Wednesday, November 11, 2009

Its a bird, its a plane its a drag!

The LLC vehicles have had almost no aerodynamic constraints. The Rocket Racers of Armadillo and Xcor used off the shelf airframes with established functional aerodynamics. Not even spacex has yet dealt with aerodynamics in any complciated way. The Spacex falcon 1 did have aerodyanmic Max Q issues, and probably aerothermal issues, but there were no aerodynamic controls and aerodyamics does not get much simpler than a long round tube with a pointy end.
(The recovery parachutes of the F1 first stage either were not included or failed)

Spacex is about to fly the dragon capsule with hypersonic aerodynamics, aero thermal, and stable parachute deployment and recovery issues. Armadillo is starting to fly to higher altitudes, Masten hopes to soon follow and or surpass what Armadillo has been doing.
Xcor is planing their next rocket vehicle where the aerodynamics include transonic and some aero thermal issues.

From a rocketry stand point the smaller New Space organizations (Masten ,Armadillo and Xcor) are nearing the level of rocketry sophistication reached by the Germans at the end of WWII. Please note that the Germans never flew a supersonic aircraft. (Yes the V2 was supersonic) Get in your time machine fly back to 1946 and ask Chuck Yeager if supersonic is a big deal?

If an LLC L2 vehicle was flown on an airless earth with zero drag and maintained a steady state 4G acceleration (3 g net) it would run out of propellant near 30Km and coast to 119Km before falling back to earth. Using my really simple aerodynamic model... assuming a Masten Glow of 900 lbs, and 25" diameter with very good aerodyamics removing the 25Kg payload and flying the same 4G flight Mastens L2 vehicle would reach about 116Kf or 35Km. Now in practice you would probably have to throttle back to reduce max Q if you picked 300Knots as max Q equivalent then you would have your velocity limited by maxQ until at least 40Kft and one would trade more gravity loss for reduced aerodynamic losses and get to about the same place just over 115K ft.

This not an up and soft land simulation, this is a up up and away simulation with a crash landing at the end. So given the stated goals of both armadillo and Masten to fly payloads to space they will need vehicles that are both higher performing and more aerodynamic than what they presently have.
(A max Q of 300 knots equivalent may not seem like very fast in a world with 500knot airliners, but a skydiver released into a 300 knot airstream would experience more than 8 g of deceleration.)

I vaguely remember Henry Spencer making a comment that the Apollo command module heat shield had an eqiuivalent ISP of 7000. If your going to build reusable vehicles that fly to space and come back its never going to make sense to kill your velocity via propulsion as long as we are using chemical fuels. So when reusable suborbital vehicles start flying to 100km they will need to use aerodynamics to scrub off the energy from the gravitational potential.

Will this look like the space ship 2 shuttle cock? will it look like NASA's hypersonic ring slot parachutes developed for viking and used on every mars landing since? Will it be airplane like?
I think this is going to be a harder problem than many think. Mr Musk of spacex has been quoted as saying that a fly back first stage booster would be a really useful thing to reduce space access costs, but it would cost > $1B to develop. Some solutions look simple like the rocket becomes winged vehicle as shown in Charles Pooley's Microlauncher presentation. I don't think that an easily fabricated simple wing will function well with supersonic shocks, flutter stiffness etc... One wants a shape that can give you decelerating lift at high altitude, does not provide much drag on the way up and is structurally stable at all points in between. This is a you pick any two sort of problem. The U2 had very long thin wings to get get lift at high altitude. Yet it it was a subsonic aircraft and as a result operated in a very narrow box where a few knots faster and it hit Mach buffet and a few knots slower and it stalled.

One can work around these issues by using a combination, like a hypersonic parachute to decelerate you to subsonic followed by rotate open wings to glide back to base.

Whatever features will be used they will require testing. With the exception of xcor all of these vehicles are unmanned. The regulatory environment for testing rockets under the amateur or experimental permit rules are now well defined and reasonable friendly. If one is developing a glide back system one would like to test the basic aero controls, flight, landing etc in an incremental manner. Just try getting the FAA to give you permission to fly such an unmanned vehicle in their airspace? The Aircraft side of the FAA is significantly less understanding than the AST. (Just ask John Carmack) So from a regulatory standpoint one is going to have to fly it as a rocket under AST's jurisdiction. This is not testing that can easily be done under tether, or even at the locations that Armadillo and Masten are currently flying from.

