February 26, 2000
The Space-Glider (February 2000, experience not real for me: Simulated in X-Plane only)
Read this chapter before attempting Space Shuttle landings in X-Plane if you want to live!
What do you think the first rule of flying a glider is?
Think about it.
The first rule of flying a glider is: “Never come up short”.
When you are bringing a powered plane in for landing, if you think you are not quite going to make it to the runway, it is no big deal.. just a add a bit of power to cover the extra distance! Need a little more speed maybe? Again no problem: Just add power.
Gliders play by a different set of rules, though: There is no engine to provide power, so when setting up your landing, you must always have enough altitude and speed to be able to coast to the airport, because if you guess low by even one foot, you will hit the ground short of the runway, crashing. You must NEVER be low on speed or altitude, because if you EVER are, you have NO WAY of getting it back: a crash is assured. (the exception is thermals, or rising currents of air, which can give efficient gliders enough boost to get the job done, but thermals will typically provide less than 500 feet per minute of vertical speed… not enough to even keep a lightweight Cessna in the air!)
Now with the Space Shuttle, it is certainly true beyond doubt that it has engines. Three liquid-fuel rockets puting out 375,000 pounds of thrust EACH, to be exact. (To put this in perspective, a fully-loaded Boeing 737 tips that scales around 130,000 pounds or so, so EACH ENGINE of the orbiter could punch the Boeing straight up at 3 G’s indefinetly… and that is not even considering the solid rocket boosters attached to the Shuttle’s fuel tank that provide MILLIONS of pounds of thrust!)
I think this safely establishes that the Space Shuttle has engines.
The problem is FUEL. The orbiter exhausts everything it’s got getting up INTO orbit, and there is nothing left for the trip down: Thus the ship is a glider all the way from orbit to touch-down on Earth. With the final bit of fuel that is left after the mission, the orbiter fires its smaller de-orbit engines to slow it down to a bit over 15,000 miles per hour (I love saying that… SLOWING DOWN TO A BIT OVER 15,000 MILES PER HOUR) and begins it’s descent into the atmosphere.
Now we have to remember the cardinal rule of gliding: ALWAYS AIM LONG (PAST your landing point, not short), BECAUSE IF YOU EVER AIM SHORT YOU ARE DEAD SINCE YOU CAN NEVER MAKE UP LOST SPEED OR ALTITUDE WITH NO ENGINES. Aim LONG since you can always dissipate the extra speed and altitude with turns or speedbrakes if you wind up being too HIGH, but you are SCREWED if you come up SHORT.
Following this rule, the orbiter intentionally flies it’s glide from orbit EXTRA HIGH TO BE ON THE SAFE SIDE.
But there is one problem: If the orbiter flies it’s entire approach too high, won’t it glide right past Edwards?
And here is why: For most of the re-entry, the shuttle flies with the nose WWWAAYYY up for EXTRA drag, and making steep turns to intentionally dissipate the extra energy. The nose-up attitude and steep turns are very inneficient, causing the shuttle to slow down and come down to earth in a steeper glide-angle… and if it ever looks like the orbiter might not quite be able to make it to the landing zone, they simply lower the nose to be more efficient and level it out in roll to quit flying the steep turns… the orbiter then glides better, and they can stretch the glide to Edwards for sure. The extra speed and altitude is the ace up their sleeve, but the drawback is they have to constantly bleed the energy off through steep turns (up to 70 degrees bank angle!) and draggy nose up (up to 40 degrees!) to keep from overshooting the field!!!
OK, I will now walk you through the re-entry process from the beginning, as it is done in the real shuttle, and all of this carries over perfectly to the shuttle landing in X-Plane, which you will fly after reading this chapter.
After de-orbit burn, the shuttle heads for the atmosphere at 400,000 feet, 17,000 miles per hour, and 5,300 miles away from Edwards. (Yes, you are landing in the Mojave desert and you are starting your landing approach West of Hawaii). Not a bad pattern entry, huh? In reality, the autopilot flies the the entire 30-minute re-entry, and the astronauts do not take over the controls of the shuttle until the final 2 minutes of the glide. The astronauts COULD fly the entire re-entry by hand, but it is officially discouraged by NASA. The reason is obvious… these speeds and altitudes are way outside of normal human conception, so our ability to “hand-fly” these approaches is next to nil.
