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jflis

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I started this same thread in ROL as a result of some discussion about the X-prize and such, and thought it would be a good discussion here.

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ok, I'm going to go ahead and ask this question even though I have heard all the arguments and understand the physics, but I don't beleive that this has been discussed in a long, *long* time and maybe it is worth tabling again...

One of the biggest problems with going into orbit is getting *out* of orbit and back to earth. Just do a google on "heat shield tiles" to get a jist of the complexity of it.

**but**, for probably the 10 millionth time, why not slow down enough while in space so that the heating effect is reduced to more managable levels.

Now, before I get flamed :) I realize that it would take a LOT of fuel. Part of what I am asking is this: Would it take less fule (in dollars) than heat sheild tiles (in dollars and labor)?

Also, why not some sort of balance? Don't try to *stop* the ship in space and drop back to earth, just take some of the edge off so that the thermal impact on your heat shield is reduced. Even a 10% savings may (i say MAY) be worth the sacrifice in payload or cost in fuel.

I don't really know today. As I understand it, we have better motors, hotter fuels. Is this a question that should be revisited??

Flame away :)
 

rstaff3

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I don't know enough about orbital mechanics. I used to work with a bunch of ppl who did this stuff for a living, but have lost track of them.

Of course the problem with slowing an orbit (or speeding it up) is that the delta-v changes other elements of the orbit (most notably altitude), which would have to be compensated for. By cost of fuel I assume you also consider the cost of the fuel that it took to lift that fuel to orbit ;)

I suspect that this is not a hard question if you are in the biz, but it would be hard for most of us to start to quantify.
 

vjp

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I think lighter vehicles with larger cross-sections are the way to go. Reduced loading, leading to reduced heat generation in the first place, so a simple reusable metallic TPS can be used, or maybe a spray-on ablative coating.

There's nothing wrong with your idea in theory, but every pound of fuel you put into orbit requires many more pounds of fuel to put it up in the first place. You couldn't tank that much fuel inside the shuttle, so you'd need to put the ET into orbit with it. More fuel there! Then, there's boil off - how do you prevent this over 2 weeks? etc., etc., etc. Overall a much bigger engineering problem, with many more failure modes than a simple TPS.
 

wwattles

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I think something that is being overlooked here is that no matter what the descent trajectory is, there is going to be significant heating as a body enters the atmosphere. The shuttle orbiters do a "reverse burn" maneuver in which they turn the shuttle around and fire the engines to slow down the overall speed prior to re-entry. Even the suborbital flight of SpaceShipOne had some minor heating to deal with. Imagine that on a flight coming down from a much higher altitude, and with a lot more angular momentum through the atmosphere.

Unless someone's got a better idea, we are still left with 2 options: a sacrificial or ablative surface like the gemini/mercury heat shields, or extremely heat-absorbent tiling that is reusable like the shuttle. If memory serves, the Soviets had a highly experimental hypersonic test program called "Ajax" that involved an accelerated particle stream in front of the aircraft that would create a plasma which would in turn develop a slipstream of sorts. The test program was intended to be used to create a hypersonic aircraft, but the same technology could, I suppose, be used to create a bubble of air around a deorbiting spacecraft. The bubble would then act as the "heat absorber" for reentry. But of course, the problem again is the weight of the equipment to create the bubble.

WW
 

rstaff3

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In reality you are right, but it is theoretically possible to re-enter at a very low speed. All you need is enough force in the engines. Maybe with wap cores :)

I guess Jim just wanted some quantification of how close you can come today. I think the answer is 'not very'.
 

illini

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I actually had this discussion offline with another forum member a few months ago.