None of this is impossible, its just another layer of problems to be tackled. Anyone have any good recommendations for a good book on supersonic aerodynamics ?


Unknown said...

Speaking as a glider pilot who's done a fair bit of amateur-level research on wings (for potential homebuilt aircraft projects), I really don't think you want to deal with any kind of "swing open" or rotate-to-position wing. The weight of such a design (with moving components and taking loads in different paths - especially after your wings open up) would really hinder your payload capability. Its too complex to go into in this short space; but for a wing to work down low (for landing) it needs to produce good lift at lower speeds. This requirement runs exactly contrary to the aerodynamic and structural requirements for a wing to work at high speeds (or at the very least, not rip off of the vehicle due to excessive load or aeroelastic effects such as flutter). The ability to control overall wing camber or thickness can help with this, but standard methods (such as flaps or other moveable surfaces) exacerbate aeroelasic effects due to their weight and balance with respect to their hinge/pivot-point. SS1 and earlier tests on a SGS 1-36 glider with a special pivoting tail/stabilizer arrangement (which may have inspired Rutan) are probably the closest to a feasible winged design that you can achieve: a low aspect ratio wing minimizes bending loads on the wing spar, and the ability to use the trailing edge and horizontal stabilizer to force the wing into a deep stall (high angle of attack relative to the oncoming air). This deep stall can happen with conventional aircraft - usually with a very aft CG position and/or improper rigging or sizing of tail surfaces - and can be fatal and unrecoverable due to the way conventional aircraft lifting surfaces are arranged. But a deep stall prevents attached airflow over the wing at high speeds. No good airflow means no lift; just LOTS of drag! Only problem is that a big low aspect-ratio wing tends to produce a lot of drag because of its large surface area, big wingtip vortices, thick airfoil (usually), and other details. And during the ascent phase you can't keep the wing aligned with the oncoming airflow because the wing is then producing lift and you run right back into the load & flutter problems! You could try to manage the wing's angle of attack so it produced zero net "lift" (force perpendicular to the relative wind) during ascent; but this could turn out to be incredibly complicated and easy to have a catastrophic failure during ascent. Note that SS1 got around all of this by being air-launched at a high altitude where relatively few air molecules would be flowing over the wing at high speeds.

Bottom line: Its a thorny problem; but wings are _really_ tricky to fit into a vehicle that has to cover a wide range of velocity and air-density regimes! If I were you, I'd look to other solutions... many folks have tried to tackle this problem over the years, yet I know of few that have been successful (NASA X-15, Orbital Sciences Corp booster, SS1, Space Shuttle).

As always, best of luck!

Unknown said...

Quick follow-up: For airspace and testing, read up on "Class G" airspace. :-) Also, folks fly 200mph radio-controlled aircraft all over the country. Some guys do "dynamic soaring" in remote locations and take unpowered R/C gliders to speeds approaching 400mph (yes, with NO motor). If you're willing to work on a small scale (say less than a 12 foot wingspan), you can probably do it all under the argument that you're flying an R/C model. Its a gray area in many legal respects, but folks do it every day...

Krishna Kattula said...

I don't think Masten & Armadillo will want wings. Some sort of drag increasing device maybe. Possibly a drogue or speed brakes.

But terminal descent & landing should still be under rocket power.

Anonymous said...

I don't think wings are a good idea either. But if someone does, you should look at the many variable sweep wing concepts that have been proposed for flyback boosters. NASA did studies for the SRBs, and the Russian looked at the baikal (sp?) flyback concept.

If you go to orbit you will want a heat shield. Terminal descent and landing is like 1-3% of the remaining energy from orbit. And as we know, if that 1-3% is a tough problem to do reliably. The other 97% is no easier.

That heat shield may be single use, ala apollo, or multi-use like shuttle. Inflatable devices, for hyper and supersonic re-entry show lots of promise, but have not been used. Chutes are only good to about Mach 2 or 3, so you still have to slow down way before then.

Take a look at the IRVE work being done by NASA, or the MIAS concepts from the Russians. These concepts are 'out there' enough that they hold lots of promise, and seem perfect for a small company to push forward.

Unknown said...

