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Old 04-15-2008, 01:36 PM   #1 (permalink)
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Question Aerodynamics of rockets vs. cars

Why do modern rockets have a blunt pencil shaped nose cone versus the streamlined 'boattailed' V2 designs of WW2 ?

I know that the boattailed approach is pointless because the smooth airflow passing over the rockets body meets up with the massive rocket behind it ... but I see a lot of pipes and other junk stuck to the sides of some rocket designs and have to wonder if it hurts the efficiency any.

I would think that up to a certain point, the same aerodynamic principles that apply to automobiles would also apply to rockets.
Once you go supersonic and then further, how do aerodynamics become affected by sleek shapes ?
If you recall, the tail on an X-15 is a wedge shape and does not taper back to a smooth teardrop profile.
Hypersonic designs and lifting body designs tend to look like a wedge as well, with what appears like no wings at all.

Further off topic, why do rockets shoot straight upwards versus flying to the edge of space and then turning on the rockets ?


The X-15 flew into very low earth orbit. What would keep the X-15 from continuing further into space ?
I understand that a certain speed must be achieved before you can break free of Earths' gravity, but why do we really need to start from ground level and fight our way upwards ?

It seems like an awfull lot of wasted fuel to me .

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Old 04-15-2008, 03:52 PM   #2 (permalink)
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The shape of the V2 was modelled after the shape of a bullet or artillary shell, which were the only supersonic vessels that the people in the 1930's had any experience with. Rockets don't need boattails because their exhaust fills in their wake at subsonic speeds and concepts like wake and rounded shapes don't apply at supersonic and hypersonic speeds. At supersonic speeds a sonic shock wave is formed at the tip of the vehicle and at each point where there is a change in the shape of the vehicle. For this reason the most efficient shape for a supersonic vehicle is a sharp pointed wedge, which will produce one shock wave at the tip and second shock wave at the back of the vehicle.

Rockets spend the bulk of their time in the supersonic and hypersonic realms so subsonic aerodynamics plays little part in their design.

Rockets initially fly straight upwards to get above the bulk of the atmosphere and its drag as quickly as possible.

The X-15 was suborbital, reaching a maximum altitude of 600,000 feet in its test flights. It didn't carry enough fuel to enter earth orbit and wasn't designed to do so. It was designed to test vehicle aerodynamics and flight controls in the supersonic and hypersonic realms and to test flight controls transitioning from atmosphere to vacuum and back.

Where else are you going to start but at ground level? You can either go all of the way to orbit via rocket power or use a carrier vehicle (air-breathing aircraft or helium balloon) to take you above part of the atmosphere before continuing the rest of the way on rocket power. But this latter approach adds additional complexity in the form of a second vehicle and the limited weight that can be carried aloft by this vehicle. There's no way that a 747 could carry the Space Shuttle and its load of fuel to 30,000 feet, but it can carry and release the much smaller Pegasus rocket that can carry small payloads into low earth orbit.
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Old 04-15-2008, 04:11 PM   #3 (permalink)
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Thanks basjoos ! That was really informative.


Frank : It's been a long time since I'd seen that one.

Waaaay off topic ( again ) : Basjoos : what is a basjoos ?
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Old 04-15-2008, 09:03 PM   #4 (permalink)
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OK, the Space shuttle tank has a big LO2 feedline running down the length of it to carry the oxidizer to the engines. It is much more complex to put this through the interior of the tank, and a huge safety risk. If either leaks, LH2 and LO2 will react to form water, but extremely vigorously So in this case, the inefficiencies are made up for in ease of build and safety.

Also, it only takes a couple/few minutes for a rocket to get high enough that aero loading isn't an issue.

As far as the liftoff-from-altitude idea, basjoos is correct about the fuel weight issue. Plus, there's there's nothing to push off from, so it's harder to get started, there's no vertical momentum. You could do it but you'd probably have to burn your engines for just as long if not longer just to get started, so there's no benefit.
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Old 04-15-2008, 09:39 PM   #5 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
I wanna know how that dude controlled the space Corvette in Heavy Metal. j/k!
What synchronism... I literally just saw that movie for the first time last night

Quote:
Once you go supersonic and then further, how do aerodynamics become affected by sleek shapes ?
Just making sure it's clear - transonic and supersonic flow does not apply to cars...




