You always hear about how space shuttles and whatnot needs to shield themselves during re-entry into the earth’s atmosphere due to friction, but why don’t rockets experience the same thing when they go into space? We never see the same kind of heat and flames going out as we do when they’re coming back. They look like a fireball coming back down. But the only fire seems to be coming from the engines when they are exiting
Bonus question (possibly related, I dunno): Generally, are rockets going faster leaving the earth or are they faster coming back down?
Off the top of my head, there’s a couple of factors here - it’s not really friction that heats up the re-entering body; it’s compression of the air in front of the vehicle. The rocket going up a) has a point, not a blunt shape, so it compresses air less and b) it’s going in the direction of less dense air anyway. The capsule coming down is blunt (it’s trying to slow down after all) so it’s really good at compressing the air in front of it (thus heating the air, and resulting in the capsule getting hot), and b) it’s moving towards ever denser atmosphere.
Rockets do not take off at the speed they are going to be going in orbit since they are accelerating all the way up. Through the lowest part of the atmosphere, they are moving relatively slowly. Rocket trajectories are mostly vertical for the first part of the flight to get through the densest part of the atmosphere while it is still going the slowest.
When re-entering, they are starting at their orbital velocity and counting on air friction to do most of the slowing down work. It is therefore necessary to be going really fast through the atmosphere.
I’m pretty sure air friction is the why. At lower altitudes there is more atmosphere causing drag and therefore friction, at higher altitudes there is less atmosphere, and therefore less drag/friction.
It takes a while for something like the shuttle to achieve orbit velocities, and most of that velocity is achieved once the thing is several kilometers high. Once in orbit the damn thing is going at 18,000+ MPH. Without any significant fuel available to slow it down (since it used it all up getting up there in the first place), that’s the velocity it’s going to have when re-entering the atmosphere.
So when leaving earth, the shuttle travels at a few hundred miles per hour for several miles up, speeding up slowly at first, then faster (as the atmosphere thins), until it reaches it’s orbital velocity.
When coming down, it’s traveling at 18,000 MPH+ and the friction at those speeds, is pretty damn huge.
One reason rockets start with a near vertical trajectory is to get through the atmosphere as quickly as possible to reduce the results of drag and friction. If they were to reach orbital speeds within the lower atmosphere they’d be in danger of burning up. This is a major problem faced by SCRAM jets and similar rockets that will try to travel through the atmosphere in order to use available oxygen instead of carrying it on board as fuel.
And in sharp contrast, when they return, they have a near horizontal trajectory to get through the atmosphere as slowly as possible to maximize the results of drag and friction, because that drag and friction is the only way to slow the thing down to avoid a crash at the bottom.
“The high heating experienced by spacecraft when entering the atmosphere is caused by a high-pressure bow wave in front of the ship. This strong shock wave is caused by the craft flying at supersonic speeds, even hypersonic speeds. Hypersonic is roughly greater than Mach 5. The shock wave is where the atmosphere is rapidly compressed by a factor of 50 to 100, depending on the speed of the vehicle. Because of this rapid compression the gas is heated to high temperatures, as high as 6000 K or more. This hot gas then impinges on the front of the spaceship, and transferring heat to the surface. That is why it has to have a heat shield.”
Right. If the space shuttle was in orbit with a full belly tank and solid rocket boosters, it could use all that fuel to slow down before getting into the atmosphere, and to then come more or less straight down gently and slowly, never going fast enough for atmospheric heating.
Andy’s point about the shock wave is correct, but it’s correct simply because the speed when ascending is relatively slow but very high when descending.
Also, as alluded to above, the energy equation. The energy put in during ascent has to come out during descent, for a net of nearly zero. The energy going in during ascent is burnt in the engines. The energy coming out during descent is mostly thermal.
Assuming more or less 45 degrees upward pitch at the time of separation, wouldn’t this mean that the booster might well have reached over 100 miles altitude at the top of its trajectory, and almost certainly burned spectacularly when it re-entered?
For comparison, the two suborbital Mercury flights likewise separated from their boosters at about 40 miles and 5000 mph. And similarly, these spacecraft didn’t heat up all that much on the way up, but they certainly needed their heat shields on the way down.
Exactly. And the space shuttle limits its speed during takeoff specifically so that it they don’t have atmospheric problems (mostly so that the pressure forces don’t rip the fuel tank and boosters off of the shuttle). They keep the engines throttled back to a little over half power. Once they get through a certain amount of atmosphere and it starts thinning out a bit, then they crank up the engines. If you listen to old shuttle launches you’ll hear them say “go at throttle up” which is basically the point at which they crank the engines all the way up.
Yep,
I heard that quote one cold January morning just as I was walking into biology class. It was 1986 and I will never forget it. We watched all the shuttle launches in HS, didn’t do much in class that day.
Famously the last words before the Challenger disaster.
I thought this must be the case, even though when playing with the Shuttle scenarios in the Orbiter space flight simulator, I can’t get it off the ground at anything less than full throttle. Still, Orbiter is highly entertaining and edifying, and well worth a look.
the rockets are suppose to separate from the shuttle BEFORE entering space? i think.
otherwise it probably would burn up. just another mystery. interresting question though.
The boosters separate before entering “space” and don’t burn up because they’re not going very fast. They’re reusable. The main fuel tank separates several minutes later, just before achieving orbit, and disintegrates during reentry because it’s traveling a lot faster.
On the blipping the throttle thing I often have to do that on my bike until it’s warmed up. It will often stall out when cold if I don’t do this. It’s an old bike though with no choke so setting the idle right so that it won’t stall when cold but will tick over when hot is fairly tricky. Hence I tend to have it set so I do need to blip the throttle when cold in order to have a reasonable low idle when hot which is how the bike will be for the majority of my riding.
On the whole loud pipes thing I like a reasonably noisy pipe but the open pipes you see on a lot of Harley’s these days seem to be taking it way too far. What I don’t understand is from a rider comfort perspective I would have thought having such loud pipes must become really tiring on a long ride. It is odd that you see so many people on Harley’s with completely open pipes as I have never seen that on a sports bike and I cannot imagine anyone getting away with it on a sports bike. I am from the UK though so might have a slightly different perspective on this. In the UK Harley riders tend to be older born again type bikers whereas sports bike riders tend to be adrenaline junkies which is probably why the law seems to crack down on them more than Harley riders.