I have to say this blog and its associated commenters are really informative for any commercial space enthusiast. I did pilot ground school so I get some of the fundamental aerodynamic forces, but the original post and especially noel.wade's comments have really grounded the launch and reentry issues in basicaerodynamics. And it was done without involving calculus, which is about the limit (get it, limit... *sigh*) of my math experience. Usually it's either so low level that the engineering is handled in a 'black box' fashion or so high level you need at least a masters in engineering to understand. I really appreciate the input here.

Good luck, Paul, with whatever direction you end up taking your development work. I'll certainly be following along.

Terence Clark

Paul Breed said...

Noel, Take a look at my last project:

I've spent some time in the land of the RC sailplane. I've actually talked to a someone that builds DS airframes about building some rocket parts. I think the problem gets a lot easier if the aero surfaces only have to get the vehicle near the landing point where one can turn the rocket back on and do a vertical landing on its tail like God and Heinlein intended. Figuring out how to keep the center of pressure behind the CG while the rocket is climbing in rocket mode, then have the CG be in the proper spot for the vestigial wings is another interesting issue.

Andre said...
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Andre said...
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Unknown said...

Sorry Paul, can't believe I forgot about your solar splinter project! :-P Been thinking of you as a Rocket-Man for a long, long time. ;-)

CG and CoP are certainly thorny issues, but even if the structural loads and aero problems are solved (maybe some sort of Freewing-like solution?) aren't you still wasting mass and drag with the wings? They're unsuitable as pressure vessels (fuel tanks), though maybe the thinking is that they weigh less than the propellant req'd to decelerate...

During the glide, do you envision the vehicle in a "nose up" attitude, or a "nose on the horizon" attitude? Horizon means careful energy management and some good control surfaces so that you can pull up to vertical as you reignite.

Do lifting bodies warrant consideration? Just tossing it out there... I loves my airplanes (I'm building one from scratch in my garage, see, but I have a hard time believing that they're the appropriate solution/model. Hate to be a naysayer, though; would love you to surprise us all, Paul!


Paul Breed said...

You don't need much L/D to control where you return, 2:1 would be doable. As long as one cn pull up to vertical it really does not matter if the thing stalls at 150 knots, so I think the "wings" really are vestigal. Maybe it looks more like a air to air missile with some forward canards than an airplane

The other interesting choice is a steerable parachute.

Unknown said...

Hey Paul,

Yep, aerodynamics are a real issue. But, I don't think it is as bad as you imply. There are some fairly simple* solutions that we are investigating.

Also, we are at a Spaceport for a reason. There does not appear to be any reason that we could not get to space from Mojave.


* where "simple" is relative to rockets in general, i.e. it is still difficult.

Roga said...

A problem I see with VTVL (which is not there with HTHL) is shuttlecocking. You need the Cp behind the Cg for stability on the way up, but you need it in front of the Cg for stability on the way down. I guess this could be managed by putting your fuel up high; but what happens in the case of an emergency abort? You have to dump most of your propellants before you reenter/pitch over.

Anonymous said...

The VTVL vehicles will probably be subsonic down low in terminal descent without any help from wings. Even when scaled up. If you're smart on the ascent, you shouldn't need too much maneuvering at descent.

That's the same from orbit or 100 km. You don't open any parachutes or anything at great mach numbers.

So maneuvering supersonically, it's a false problem, a red herring!

If you still want to fly instead of just fall at super/hypersonic speeds, the space shuttle orbiter is stable in a large range mach numbers IIRC. A direct copy or only slightly changed shape might be good. Very low lift at low speed though. The X-24B is another. Even simple Apollo cones can fly a little with CG offsets. Or you can have a sphere with offset cg. (Wink wink!)

Lynx does have a problem (and XCOR was looking for senior aerodynamicists years ago), in that it has to lift off horizontally meaning it needs some considerable lift (since it's heavy) at sea level and yet operate at some mach numbers at altitude. 60 km is not hugely high and IIRC they wouldn't need to actually go very fast. And you don't have to really maneuver at those high speeds.

Also, modern materials like carbon fiber enable far superior rigidity in thin structures compared to the forties or even early sixties. As everybody knows gliders have completely transformed in the mean time. Hence flutter could be less of a problem.

