on the way down.

[Forty Rod SASS 3935]

19 hours ago, Texas Joker said:

If you are using conventional fuel, some form of advanced propulsion may allow it.  The sr71 created enough friction once up to speed to glow.  

 

Reverse thrust and big s turns might slow you down enough to avoid a big poof

Uh, I got it from a SR-71 pilot that that wasn't from friction but massive compression of the air that couldn't get out of the way quickly enough.

 

The space vehicles are coming in with practically no fuel left beyond what is necessary to maneuver.  They aren't being driven, they are just falling really, really  fast.

[Texas Joker]

Plasma superheated by friction upon re entry due to speed IS air that can't get out of the way fast enough. Or so I thought.

 

[Black Angus McPherson]

19 hours ago, Chantry said:

If I understand things correctly, with the current technology, any attempt to re-enter the atmosphere at a shallow enough angle to avoid needing a heat shield would cause the object to "bounce" off the atmosphere.

 

I thought about that.  But, why couldn't you just keep bouncing off the atmosphere until you slowed down enough to, kind of, fall thru the atmosphere and fly your way down to the surface?  Assuming the bounces are not so severe as to destroy the vehicle.

 

As for the SR-71, what if, instead of "getting up to speed", it only accelerated to ~200-300mph and climbed up out of the atmosphere?  Why couldn't it just maintain speed and slowly lose altitude and land?  (OK, the SR-71 can't do that, but you know what I mean.)

 

I'm not sure I understand why you need so much fuel to slow down enough to re=enter the atmosphere safely.  What happens if you fly up to the edge of the atmosphere and drop a rock?  Are you saying that the velocity of the falling rock will increase until it begins to burn?  If so, what about a plane that is flown up to the edge of the atmosphere?  Since it doesn't fall back to Earth at X times Xsquared (Whatever the formula is) shouldn't it be OK?  How does that change if you fly up just past the edge of the atmosphere and drop the same rock?  Assuming the direction of the rock starts towards the Earth and it is not outside the Earths gravitational pull.

 

To me, a non-physicist, it seems simple.  You didn't burn up on the way out, why would you burn up following the same trajectory back down?

 

 

[Trailrider #896]

First of all, you say you are building a rocket in your backyard.  What kind of rocket motor/engine are you using? Liquid propellant(s) or solids? How high do you expect it to fly? Are you planning to have it just go straight up and fall back down, or tilt over for some range?  Have you checked with the FAA, because if you are flying high enough you may need a license?

 

As to your question about the rock being dropped from just at the "edge of the atmosphere", assuming you just dropped it from zero upward velocity, it might heat up some, but probably not enough to burn up. until it hit the ground.  But if you are moving horizontally with sufficient speed,, say 2,000 mph at, say 80,000 - 100,000 feet, you are going to need some type of shielding, titanium, for example, and possibly using the fuel in your fuel tanks to absorb some of the heat.  I am talking about the SR-71 family, of course. 

 

As far as bouncing off the atmosphere is concerned, that was recently done by the Artimus I  Orion capsule coming back from the Moon at 25,000 mph, to slow it some to reduce the heat shield requirement. It only did it enough to slow it down, not enough to send it back into space. Neil Armstrong did it in the X-15, accidentally, coming back from an altitude of 50 miles, at about Mach 5.  He slowed down enough to resume his descent. 

 

The Space Shuttle orbiter was coming back from orbital velocity (17,500 mph or Mach 25), and to save weight, the main structure was made of aluminum, which would melt without the silica head shielding tiles. When the tiles on the leading edge of Columbia's wing were knocked off by ice falling from the External Tank, friction destroyed the wing, resulting in the fatal crash that followed. 

 

We used to recover the Shuttle's Solid Rocket Boosters (SRB's) using parachutes, without much thermal protection, because they didn't fly that high, and the parachutes slowed them enough not to need extensive insulation.  But using parachutes doesn't permit pinpoint accuracy in landing.  Falcon 9 OTOH, needs to land the 1st stage on either the barges at sea, or at the landing sites at the Cape. So they use a series of engine firings and combination of a certain number of engines to land on the X.  The penalty there is they have to keep enough propellants onboard to do the job, which reduces the amount of payload they can lift. The exception is when they need to use enough propellant for a heavy payload, in which case, like on the last Falcon 9 Heavy launches, they recovered the side boosters, but had to use all the propellants in the core stage, so the core isn't recovered.

