Landing the Shuttle

[Ed Burke]

An interesting look at Shuttle training.

www.nasa.gov/multimedia/podcasting/flyout_sta.html

and Atlantis doing the real thing,

www.nasa.gov/multimedia/podcasting/sts135landing.html

[Tom Goodrick]

As you might remember, I used to make Shuttle simulations available for FS. But I have not done one for FS2004. I used the Shuttle as a base line for performance of all flying landings from space when I worked at NASA. One of my jobs was to be familiar with Shuttle aerodynamics and to make comparisons to all vehicles we were considering. Incidentally, We had several vehicles on the "drawing boards" to replace the Shuttle and could have had one ready as a smooth follow-on except, beginning with Clinton, they started looking for low cost designs (that don't work).

I could not see airspeeds on these videos. That is where flying a shuttle approach gets interesting. You keep the airspeed locked on 300 KIAS flying the HAC until you get on final. Then you pull the nose up very slightly and drop the gear short of the fence. Touchdown should be at 195 to 210 KIAS.

Just as the GII is used to simulate the Shuttle in flight, you can use most bizjet models in FS to simulate the shuttle from 28,000 ft. The trick is trying to fly the HAC. NASA uses a computer based on an MLS to provide a nav bug for the HAC (Heading Alignment Circle).

[louross]

Speaking about the Shuttle and FS, a bit ago I downloaded Orbiter- the new version. Downloaded it 2 times, one for the DG IV programs and the other for- I don't remember- prgograms. Seems to be pretty good, especially according to the intro material about its use and designer(s). The learning curve is tremendous.
Any opinions, thoughts, concerning this program?
Anybody else using it?

I can get the DG IV close to Low Earth Orbit by hand flyiing it, but just before orbit height I begin losing control. So I need to bring up the orbit insertion autopilot. I've yet to make a de-orbit successfully free flying- I have to use the re-entry auto pilot. Remember now- the other install is for the Shuttle.
lou.

[Tom Goodrick]

What is the DG IV ? I have not heard of it. I know nothing about an Orbiter program. But I have worked a lot of problems in orbital mechanics. Orbital maneuvering in general is pretty easy if you have something that shows your orbit parameters when you finish every burn. The orbit parameters can be computed from your speed components (radial and tangential) at any point in your orbit. As long as you are well above the atmosphere, the orbit parameters won't change. These orbit parameters would be apogee and perigee (the same if in a circular orbit). Whenever you stop firing an engine or thruster, you are in an orbit. I don't understand how it can be difficult to get into an orbit. If you don't like the orbit, just correct it with another burn. Burn along your path briefly to increase you apogee. Burn in retro (flip around and burn against your path to decrease your apogee. In climbing from Earth, you would burn while in the atmosphere until you get an apogee that is above the atmosphere then you quit burning an watch your apogee. It will change because of drag and needs another burn to keep it above the atmosphere. When you get above the atmosphere you burn to get an apogee equal to the height of your desired orbit. Then you coast until you reach the desired height. Then you give it another forward burn (or several short bursts) to raise the perigee to the orbit height. Then the rest of the ride is free.

There was a time in the distant past, when we could actually program our own computers. BASIC was a very good language for that. I used it and got many more programs to run early in their development that when I wrote FORTRAN, PASCAL, FOCAL, C or assembly codes. I used it 30 years ago to write a simple program that integrated the simple orbit equations. This taught me a lot about how orbits work. About 12 years later I joined NASA and took a course in orbital mechanics so I could converse with my co-workers. At first I just did aerodynamics related to launch and entry. But I was in an office where other guys worked launch problems, other people worked orbital mechanics problems (changing orbits and rendezvous) and others did thermodynamics and life support. So I gained the set of equations that define orbits and had some practice understanding things like how speeds determine orbits. I ended up writing a new code in compiled BASIC that showed orbital maneuvers and entry with heating and aerodynamic flight to the landing.

A primary difficulty is giving the pilots good cues to control the flight. If the cues are hard to follow when you get close to the proper orbit then the gauges are bad. Also, they may have given the thruster too much thrust to close on the proper orbit. But you should also know that, unless a rendezvous is involved, the height of an orbit is not critical. As long as the perigee is above the atmosphere (>150 km) the orbit is safe.

De-orbiting to get close to a particular spot on the ground takes a lot more planning and special equipment like a gliding re-entry vehical. A retro or reverse burn is made (on the opposite side of the Earth) that produces a perigee in the atmosphere. Then various means are used to control heating and aero g's to keep the ride cool and comfortable. My program I wrote for NASA allowed manual entry control but provided vertical and lateral guidance for a "nominal" path.

[Tom Goodrick]

Sorry, I should have given a little more basic info. Not sure what comes with that program. Any craft in space will be drawn toward the center of the earth and will move on a path called an orbit. Depending on the radius of its position from the center of the earth, there is a speed called an escape velocity. If the horizontal speed exceeds the escape velocity, then orbit will not be a closed curve but will extend endlessly into space. It will never return unless someone fires its engines in reverse to reduce its speed. Slower craft will move on an orbit that has a high point - its apogee - and a low point - its perigee. These points are on opposite sides of the earth. Many "sub-orbital flights" that go up steeply and fairly fast but without much of a horizontal speed component will come down so steeply they will hit the earth. These are actually on orbits but the perigee is well below the surface of the earth. To get on an orbit that will go unhindered around the earth takes a very high horizontal velocity (about 8,000 m/s, sorry I did all orbital calcs in km and m/s). You must burn until the apogee has an altitude of about 250 to 400 km. This is in Low Earth Orbit or LEO. Then when you get close to that altitude (radius minus msl), you burn again raising perigee to about the same value.