You could do supersonic testing by dropping off some very heavy instrumented models from a suborbital VTVL/HTHL flight. :) That way the main vehicle would still be subsonic but the model would have a higher ballistic number meaning it would go supersonic.

Lockheed Martin is developing spaceplanes and buying supersonic flights from the UP Aerospace sounding rocket company at some spaceport. It's a tricky problem - you are using expendable hardware to develop reusable...

One way would of course just to have a stupid simple absolutely supersonically stable reusable VTVL falling and have a more refined model attached to the front (ie not lee side) for testing. This way testing should be cheaper as you don't throw away a sounding rocket or your model every time.

Also, the unit mess in looking at this is horrible. Why not keep all altitude in meters, all speed in m/s? Everything really is the same physically, it's crazy to use ft for altitude, knots for speed and who knows what for something else. Mach 1 is around 300 m/s. 1 gee acceleration is about 10 m/s^2.
Also kilo is written with a small k. K is Kelvin, for temperature.

Paul Breed said...

All my sims were done with a Cd of 0.3 and a very simple table driven atmospheric model. With "simple" aerodynamic fixes what do you think zoie can do and still soft land?

Even with active stabilization the vehicle needs to be close to neutrally stable going up and going down. Do you plan a return to landing swoop? If so the propellant settling issues are non-trivial for LOX. For HTP can you say "polyethelene bag"?

Given Delta V == 200 seconds of hover and propulsive braking with no atmospheric drag 4G max load I calculate peak altitude for soft landing with zero margin at 36Km or 120K ft.

To do a soft land with 4G max acceleration zero margin and hit 100km one would need performance to hover 300 secs assuming zero drag.

Unknown said...


we're using Missile DATCOM to get initial estimates of aero coefficients over a range of Mach numbers, Reynolds numbers, and angle of attack. Then run it through our trajectory software.

Your analysis of what I call Frank (Xoie with an aeroshell) is pretty close to what we get. The aeroshell is 31" in diameter.

For suborbital flights (100km) we are building a new vehicle with a bit more propellant, total impulse around 190,000 lb*s. And it will use the 3000 lb engine we have been developing. The aeroshell will be the same as whatever we end up with for Frank.

At any rate, I'm currently working on dive brakes and looking at the trades between tail first reentry, high alpha reentry, DC-X style swoops, etc.


Anonymous said...

Bottom line: Its a thorny problem; but wings are _really_ tricky to fit into a vehicle that has to cover a wide range of velocity and air-density regimes!

How about rotary wings or ducted fans ?

Paul Breed said...

>Your analysis of what I call Frank >is pretty close to what we get.

Thanks for the validation, my sim is just 150 lines of C code written from first principals with no real validation. It would be real easy for me to be off somewhere by a factor of 9.8 or so...

Anonymous said...

Openable drag brakes at the top of the vehicle for stability and lower terminal velocity...
They'll pay their weight in reduced landing propellant mass.

Anonymous said...


How about a new X-Prize called the "Mars Lander Challenge" that rewards a winning team for getting to 100-km twice within an 8-hour period of time. The winning prize should be for ~ $2 Million, because it would probably cost a team $5 Million to win this prize (and X-Prize foundation says it usually takes 2.5-times the prize purse to win an X-Prize).

Because Mars has an atmosphere and more gravity than the Moon, I am assuming that the new challenges that you would take on for sub-orbital flights would be similiar to the differences between the Lunar Lander Challenge and my hypothetical "Mars Lander Challenge".

Robert Horning said...


The "steerable parachute" concept does seem to be at least something to research and to consider as an option. If my very faulty memory is correct, The Big Gemini proposal had the steerable parachute as a serious consideration for a land-based landing requirement for this vehicle. Some significant research did go into this proposal by NASA, the USAF, and McDonnell Douglas for this concept... even though it never actually got beyond the initial test article/prototype stage of development.

That metal was bent on the idea should at least demonstrate that some serious effort went into the idea. The Wikipedia article has some links and references to the concept, and I've seen other references in relationship to this vehicle on other discussion forums too. The Wiki article on the Rogallo wing even shows a snapshot of the "parawing" being deployed as a part of a static test by the engineers.

An even more unconventional concept was to deploy a wing that was essentially a reinforced hang-glider made of fabric that would be deployed at sub-sonic speeds. Essentially more of the same concept, it had some much more recent NASA research that I'm sure you could wade through if you wanted to take the time.