First, some basic physics. A spacecraft in orbit has a total "kinematic" energy of kinetic energy + potential energy. A "re-entered" spacecraft also has kinetic and potential energies, but much less of both. Therefore, the energy that must be dissipated is the difference between the total energy in orbit and the total energy after re-entry. KE = 1/2 mV^2; PE = mgh, where m is mass, V is velocity, g is gravity, and h is height. Since it requires about 34 times the energy to get into orbit than it does for a Spaceship One lob, it also requires that we dissipate about 34 times more energy...i.e., that is roughly the difference between the total orbital energy and the total "re-entered" energy. The vast majority of that is KE since orbital altitudes are ~twice suborbital and, therefore, PE of orbit is only double PE of sub-orbit. To reduce the speed, in the extremes we have two mechanisms. One is a long controlled thrust burn - similar to an Apollo LM - that decreases speed and then descends at a moderate speed with low aerodynamic heat transfer. The other mechanism is aerodynamic drag which requires you to be able to tolerate the heat transfer that goes along with that (my previous offline discussion forced me to crack open my old hypersonic aerodynamics book to figure out what kind of heating we're dealing with here. Believe it or not, the *ascent* of an aerospace plane produces nearly 10 times the heat of the *descent* of a space shuttle). Each of these mechanisms requires hauling extra weight into orbit and weight - by virtue of the rocket equation - translates directly into additional propellant. Because of the enormous delta-v involved here, I believe the amount of propellant required for a "chemical" deceleration is far, far more than that required to haul a heat shield into orbit. The Apollo LM had no other choice due to lack of atmosphere. But where atmosphere exists, it is usually going to be more efficient for deceleration than a burn.
 

illini

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Here's another way to think about it (actually, just wanted to up my post count ;) ):

If it takes a Saturn IB to put an Apollo CM and SM into earth orbit with a delta-v of 17,500 mph, then it requires *nearly* a Saturn IB to reduce the speed from 17,500 mph to ~2,500 mph. If I need a Saturn IB to de-orbit, then what do I need to get the whole thing into orbit in the first place? Our friend the rocket equation tells us we'd need something monstrously huge.
 

bobkrech

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Jim

It really doesn't take much delta v to de-obrit, a few hundred feet per second is all that is required to cause re-entry from low earth orbit, compared with a delta v of more than 26,500 ft/sec to get into orbit. That's only about 1% of the fuel requirement to get into orbit. If you want to do a larger delta v, you need larger tankage to get that fuel into orbit and then use a lot more to get down if yu don't use aerobraking.

The problem is that if you slow down too much your re-entry is much steper and the heating rate goes way up unless you fire the rocket motor to slow down, so you would have to continually burn the retro-rockets virtually all the way down. This would significantly increase the fuel required probably by a factor of 3 to 4 over the conventional amount consumed for a getting a given mass into orbit and back again with aerobraking. If you were landing on the moon where there is no atmosphere, you don't have a choice, and consequently you have a small payload fraction. If you can take advantage of aerobraking, you do, and from a practical payload fraction standpoint, it's not a good idea to do an engine braked re-entry when you can use aerobraking and that's why no one does it.

As several have already stated in this post, the most effective way to reduce the re-entry heating is to cause re-entry to occur at the highest altitude possible. In this manner, the spacecraft reaches terminal velocity at a very high altitude, and simply reduces it's velocity further as the density of the altitude increases.

There have been a number of studies of "parachuting" from orbit as a way of astronaut rescue from a disables space craft. Check out http://www.astronautix.com/craftfam/rescue.htm for details.

The trick to perform a high altitude re-entry is to minimize the total package weight and to reduce the average sectional density of the spacecraft by inflating a baloon, drag disk, etc. either in front of or behind the passengers. I have looked at this problem in the past, and for a single person bailing out could reach terminal velocity at ~400,000 feet at a velocity of ~12,000 ft/second. You, in theory, can even inflate a baloon behind the astronaut and the drag of the baloon will provide enough delta v to cause re-entry in a few orbits without a retro fire. Maximum temperatures can stay below 1000 C so there are no insurmountable materials issues. On the down side, since the re-entry time is very long and the atmophere is somewhat variable, the landing zone can not be precisely predicted.

Bob Krech
 

jflis

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Yea, I knew this was going to generate some interesting discussion.

Oh, I am aware of the physics issues and the orbital mechanics that are involved, just not aware of advances in liquid fuel motors or even solid strap on's.

I have heard about and like the idea of doing a high orbit reentry and spread the reentry over a greater area (2-3 orbits) instead of coming in so much faster, if that could be controlled without bouncing back out to space. But it seems to me that, even if it is impractical to slow to a crawl to elliminate reentry heating, that it may be practical to at least reduce the reentry speed to help the process.