 

Does this help? BTW, I was never a rocket scientist! No, I was a rocket engineer!  

[Sedalia Dave]

The biggest difference between reusable rocket boosters like Space-X uses and the Dragon Capsule or the shuttle is that they are only going up. They are not attempting to orbit the earth. To maintain low earth orbit requires a velocity of about 17,500 miles per hour. The 2nd stage is what Space-X uses to obtain orbital velocity.

Once in orbit you have to find a way to slow down. Atmospheric drag is the cheapest way to do this.

 

Gliding in slowly doesn't work as you lose the ability to maintain altitude long before the atmosphere is dense enough to generate the lift required to support flight. By the time the atmosphere is dense enough to support forward flight your forward speed is still so great that you are going to generate more heat than a craft without a heat shield can withstand.

[Black Angus McPherson]

23 hours ago, Trailrider #896 said:

First of all, you say you are building a rocket in your backyard.  What kind of rocket motor/engine are you using? Liquid propellant(s) or solids? How high do you expect it to fly? Are you planning to have it just go straight up and fall back down, or tilt over for some range?  Have you checked with the FAA, because if you are flying high enough you may need a license?

 

I've got a couple V-12s out of old Jaguars I'm going to link together.  I'm thinking 90 Octane with a NOS(?) booster.  I understand that NOS stuff can really up the horsepower.  I'm planning on going really, really high.  Probably pretty much straight up and straight down.  All that tilting might give me motion sickness.   FAA?  Pshaw!  I don't need no stinkin' license.

 

23 hours ago, Trailrider #896 said:

 

As to your question about the rock being dropped from just at the "edge of the atmosphere", assuming you just dropped it from zero upward velocity, it might heat up some, but probably not enough to burn up. until it hit the ground.  But if you are moving horizontally with sufficient speed,, say 2,000 mph at, say 80,000 - 100,000 feet, you are going to need some type of shielding, titanium, for example, and possibly using the fuel in your fuel tanks to absorb some of the heat.  I am talking about the SR-71 family, of course. 

 

As far as bouncing off the atmosphere is concerned, that was recently done by the Artimus I  Orion capsule coming back from the Moon at 25,000 mph, to slow it some to reduce the heat shield requirement. It only did it enough to slow it down, not enough to send it back into space. Neil Armstrong did it in the X-15, accidentally, coming back from an altitude of 50 miles, at about Mach 5.  He slowed down enough to resume his descent. 

 

So, maybe it is theoretically possible.  Interesting.   2000mph?  Dang that's fast.

 

23 hours ago, Trailrider #896 said:

The Space Shuttle orbiter was coming back from orbital velocity (17,500 mph or Mach 25), and to save weight, the main structure was made of aluminum, which would melt without the silica head shielding tiles. When the tiles on the leading edge of Columbia's wing were knocked off by ice falling from the External Tank, friction destroyed the wing, resulting in the fatal crash that followed. 

 

We used to recover the Shuttle's Solid Rocket Boosters (SRB's) using parachutes, without much thermal protection, because they didn't fly that high, and the parachutes slowed them enough not to need extensive insulation.  But using parachutes doesn't permit pinpoint accuracy in landing.  Falcon 9 OTOH, needs to land the 1st stage on either the barges at sea, or at the landing sites at the Cape. So they use a series of engine firings and combination of a certain number of engines to land on the X.  The penalty there is they have to keep enough propellants onboard to do the job, which reduces the amount of payload they can lift. The exception is when they need to use enough propellant for a heavy payload, in which case, like on the last Falcon 9 Heavy launches, they recovered the side boosters, but had to use all the propellants in the core stage, so the core isn't recovered.

 

Does this help? BTW, I was never a rocket scientist! No, I was a rocket engineer!  

 

Yes, thanks.