Both of these concepts could be relatively lightweight and wouldn't take up "comparatively" much room in terms of payload for launching purposes. That certainly seems to be a major consideration for why these were explored as ideas in the first place. It is certainly thinking "outside of the box" and doing a follow-up to previous research that deserves some attention.

Anonymous said...

I'd second the lifting body.
The problem with rocket landings and parachutes is if the engines don't get up to speed or the chute doesn't open, your done.
A fixed wing aircraft is always ready to fly (if it can survive reentry) and a lifting body gives you the most internal capacity.

When the X-33 project was being canned I always wondered why they didn't pursue it as an unmanned booster. The added weight wouldn't matter as much if the ship was only going halfway up, and a runway landing is the fastest way to get it back to the launch facility.

Anonymous said...

Why 4G?

Anonymous said...

3g net

Anonymous said...

Yes, 4G/3G net, that's not my question. Why this constraint?

Anonymous said...

I rarely comment on the internet on anything, but this comment really caught my eye.

"The VTVL vehicles will probably be subsonic down low in terminal descent without any help from wings. Even when scaled up. If you're smart on the ascent, you shouldn't need too much maneuvering at descent."

Pat Bahn
This is a very dangerous conclusion.

Paul Breed said...

4G was a tradeoff between lots of thrust calling for larger motors and thrust system masses and less thrust giving more gravity loss.....I actually thought it might be a bit optimistic given the present vehicles.

In reality just a random number pulled out of the air. The drag free numbers were done with a calculator and parts of x=0.5*a*t^2

The numbers with drag were done with my simulation that does integration while adding dynamic pressure, max thrust and max acceleration limits. I think max thrust was set to 1.5 or so x gross initial vehicle mass...
(Don't have sim in front of me at the moment)

Just Jerry said...

The physics of small wings is well understood and if horizontal landing is a requirement, this approach makes it practical. Here is an image of concepts from the late 60's still appropriate today.

Skids are far less mass than landing gear. CF tubes for low mass?

Who says Armadillo, Masten, and Unreasonable have to have SEPARATE efforts anyway?

Unknown said...
This comment has been removed by the author.
Unknown said...

[reposting to edit typos]

Just Jerry - Not to be a buzzkill, but AFAIK most of the designs in that photo are based on lifting-body concepts; the wings are more for stability than actual lift. If you want to soft-land the thing you will need a certain amount of low-speed lift production. Of course, this could be obviated somewhat by coming in at high speed and swooping to a vertical position for re-light and a vertical landing.

BUT if you're going to do all that, why not use some sort of deployable aerobrake (even if its just ablative material) instead of a wing to slow you enough to reduce your required propellant load (versus a pure VTVL rocket)? Whether its a series of drag brakes or a wing, in both cases you're carrying added mass. Brakes or heat-shields seem to be a simpler solution. Is there any study or factual basis for the hypothesis that a wing would be a lighter-weight solution?

Personally, I have come to believe that the whole "Reusable SSTO" concept is a bad idea. Carrying all that mass to orbit and back seems wasteful, when you look at the three fairly distinct operating environments that you pass through in a suborbital or orbital shot: Low atmosphere, High atmosphere, and space. 50% of the Earth's atmosphere is below 17,000 feet. You don't have to go super-high to gain a noticeable advantage in terms of drag or pressure. A reusable non-space-capable vehicle (as a base for high altitude launches) is much easier to design than one that has to go all the way to space and back. An air launch itself is trickier than a ground launch, and certainly you run into complexity issues when you start stacking disparate vehicles and systems together to make a multi-part launch system; but personally I am coming to believe that for small vehicles/payloads it may very well be worthwhile, given modern technology and materials... Of course, I've spent all of 15 minutes noodling on this concept; so I could be very very wrong. ;-)

Just Jerry said...

Let's take the instructive case, a wing that swings out and is very close to the airframe until deployed. Its purpose is terminal landing direction change, to make the flight follow an asimatotic line rather than a linear one with thrust reversal, parachute, or other method.

By changing the direction of flight from vertical to horizontal, which can be done at any speed, altitude, or mass, one changes the "style" of recovery to a horizontal slide rather than a vertical stop. Lowering the vertical component reduces the damage risk.