Even if just from a safety margin, it may be worth looking at the cost/value of doing a dual launch. One launch carries the space vehicle (shuttle, capsule, whatever) and a second launch carries the reentry fuel/motors that are parked in orbit to be picked up when needed by the space vehicle.

Again, maybe not enough to *stop* in orbit and drop down, but reduce the reentry speed by 10%, 20%, 50%, whatever you could do while balancing cost and safety considerations.

Seems that such a system in place would have saved a shuttle mission.
 

illini

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Originally posted by jflis
Oh, I am aware of the physics issues and the orbital mechanics that are involved, just not aware of advances in liquid fuel motors or even solid strap on's.
Start by taking a look at the thrust equation:

T = m-dot * v-exit + (p-exit - p-ambient) * Area-exit

Primary contribution here is m-dot * v-exit...mass flow rate times exit velocity. Improvements in liquid/solid technology without changing total propellant mass over a given period of time would have to manifest themselves in a higher exit velocity by virtue of greater energy release and higher pressure in the combustion chamber. Any such advances would be in the range of a few percent. That is, Isp of these motors is in the range of 300-450 and that would change only incrementally with more energetic propellants. To make the "big leap" would require you to look at things like electric propulsion systems with Isp's around 3000. While much more efficient with a given propellant mass, their thrust is so low that it would take a very long time to significantly impact delta-v.
 

rstaff3

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Originally posted by bobkrech

There have been a number of studies of "parachuting" from orbit as a way of astronaut rescue from a disables space craft. Check out http://www.astronautix.com/craftfam/rescue.htm for details.

The trick to perform a high altitude re-entry is to minimize the total package weight and to reduce the average sectional density of the spacecraft by inflating a baloon, drag disk, etc. either in front of or behind the passengers. I have looked at this problem in the past, and for a single person bailing out could reach terminal velocity at ~400,000 feet at a velocity of ~12,000 ft/second. You, in theory, can even inflate a baloon behind the astronaut and the drag of the baloon will provide enough delta v to cause re-entry in a few orbits without a retro fire. Maximum temperatures can stay below 1000 C so there are no insurmountable materials issues. On the down side, since the re-entry time is very long and the atmophere is somewhat variable, the landing zone can not be precisely predicted.

Bob Krech
It appears I have forgotten more about this subject than I originally knew :rolleyes: I had heard of this technique (possibly even from that site) but had totally forgotten. Thanks for the reference...I gotta find time to scour the encyclopedia!
 

rstaff3

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Originally posted by illini
Here's another way to think about it (actually, just wanted to up my post count ;) ):

If it takes a Saturn IB to put an Apollo CM and SM into earth orbit with a delta-v of 17,500 mph, then it requires *nearly* a Saturn IB to reduce the speed from 17,500 mph to ~2,500 mph. If I need a Saturn IB to de-orbit, then what do I need to get the whole thing into orbit in the first place? Our friend the rocket equation tells us we'd need something monstrously huge.
Thems one big set of retros!
 

thomasrau

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Originally posted by jflis

Seems that such a system in place would have saved a shuttle mission.
Jim,

What would have saved the shuttle was not having those stupid wings on it. NASA was going to go with a lifting body design until congress forced them and the Air Force to combine their programs. The Air Force required a large cross range capability on landing (orig. spec was for an ability to do a single polar orbit and return to launch site), that required large wings. There were other silly design decisions but thats another discussion.
 

bobkrech

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There is a big difference between a powered descent to the moon and a powered decent to the earth. It's the atmosphere.

When the LEM was descending to the moon, there was no aerodynamic heating to the LEM nor was there an atmosphere to backscatter the exhaust plume. I actually think a motor assisted re-entry would cause more heating to the base of the vehicle rather than less. For an engine to be efficient at high altitude, it needs a very long nozzle. This is typically accomplished by extending a nozzle skirt down over the primary nozzle. This skirt is cooled by radiation, but there is no way to cool the rocket nozzle skirt in a descent mode since when the rocket is going backward, the entire base is in the shock heated airflow and the nozzle will fall apart from the convective thermal load.

Just a comment on the Shuttle.

Basically there's nothing wrong with the Shuttle from an engineering standpoint. It work well, but it requires a lot of maintenance and is expensive to operate. What caused both Shuttle accidents was politics, not engineering.

