Thread: Why spacecraft are blunt :?:

Spaceships that travel at hypersonic speeds are designed to have more drag while conventional supersonic and sub-sonic aircrafts are designed to minimise it. Why the difference

I am surprised a 'Senior reesearch fellow' would ask such a question, so in the same frivolous vein in which you have asked it I will attempt to answer.

1) It's cheaper,
2) Vacuum wind tunnels are few and far between.
3) They need to slow down when they re-enter the Earth's atmosphere.

:wink:

i reckon its a lot to do with the re-entry how the heat hits a curved bottom as opposed to having sharp edges. look at the shuttle or early pods when you see how the heat "laps" at the underside

a blunt shape (high drag) makes the most effective heat shield compared to a tiped one. From simple engineering principles, the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater the drag, the less the heat load. Through making the reentry vehicle blunt, the shock wave and heated shock layer were pushed forward, away from the vehicle's outer wall. Since most of the hot gases were not in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere. Don't mind Billco, it was a good question and his answers are basically wrong.

Originally Posted by nanorman

a blunt shape (high drag) makes the most effective heat shield compared to a tiped one. From simple engineering principles, the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater the drag, the less the heat load. Through making the reentry vehicle blunt, the shock wave and heated shock layer were pushed forward, away from the vehicle's outer wall. Since most of the hot gases were not in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere. Don't mind Billco, it was a good question and his answers are basically wrong.

You obviously didn't get the joke.

Yes I did, your answers were hilarious, thanks for the laugh

Slightly off topic.

When you look at photo's of the old Gemini re-entry pod, it has to be the most un-aerodynamic shape i.e a shape designed to create maximum drag, with the amount of drag being created what is preventing the craft from flipping upside down which would be the position of least resistance. i dont think small stabilizing thrusters would have much impact at the velocity the craft is travelling when it re-enters the earths atmosphere. The only explanation that I can think of is that the craft's centre of gravity is very low with most of the weight being at the bottom close to the heatsheild.

The heat shield was a huge thick lump of iron I think, so it will always face forward, rather like strapping polystyrene onto a metal plate and dropping it into water it will sink with the polystyrene upper. That's what I think, it could be all BS though.

a blunt shape (high drag) makes the most effective heat shield compared to a tiped one. From simple engineering principles, the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater the drag, the less the heat load. Through making the reentry vehicle blunt, the shock wave and heated shock layer were pushed forward, away from the vehicle's outer wall. Since most of the hot gases were not in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere. Don't mind Billco, it was a good question and his answers are basically wrong.

That was an excellent reply. I only have a couple of minor corrections to bring it closer to perfection.

The heat load is not really "inversely proportional" to the drag coefficient. What you are trying to say is: The heat load is reduced as the drag is increased. Saying "inversely proportional" implies a specific mathematical relationship, i.e. heat load = C/(drag coefficient). The phenomenon is too complex to be described by such a simple "inverse proportionality", but your point was taken, anyway.

The heat energy does not "dissipate into the atmosphere", it is diffused. People like to confuse dissipation with diffusion, maybe because of the linguistic similarity, but they are entirely different physical processes.

Originally Posted by billco

The heat shield was a huge thick lump of iron I think, so it will always face forward, rather like strapping polystyrene onto a metal plate and dropping it into water it will sink with the polystyrene upper. That's what I think, it could be all BS though.

The ablative heat shields on the early spacecraft were ceramic and I think the rest of the craft was aluminum. Once the craft entered the atmosphere aerodynamic forces would act to hold it in it's bottom down orientation.

for one, during re-entry if the tip was pointed, it would be more likely to come off. and also, during re-entry, the space shuttle doesnt go front-first, and the tip would have less drag curved. and, even if you argued it would save fuel on the launch into space, the aerodynamics of the shuttle itself are somewhat undermined by the huge external tanks. once they come off higher in the atmosphere, the air is thinner anyway, and, of course, space is a vaccum, and thus no drag.

Originally Posted by chamilton333

for one, during re-entry if the tip was pointed, it would be more likely to come off. and also, during re-entry, the space shuttle doesnt go front-first, and the tip would have less drag curved. and, even if you argued it would save fuel on the launch into space, the aerodynamics of the shuttle itself are somewhat undermined by the huge external tanks. once they come off higher in the atmosphere, the air is thinner anyway, and, of course, space is a vaccum, and thus no drag.

The space shuttle re-enters the earth's atmosphere front first. otherwise it would break up.
The SRB's detach at appoximately 2min 2 Seconds, some 40 miles above the surface. The External tank detaches appox 8 mins after launch.

Originally Posted by billco

Originally Posted by chamilton333

for one, during re-entry if the tip was pointed, it would be more likely to come off. and also, during re-entry, the space shuttle doesnt go front-first, and the tip would have less drag curved. and, even if you argued it would save fuel on the launch into space, the aerodynamics of the shuttle itself are somewhat undermined by the huge external tanks. once they come off higher in the atmosphere, the air is thinner anyway, and, of course, space is a vaccum, and thus no drag.

The space shuttle re-enters the earth's atmosphere front first. otherwise it would break up.

it goes bottom first
http://newsimg.bbc.co.uk/media/image...e_land_203.jpg

When you said it does not go 'nose first' I though you were implying it went 'ass first' especially since that is it's orientation when it does the de-orbit burn. Also since you said tanks, as opposed to tank [there is only one external tank] I thought you might not have fully understood the subject. A slight mis-understanging me-thinks.

Originally Posted by billco

Also since you said tanks, as opposed to tank [there is only one external tank] I thought you might not have fully understood the subject. A slight mis-understanging me-thinks.

i was including the tank and the boosters in the same word.