 

Angus

[Black Angus McPherson]

16 hours ago, Sedalia Dave said:

The biggest difference between reusable rocket boosters like Space-X uses and the Dragon Capsule or the shuttle is that they are only going up. They are not attempting to orbit the earth. To maintain low earth orbit requires a velocity of about 17,500 miles per hour. The 2nd stage is what Space-X uses to obtain orbital velocity.

Once in orbit you have to find a way to slow down. Atmospheric drag is the cheapest way to do this.

 

Gliding in slowly doesn't work as you lose the ability to maintain altitude long before the atmosphere is dense enough to generate the lift required to support flight. By the time the atmosphere is dense enough to support forward flight your forward speed is still so great that you are going to generate more heat than a craft without a heat shield can withstand.

 

I think that's the best answer, for me, so far.  I think I can actually understand that.  17,500 mph!?  Dang!  I thought 2000 mph was fast.

Maybe I'll have to invest in heat shields after all.  Or, maybe a parachute.  I could jump out before I go too fast and too hot and just float down.   I wonder if I could sell that idea to NASA?

 

Angus

[Sedalia Dave]

1 hour ago, Black Angus McPherson said:

 

I think that's the best answer, for me, so far.  I think I can actually understand that.  17,500 mph!?  Dang!  I thought 2000 mph was fast.

Maybe I'll have to invest in heat shields after all.  Or, maybe a parachute.  I could jump out before I go too fast and too hot and just float down.   I wonder if I could sell that idea to NASA?

 

Angus

 

I recommend you buy Joe Kittinger, Felix Bumgardner and Alan Eustace a few beers and get their take on high altitude parachute jumps.

 

 

[Trailrider #896]

4 hours ago, Black Angus McPherson said:

I've got a couple V-12s out of old Jaguars I'm going to link together.  I'm thinking 90 Octane with a NOS(?) booster.  I understand that NOS stuff can really up the horsepower. 

Wait a minute! Are you talking about a pair of automobile engines? Just exactly how are they going to lift off?  That doesn't sound like rocket engines at all. Do I feel one of my legs being tugged at, just a skosh? 

[Sedalia Dave]

1 hour ago, Trailrider #896 said:

Wait a minute! Are you talking about a pair of automobile engines? Just exactly how are they going to lift off?  That doesn't sound like rocket engines at all. Do I feel one of my legs being tugged at, just a skosh? 

 

I'm telling you this because you appear to be capable of keeping a secret. In actuality the V-12 is not an automobile engine. It's a red herring used to prevent snow flakes from discovering that the V-12 t is actually a 10th generation rocket based on Von Braun' original V-2 design.  It was designed by Von Braun's illegitmate great grand daughter Hilda.  Remember that this is a secret so after you read this, destroy your monitor so it doesn't fall into the wrong hands. 

 

[Trailrider #896]

If that's the case, then I would recommend using liquid oxygen and liquid methane. While you have to be careful handling the LO2 (no hydrocarbons in close proximity). But you would certainly be helping the environment if you collected the methane from cows and sewage treatment plants. Sewage plants aren't a real problem. I used the methane off one for a college project. Collecting methane from cows farts might be more problematical. 

[Black Angus McPherson]

19 hours ago, Trailrider #896 said:

Wait a minute! Are you talking about a pair of automobile engines? Just exactly how are they going to lift off?  That doesn't sound like rocket engines at all. Do I feel one of my legs being tugged at, just a skosh? 

 

OK, I'll admit I've still got a couple bugs to work out.  ( just a skosh?  HA!  I'm about to rip one out of the socket. )

 

OR

 

What SD said.

 

17 hours ago, Sedalia Dave said:

 

I'm telling you this because you appear to be capable of keeping a secret. In actuality the V-12 is not an automobile engine. It's a red herring used to prevent snow flakes from discovering that the V-12 t is actually a 10th generation rocket based on Von Braun' original V-2 design.  It was designed by Von Braun's illegitmate great grand daughter Hilda.  Remember that this is a secret so after you read this, destroy your monitor so it doesn't fall into the wrong hands.