NASA management simply chose not to follow their own safety rules and the advice of their engineers. The o-ring problem was observed on many flights previous to the Challenger accident, but nothing was done about it. On the day of the Challenger launch, the engineers advised against launching, but the very political NASA management overruled them because Regan's state of the union address featured the Teacher in Space program.

The Columbia accident was caused by a thermal control system failure cause by External Tank foam impact that broke a leading edge tile. The loss of foam had occurred on scores of flights, and "nothing happened" so it must have been ok. Right?

Wrong! NASA's own flight safety rules demanded that anything that might result in the loss of the vehicle (and a chunk of foam impacting the leading edge that lead to a thermal protection system failure failure was one of those mission critical item) should ground the vehicle until the problem was fixed. Since nothing happened the first time it happened, or the second, ... or the twentieth, ... it must be ok. Management simply did not stop and fix the problem, and it finally bit them in the butt. Basically several bad management decisions cost 14 lives and several billion dollars just to adhere to a schedule. They didn't have time or money to fix it the problems the first time they found them, but they sure have lots of time on their hands now and are spending a ton of money to finally fix it.

The worst part about the Columbia accident is that NASA management didn't learn anything from the Challenger accident. The same systematic problems that existed in NASA in the 80's are still there 20 years later.

NASA engineers and technicians are hard-working, dedicated people, but unless the NASA management mindset is changed, it won't matter what NASA flies, and avoidable accidents will continue to happen.

Bob Krech
 

wwattles

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The recent flurry of press coverage of the X-Prize has gotten me wondering if something like that could spur commercial development of a shuttle replacement. Here are my design parameters:

$50 Million and a 5-year exclusive contract for cargo/personnel transfer for the first commercial enterprise to build and demonstrate a vehicle to:

Carry 10 passengers
Attain orbital flight and dock with ISS (Or whatever they are calling it today)
Repeat 2 more times within a 2 month period with same vehicle.

This kind of prize will be enough to get the biggies (Boeing, Lockheed/Martin, etc) interested, since the 5-year contract would be potentially rather lucrative!

WW
 

illini

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Been there, doing that. Go take a look at the America's Space Prize on space.com
 

DynaSoar

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Originally posted by jflis
I started this same thread in ROL as a result of some discussion about the X-prize and such, and thought it would be a good discussion here.
They get up there without burning up because they don't get up to that speed until they're out of the majority of atmosphere.

To get them down the same way would require the same equipment. You'd have to have a fully assembled and fueled booster already in orbit capable of slowing the craft down sufficiently at first, and then a sustainer (whether two burns or two stages makes no difference) capable of preventing free fall velocity from reaching those speeds (which they could from LEO, even with a "dead stop drop").

Consider the amount of booster you'd need to orbit a complete and fueled booster, riding up as payload. Or, the number of launches required to get the components and fuel up there in pieces and then build it.

Given a light, very high impulse propulsion technology, it would be possible. So far we haven't been able to build much beyond the "minimum energy" design for ground to orbit and back.
 

bobkrech

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I've actually talked to Edwards. He's a nice guy but his concept is far-fetched, and his cost projection and time schedule are absurdly optimistic.

This is a summary of what they propose to do that I abstracted from a manuscript entitled "The Space Elevator by Bradley C. Edwards, Ph.D." that is the result of a six-month investigation he conducted for NASA under the NASA Institute for Advanced Concepts (NIAC) program.

In his manuscript, Edwards states:

"In considering the deployment of a space elevator we can break the problem into three largely independent stages: 1) Deploy a minimal cable, 2) Increase this minimal cable to a useful capability, and 3) Utilize the cable for accessing space."

The initial “string” we deploy from orbit is actually a ribbon about 1 micron (0.00004 inches) thick, tapering from 5 cm (2 inches) at the Earth to 11.5 cm (4.5 inches) wide near the middle and has a total length of 55,000 miles (91,000 km). This ribbon cable and a couple large upper stage rockets will be loaded on to a handful of shuttles (7) and placed in low-Earth orbit. Once assembled in orbit the upper stage rockets will be used to take the cable up to geosynchronous orbit where it will be deployed. As the spacecraft deploys the cable downward the spacecraft will be moved outward to a higher orbit to keep it stationary above a point on Earth (a bit of physics we will explain later). Eventually the end of the cable will reach Earth where it will be retrieved and anchored to a movable platform. The spacecraft will deploy the remainder of the cable and drift outward to its final position as a counterweight on the end of the cable. This will complete deployment of a stable, small, initial cable under tension that can support 2724 pounds (1238 kg) before it breaks.