The space station flies at 450 km. But I had to do drag calculations on it to determine how long it could coast before its perigee would get so low it needed a boost (from the Space Shuttle). It hits a couple of air molecules each day. The force depends on the configurations of the solar arrays.

The speed of a circular orbit is constant. At lower altitude the speed is higher than at higher altitude. For an elliptical orbit, the speed at apogee is slow and the speed at perigee is fast - faster than the circular orbit speed. To raise a circular orbit to a greater altitude, you must make two identical forward burns. This called a Hohmann transfer. The first burn sets the apogee for the new altitude. This puts the craft on an elliptical orbit. When apogee is reached, a second forward burn is made, equal to the first. This circularizes the orbit (perigee = apogee). To come back down to a low orbit, two reverse burns are made. Note that it takes about as much fuel to go up as to go down. Many find this surprising. A few sci-fi authors have screwed this up.

The speed is higher on a low orbit. This means both the horizontal component of speed and the angular component (degrees per second around the circle). Thus the orbital period (time to do 360 degrees) is less at low altitude. A craft coming back from the Space Station will typically go to a low orbit (180-220km) for "phasing".

Phasing can only be understood when you consider that, while the craft flies a simple orbit in a geometric plane in space, the Earth rotates beneath it. So if you want your last orbit to pass over your intended landing site (a good plan), you must start watching the "phasing" of the orbital track on the Earth's surface. A key point on an orbit is the point at which it crosses the equator going northward. The longitude of that point is determined for a landing at a particular runway and then the progress of your orbit toward that crossing point is watched to determine when to do the de-orbit burn.

The de-orbit burn is made generally on the opposite side of the earth from the landing runway. The exact positioning depends on the aerothermal flight characteristics of the craft and the heating and loading profile desired. A steep descent would be 100 to 120 degrees of longitude from the de-orbit burn. That would be hot with a substantial time at 4 g's. Using more like 150 to 160 degrees of longitude means the heat rate would be low allowing cheaper material, but you might have to use active cooling because the heat would build up over the longer time. It would be very comfortable at little more than 2 g's. Good for us old folks.

I flew manual profiles looking at heat rate and aero g's to determine which angle of attack to fly on the profile. Some craft do not change angle of attack (they prefer to keep the hot spot in one place) and roll the lift vector left or right of the path to control descent rate. In any even, glide ratio during hypersonic flight, which applies until very near the target, is very low - about 0.6 to 1.1 (shuttle). You are not going to curve the flight path until very near the target. The de-orbiting momentum is too high.

[Tom Goodrick]

People often wonder how high you have to go to reach zero gravity.

Height has nothing to do with it. Gravity at the Space Station would only be 2% weaker than on the surface. We call the free-fall condition "zero G". In an orbit you just keep falling around the Earth and never come to a stop. To hold still in space, you'd have to stop all motion relative to earth and then fire a 1 G thruster straight up from the center of the Earth. Then sitting in your craft you would feel about the same as in your living room.

The simple attempts at making "commercial space ships" so far have achieved only steep trajectories to a point that then fall back to the surface. Riders can experience "zero G" for the minute or so it takes after the engine stops firing the last time as the craft turns over and falls steeply back down. When it starts pulling out of the descent you feel some G's.

[Ed Burke]

I took a great interest in STS-134 and Discovery's last flight. This carried over into following STS-135 with Atlantis doing the very last shuttle flight.

Thanks to Chris Peat and his fabulous web site www.heavens-above.com/ I was able to see a great pass of the ISS almost directly above with Atlantis docked. I took a lot of photos but was not able to make out any detail, just a very bright satellite pass.

Then Atlantis's second last orbit before landing was away to our west, technically visible but unfortunately the sun was only 4 or 5 degrees below the horizon with the result that there was still too much light to be able to spot a faint object. Nevertheless we cheered them on and were delighted when they greased it onto the runway some two and a quarter hours later.
The end of an era and time for reflection.

And along relective lines Chris's site gives lots of info about spotting Iridium Flares. This is reflection of the sun's rays from solar arrays which power the vehicles. Many of them are quite spectacular while some are even visible in daylight. Interesting stuff if your skies are clear as are ours at the moment and will probably stay so for a few months. Sorry to tell that to folk who are a bit clagged in !!!!!

[louross]

Here I am,

I'm sure everyone's busy, but everyday I've had off my boy and I have been finishing up some work on the roof, and we're about done with that. I've had 2 workers out there the past 2 days, and they'll be there all this week and into next week. Plud regulsr work, so ... busy. Anyhow-

Ed, I have a friend who's a chemical engineer and his boy is really into astronomy, so I'll tell him about the web site you told us about- Thanks!

Tom- The DG (Delta Glider- although it's not a glider) is a default vehicle that comes with the program. It is ficticious. Another developer designed the DG IV, as a more suffisticated model.
Apparently, the instrumentation is based on real isntruments, various pages on the computer display show different paramenters concerning orbit, or what ever your doing based on how you've set it up for the particular function or mode of flight. I uinderstand what ou are saying in the first paragraph of your first post, and I understand and can read and use the instrumentation depicting such. The problem is the actual contoling of the vehicle. Using the auto-pilot for insertion works fine, but I think I hould also be able to do it manually. At this point, I can't. The manual de-orbit problem was my fault as at the time I didn't knoe anything about figuring the pitch and descent angles into the atmosphere. So now I know I need to learn how. At this point it appears pretty complicated, so am assuming Orbiter has a program to do it for me. However, I will study your explanation more. I burned up on the re-entries. Again, the auto-pilot makes it simple.

I willalso study the first paragraph of your second post (second after my post). By the way, the program also uses m/s.

I will get back to the Orbiter program when time permits, and I will also be going over your posts here. in the mean time- Thanks!
lr.