In the history of Shuttle missions (the 100th mission has just come to a close as I write this), the real space shuttle has been hand-flown for the entire re-entry only ONCE, by an ex-marine pilot, as I understand it, who was ready for the ultimate risk and challenge.
Oh yes, did I mention you will be hand-flying the entire mission in X-Plane, a well?
I have not gotten around to writing an autopilot for the Space Shuttle in X-Plane yet… I will have to do that some day… maybe after I sort my sock drawer…
Anyway, you start in X-Plane around 400,000+ feet, in space, coming down to eat air like a bag of bricks at Mach-20. Your control will be limited in space (you are operating off of small reaction jets on the Orbiter, set up as “Puffers” in Plane-Maker), but once the shuttle hits atmosphere, there will be some air for the flight controls to gret a grip on and you will actually start to be able to fly the thing. You will first hit air at about 400,000 feet, but it will be so thin it will have almost no effect at all. Your airspeed indicator will read around ZERO. Kind of odd since you are actually doing over 17,000, huh? Not really. The airspeed indicator works just like the wings of the orbiter: based on HOW MUCH AIR IS HITTING IT! And in space, that ain’t much! It will build gradually as you descend. The odd thing is that even though you are actually SLOWING DOWN, the airspeed indicator will RISE as you descend into thicker air that puts more pressure on the airspeed indicator! You LIKE this oddity of the airspeed indicator, though, since the air is also putting more pressure on the WINGS, so the airspeed indicator is really measuring how much force the WINGS can put out for you, which is really what you are interested in!
Bottom line: THE AIRSPEED INDICATOR INDICATES YOUR TRUE AIRSPEED TIMES THE SQUARE ROOT OF THE AIR DENSITY, SO IT INDICATES LOWER IN THIN AIR, BUT THE WINGS PUT OUT LESS LIFT IN THIN AIR AS WELL, SO THE AIRSPEED INDICATOR WORKS VERY WELL TO TELL YOU HOW MUCH LIFT YOU CAN GET OUT OF THE WINGS.
(Word to the wise: If the airspeed indicator is putting out MORE than about 250 knots, your wings can have plenty of lift to carry you.. if the airspeed indicator is indicating LESS than about 250 knots, then the wings do not have enough air hitting them to lift you, and you are still more or less coasting in the thin upper atmosphere where the air is too thin to do much for you.)
So as the airspeed indicator on the HUD gradually starts to indicate a value (as you descned into thicker air), you know it means you are starting to ease down into the atmosphere at 15,000 mph like a sunburned baby trying to ease into a boiling-hot chacuzzi: VERY CAREFULLY AND SLOWLY. Remember, if you were going 15,000 mph in the thick air of sea level, you would break up into a million tiny pieces in a microsecond… the only reason you can survive 15,000 mph up here is the air is so thin it has almost no impact on the ship. (And again, the airspeed indicator tells you how much the air is really impacting the Orbiter… 250 is a “comfortable” amount). The trick is for you to be going a lot slower than 15,000 mph by the time you get down to the thick air of sea level. And be at Edwards Air Force Base. And that is what the re-entry is for… dissipating speed as you descend so that you are never going too fast for the thickness of the air that you are in… you only descend into the thicker air once you have lost some speed in the thinner air up higher… the whole thing is a smooth process where you never ram the ship into thick heavy air at too high a speed.