The next stage is to increase this ribbon we just deployed to a useful size. During this stage climbers will ascend the cable and epoxy additional ribbons to the first one as they climb. At the far end of the cable the climbers themselves will become counterweights for the space elevator. One problem is how to get power to these climbers. Gasoline engines don’t work well in space where there is no air and wouldn’t have the required range, solar cells are too inefficient for their mass to be feasible, nuclear reactors are too heavy, an extension cord just simply wouldn’t work, etc. The best option is to beam up the required energy. By using a large laser directed at solar panels on the bottom of the climber, we can efficiently send up lots of power to the climbers. This power is easily converted to electricity for running an electric motor to climb the cable. As each climber completes its ascent the cable would be 1.5% stronger. After 207 climbers (2.3 years), the cable would be capable of supporting a 22 ton (20,000 kg) climber with a 14 ton payload (13,000 kg). This cable will have a cross sectional area forty times the initial cable I mentioned above. Payloads can be taken up the elevator to any Earth orbit or if released from the end of the cable be thrown to Venus, Mars or Jupiter. These payloads (large satellites, cargo, supplies, etc.) can be launched every four days. Additional cables of comparable capacity could be produced every 170 days using this first cable and “shipped” to other sites along the equator by dragging the lower end of the cable. In 2.8 years the capacity of any individual 22 ton (20,000 kg) cable could be built up to 1100 tons (1,000,000 kg) or roughly the size of a shuttle orbiter. And again, payloads as large as the shuttle orbiter can be sent to Earth orbit, Venus, Mars or Jupiter every four days from one of these larger elevators."

There are a few major technical hurdles that must be overcome before it could work.

1.) Making and deploying a 55,000 mile long 1 micron (0.00004") thick cable made from carbon nanotubes.

Saying that's a long way off is being polite.

2.) Manufacturing of 1 MW average power free electron lasers (FEL) to power the crawlers. A beam me up Scottie idea. 3 are required initially for his nominal system, and a full 1100 ton capacity system would require 120 MW average laser power.

Folks have been working on the FEL for more than 20 years and can get about 10 KW out reliably. Consider that the largest military lasers are MW class devices, it's quite a stretch to assume that a commercial MW class FEL device can be in 5 years. In spite of his optimism, a MW class FEL is decades away.

3.) The crawler uses advanced photovoltaic technology to convert the laser power into electrical power at the crawler. He assuming he can collect and convert the laser energy to electricity at 50% system efficiency which is an unrealistic level and doesn't say how he's going to dissipate the other half of the laser beam energy that is converted to heat.

There are other problems, so don't get your hopes up for a ride in this elevator in your lifetime.

LOL

Bob Krech
 

wwattles

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In a way, I feel almost like I should be looking for the reference to the Unobtanium that would be used to house the Illyrion and Upsidaizium for the power source... ;)

WW
 

DynaSoar

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Originally posted by wwattles
In a way, I feel almost like I should be looking for the reference to the Unobtanium that would be used to house the Illyrion and Upsidaizium for the power source... ;)

WW
If it's a government project, the much heavier Administratium would absorb those substances. It's strange stuff. Everytime you look at it it seem to have more and more neutrons. It's not really inert, but the more you want it to react the more inert it acts. Get enough of it in one place, say somewhere between Maryland and Virginia, and it forms that strange pseudo-molecule Bureacracy-balls. Every 4 years it's supposed to get dissolved and recrystalized, but the solvent is based on placebic acid and it doesn't really do anything.
 

thomasrau

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Originally posted by wwattles
In a way, I feel almost like I should be looking for the reference to the Unobtanium that would be used to house the Illyrion and Upsidaizium for the power source... ;)

WW
Disney had the answer long ago, FLUBBER.
 