So a huge amount of compression creates bow shock and eventually a shock wave (sonic boom). But, if the geometry cross section becomes smaller, there's a fan area (it's called fan something - I'm having trouble remembering the name). As the flow tries to expand into the smaller cross sectional areas, these little fan out ripples show up. Rockets have to deal with that AND turbulent flow separation (see the boat tailed bullet versus the non tailed version). IIRC, the expansion fanning is a source of aero losses...



I don't have too much info on superflow. But one of my professors gave an awesome analogy.... Imagine a crowded lecture hall - and class is done. 200 students are trying to get out of the two exit doors. The professor yells "coming through" and the students get out of the way as he passes. That's subsonic flow.

Now, imagine the same situation - but the professor doesn't say anything, and plows through everyone. How do you get more things out of the way? Make the door opening wider so things can compress out of the way.

This is why fighter jets have nozzles that get smaller (subsonic flow) and the space shuttle has nozzles that get bigger (supersonic flow). At least, this is how it was explained to me
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Old 04-16-2008, 02:03 AM   #6 (permalink)
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There are orbital air-launched rockets, but the amount of energy needed to get into orbit is still vast. Burt's SS1, which uses the concept for suborbital flight, can't be scaled up for orbital flight due to this issue. I used to know exactly why it was done...but don't anymore. Damn memory...

Pegasus


To cut on fuel costs, rockets are launched near the equator: Cape Canaveral, Kwajalein Atoll, French Guyana (sorry for any spelling errors). By using the rotation of the earth, rockets get a velocity boost. Supposedly Israel gets royally screwed, having to fly west against the rotation of the earth and then make a coarse correction for orbital flight due to airspace restrictions.

Ramjets have been theorized, but not capitalized on. Hypersonic aircraft (NASA's X-43 Scramjet) will probably be the future. Essentially, engineers are trying to reduce weight by substituting some stored oxidizer with atmospheric air.

Aerodynamics are of concern especially in regard to engine efficiency. Rocket nozzles are generally fixed in size and based on an average atmospheric pressure experienced during the flight. This means that on launch, where pressure is higher, the rocket plume detaches from the nozzle walls and has reduced efficiency (less pressure recovery?). At high altitude, where pressure is extremely low, the rocket plume exceeds the width of the nozzle and causes air drag.

To combat this issue, novel nozzle designs have been developed. The coolest is the Aerospike engine, where atmospheric pressure solely regulates plume size. Trident SLBM's use aerospike nozzles to increase range.

Aerospike


As far as aerodynamic loads, as others have said, it mainly lies in supersonic flow. Rockets only stay in the atmosphere for a few minutes at most, and max-q (max dynamic pressure, aka max aerodynamic load) usually happens seconds after launch (~30 I believe). Fairings are usually jettisoned well before the rockets quit, just to give an idea of how much tangible atmosphere the rocket experiences during its flight.

I used to know more about escape velocities and elementary orbital mechanics, but all that has gone the way of the buffalo.... If I remember right, LEO velocity is 7km/s and escape velocity is 11km/s...just to get an idea of the amount of energy needed. Unfortunately, rocket scientists have a reputation for a reason. Trying to learn and master that stuff is about as much fun as putting you nuts in a vice.

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Old 04-17-2008, 01:04 PM   #7 (permalink)
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V-2

The other members have laid out great explanations for the rocket vs car aero question.My only thought with respect to the V-2,was that its re-entry was of paramount concern to project designers.As a vengeance weapon,and as something to strike terror in the hearts of the British,designers were interested in the device arriving ahead of it's accoustic bow shockwave.Those on the ground would never hear it coming,terrifying indeed! The special attention to the ogival nose may have been added insurance for stealth,and may also have allowed thinner and lighter structure to the rocket,as the magnitude of the leading shockwave might allow lighter loading,something of concern for a country strapped with ever-diminishing resources.

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