Now as you begin to feel the out tinges of the earth’s atmosphere, you will notice a slight ability to fly the ship as you get some air over the wings and speed on the HUD.. now look at the picture of the orbiter on the right-hand EFIS display… the Atlantis already has this display retrofitted over it’s old steam guages (the EFISs from the Atlantis are modelled very accurately in X-Plane.. astronauts could use it for familiarization for sure). You see yourself and the path down to Edwards. Your goal is to stay on the center path. If you get above it, you are too fast or too high… you might overshoot! If you get below it, you are too slow or too low: You might not make it! (Remember ,the line is drawn with a large margin for error, so if you stay on the line, you have plenty of extra energy… getting BELOW the line a LITTLE will only tap into your speed/altitude reserve… getting BELOW the line a LOT will keep you from making it to Edwards) You must stay right near the center green line. The green line represents the desired SPEED for the early part of the re-entry, the desired TOTAL ENERGY for the middle part of the re-entry, and the ALTITUDE for the final phase of the re-entry. Don’t blame me, that is the way NASA set it up. If you are too FAST OR HIGH (above the center line) then it is time to dissipate some energy: put the thing in a steep bank, pull that nose up and hang on!
The REAL orbiter will be about 40 degrees nose up, in a 70 degree bank to try to lose energy, going 14,000 mph, glowing red hot, hurtling through the upper atmosphere on autopilot leaving a 10-mile long trail of ionized gas behind it while the astronauts just watch.
So how was YOUR day?
Anyway, you will do steep turns to dissipate energy as needed to keep the orbiter from going above the center green line. Look at the little blue pointer on the far left-hand side of that right-hand display. That indicates how high the nose is supposed to be. The green pointer is where the nose is now. Get that nose up. The pointers just to the right indicate the desired and current deceleration… you will not fly those, though. Look at the little pointer up top on the horizontal scale. That is the computer’s estimation of how much bank angle you probably need to stay on the center green line. Follow the computer’s recomendation or your own intuition for how much bank to fly, but keep that nose up for sure to keep you in the upper atmoshpere and fly STEEP BANKS to dissipate the extra speed and altitude. You might be tempted to just push the nose down if you are high. Don’t. You will drop down into the thick air and come to an abrupt stop from the tremendous drag, and then you will never make it to Edwards. You will wind up swimming in the Pacific somewhere around Hawaii.
Now, as you make your steep turns, you will be pulled gradually off course. Switch your turn direction from time to time to stay on course… turn left a while, then right, then back to the left again. That is what they do in the real Orbiter… you are slalom-skiing through the upper atmosphere at Mach-20. Not too shabby. Watch Edwards on your center EFIS display. You want to go there. Hit the “@” key to see yourself on a flyby. Fast enough for you? Hit the “w” key to get back in the cockpit. Caps lock off! Caps lock off!
As you approach Edwards, right on your center green line on the right-hand display, you will notice there is sort of a circle or something out past Edwards. This is your Heading Alignment Cylinder, or H.A.C. You will fly PAST Edwards at about 80,000 feet or so, fly AROUND THE OUTSIDE OF THE H.A.C. like you are running around a dining-room table or something, and then after you come around you will be pointed right at Edwards. And if you are on the green line still, your altitude will be just right for landing as well. This is usually where they turn off the autopilot and hand-fly the real Shuttle.
Now you are doing about 250 or 300 knots, coming down at about 15,000 feet per minute or so… about 125 miles per hour of descent rate. Do I really need to tell you what will happen if you hit the ground with that 125 miles per hour descent rate? Do not aim for the runway or you will wind up smeared along it in a thin buttery paste. Aim for the flashing glideslope lights 2 miles SHORT of the runway that I (and NASA) have thoughtfully provided for you. If they are all red, you are too low. (oops) If they are all white, you are too high (hit your speedbrakes, key “6” or use the mouse). If the lights are half red and half white, you are right on your glideslope. (about 20 degrees… airliners fly their approach at 125 knots, 3 degrees descent angle.. we use 250 knots, 20 degrees descent angle… not too unusual when you consider pattern-entry started West of Hawaii, actually).
OK so you are at 250 knots, on the green line, lined up with the runway, looking at half red, half white glideslope lights with the flashing strobes by them. Hold that approach configuration until the you are pretty close to the ground (3-degree glideslope to the runway), then level the descent and get your gear down. (“g”-key or mouse) Get the nose up for a flare as you approach, and touch down smoothly. Now lower the nose. Now hit the parachute and even the brakes if you want and let it roll out.
Now do it 100 times in a row without a single hitch and you are as good as NASA.
PS: Special thanks to Sandy Padilla for most Shuttle re-entry information!