I have not done the tradeoff but I suspect a parachute large enough to provide a limited damage recovery is probably actually heavier than the residual propellant requirement to do a last minute thrust. But that does not address risk.

By having a horizontal landing it is not propellant capacity dependent and is less suseptible to common parachute deployment problems. Even a partially deployed wing can change the orientation and direction of a flight sufficient to slide to a landing.

The shape of the vehicle is not particularly critical. Aircraft are all sorts of shapes and sizes. The vestigule wing is itself the answer to provide deflection not lift. A near stall speed landing is going to be modest speed.

The only downside is a runway is a larger facility than a landing pad for a powered landing or an air recovered parachute. A conventional parachute tends to have a landing area error at least as big as an airport without the infrastructure cost. But a skid based landing doesn't need a runway. Any improved, or dirt, or grass runway will suffice.

I urge it.

Just Jerry said...

Taking Mr. Bahn's helpful comment I will now argue against my own position. If you have a vehicle returning from orbit or high sub-orbit, and it free falls to an aerobrake reentry, it will hit the thick part at about 95000 feet and decelerate to subsonic above 35000 feet, with significant heat, and G force, and buffeting. If you deploy a reverse umbrella, presumably near the nose, but works just as well near the tail, it increases the subsonic altitude, the buffeting power, and the heating.

Between 35000 feet and 0 feet it will travel at subsonic terminal velocity until it hits the ground hard, deploys a chute and hits the ground softer, fires a solid and hits the ground softer, or fires a liquid and hits the ground at a throttled speed. The parachute is the heaviest option. The throttled liquid is the most complicated option. The solid is what is in use by Soyuz and some military RV's. The solid would actually be best if you could get the ignition timing right.

If any of that fails you have loss of vehicle. At least with the deployed wing the loss probability is considerably less and you can more fully select the landing point, perhaps except the steerable liquid, which is far more cross range (propellant mass) limited.

Anonymous said...

Great story as for me. It would be great to read a bit more concerning that topic.
By the way look at the design I've made myself High class escorts

Anonymous said...

Paul, I see. So it was a result of the thrust and drag that you were considering. That makes sense. I believe that with the 2500lb engine and other numbers that Dave is using it's more than 4G by engine shutdown.

We're currently haggling what are we going to do about aero, lots of options. I never did any of this before, and it's pretty sweet! Then again, I never did hover either, so we'll see.


Unknown said...

Just Jerry -

While you make good points about loss-of-vehicle issues, you've never flown an airplane have you? Horizontal landings (even on an airport surface and not an off-nominal landing) are not automatically easier or softer. Furthermore, if you build a vehicle to take off vertically and land horizontally you have increased the structural engineering requirements because of the new/different load paths.

I've built and flown R/C gliders (from 24" to 10 feet in wingspan), have owned two full-scale gliders/sailplanes, and am in the process of building a homebuilt aircraft in my garage. I've landed my gliders in farmer's fields and have an intimate understanding of not needing a runway (as you commented on). ;-)

If you begin to use the wings for lift and/or directional control, then the shape of the rest of the vehicle DOES matter. Also, the size and shape and lift-generating characteristics of the wing and fuselage/body of the craft will have a large effect on the stalling speed. And as basic physics tells us, small changes in velocity equate to big changes in impact energy.

Finally, I take exception to the blanket comment about parachutes having more deployment issues... A deploying wing has moving parts and failure modes that are just as problematic as a faulty 'chute deployment! Its harder to carry a spare wing than a spare 'chute, too... :-)

Gliding back to a landing is a wonderful solution that requires passive structural integrity rather than active rocket control; but there are challenges inherent in both! Space elevator, anyone?? ;-)


Anonymous said...

Pat Bahn, I certainly bow to your expertise on these matters - I'm probably wrong since I haven't thought about the issue very much.

Please post more! I'm your fan. Especially regarding the whole design dimension cube way of thinking... and many in the newspace circles are working on that. Schedule, operability and low cost first and higher capabilities later.

Anonymous said...

Why constrain the wing options to fixed wings only ?

Unknown said...

Anonymous - see "Rotary Rocket", if you think fixed wing solutions are too elementary. ;-)

Jonathan said...

John Anderson books on aerodynamics (several of them... I have only dabled in the few needed for school).

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