Chilly

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What about the Eludium PU-36 Explosive Space Modulator? They stole the space modulatorrrrrr......:p
 

graylensman

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We just may have to go for a jaunte, when all is said and done...
 

Lugnut

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Originally posted by wwattles

Unless someone's got a better idea, we are still left with 2 options: a sacrificial or ablative surface like the gemini/mercury heat shields, or extremely heat-absorbent tiling that is reusable like the shuttle. If memory serves, the Soviets had a highly experimental hypersonic test program called "Ajax" that involved an accelerated particle stream in front of the aircraft that would create a plasma which would in turn develop a slipstream of sorts. The test program was intended to be used to create a hypersonic aircraft, but the same technology could, I suppose, be used to create a bubble of air around a deorbiting spacecraft. The bubble would then act as the "heat absorber" for reentry. But of course, the problem again is the weight of the equipment to create the bubble.

WW
http://www.electrofluidsystems.com

or more specifically:

http://www.electrofluidsystems.com/news/DGLR-2003-257.pdf

Make sure you have your propellor beanie screwed on tight before you click on the link. If your heads still intact, have a gander at this:

http://hypersonic2002.aaaf.asso.fr/papers/17_5115.pdf

Counterflow jet nozzles emitting plasma can help form a boundry layer around the surface of the object, and effect the relevant Mach number it experiences (I guess). Way over my head...

My thought would be pre-orbited recovery vehicles, ablative cones, with plasma generation units built into their nose that the de-orbiting vehicle could couple to and recover back into the atmospehere. Might be impractical for normal use, but maybe have a couple in orbit as last resort re-entry systems, for utilization by damaged vehicles, or maybe just for crew recovery within the cone itself. Just a fuzzy ill considered thought.

Lugnut
 

wwattles

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If we're looking at emergency contingencies only, I think the best approach is simple: multiple space stations! If there is a catastrophic failure requiring the evacuation of one, transfer all personnel to another station for further evacuation via normal transfer means. Forget all that "lifeboat from space" stuff - just get them over to a safe place, then worry about getting them down.

WW
 

bobkrech

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Lugnut and wwattles

Plasma generators are designed for drag reduction. They work by heating the air ahead of the vehicle which effectively reduces the Mach number since the speed of sound is proportional to the square root of the absolute air air temperature.

If you increase the air temperature ahead of the vehicle to 1200 K vs a normal 300 K, Mach 20 becomes Mach 10, and the aerodynamic drag is reduced accordingly. However it does not necessarily reduce the aerodynamic heating and heat transfer, since the base air temperature is much hotter.

A much simpler mechanical implementation of the air heating mechanism is the deployable aerospike on the Trident missile.

http://www.fas.org/nuke/guide/usa/slbm/c-4.htm

This method require no plasma generators or electrical power. A simple PAD deploys the aerospike upon launch acceleration. It reduces the blunt nose drag by 50%.

Bob Krech
 

r1dermon

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ok, i didnt read the first page, so i dont know exactly what you're talking about, but i think its about the heat generated on re-entry....the X-prize space ship one crew figured this out. no tiles, no nothing. they have retractable wings that point into the direction of the ground, so its like a controlled freefall, reducing the surface area and making heat generation really minimal.
 

Weekends

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There was just a several hour show on The Discover Channel detailing Space Ship One (the winner of the X prize). Now if I have this right, Space Ship One uses what's they call a "feather" to make reentry.

The short wing assembly of Space Ship One pivots about the body of the ship. I believe the wing assembly goes to perpendicular to the ship's body. The ship's body is parallel to the earth while the wings are perpendicular. This creates enough drag and stability to slow the vehicle down to make a safe reentry. There is no need for extensive heat shielding. I think they even stated that there was no heat shielding (but don't quote me on that). One of the pilots had to wear more socks because his feet kept getting cold. His feet are positioned in the vey front part of the nose of the space craft.

So according to the Discovery Channel series, the "feather" is the means for slow reentry and not heat shielding.

Weekends

p.s. They will be replaying the episode, but I do not know when.
 

rstaff3

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I heard Brian Binnie talk and I think he mentioned heat shielding on the leading edge of the main wings. He was referencing a photo out the porthole so I may be off on the location.

The reentry SS1 made is far different than reentry from orbit (this has been discussed above).
 
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