On Thu, 01 Jul 2004 12:39:07 -0700, pacplyer wrote:
Kevin Horton wrote in message
...
On Mon, 28 Jun 2004 20:23:11 -0700, Tim Ward wrote:
Stall speed at extreme altitude would not the benign 180 knots, but
something appreciably higher (can you help me out with the high
altitude 747 data - actual stall speed at FL 450?).
I actually want to fly the 747 pretty fast. If its speed at 45000
feet is fast enough so that the spacecraft's airspeed at 100000 feet
is at the spacecraft's best rate of climb speed, then the turning
maneuver isn't required.
pacflyer gave a 1g speed range for the 747 at 45,000 ft and 580,000 lb
of 208 kt to 251 kt. I'm assuming those numbers are in KCAS. If so,
that works out to 444 KTAS to 524 KTAS under standard day conditions.
Goddammit. That's not right. I interpolated backwards on both numbers.
Good thing airlines use tabular data so you don't have to interpolate
Vref. Sure hate to go 10 kt the wrong way on that one
The 1g chart only allows a max weight of 550,000 lbs for low speed buffet
of 218kts and high speed of 242kts. and this being test pilot land, I was
trying to interpolate another 30,000 lbs into the equation (for total of
580K: which is just outside the envelope.) Sorry Guys. I should have
ADDED the ten knots to the low speed number and SUBTRACTED nine on the
high speed mach buff number. (hence the term coffin corner; range narrows
as you go up.) So 580K at FL450 would be: 228-233 interpolated out of the
envelope. Or you can just go with the stock 550,000 lbs numbers of
218-242.
So could you recalculate the below values Kevin? I promise not to play
test pilot any more!
If you've got 1 g buffet numbers of 228-233 at FL450 at 580,000 lb, you
don't have a practical envelope to do this mission. You'd be in buffet
the first time someone sneezed. So, I'll use 550,000 lb.
218 KCAS at FL450 equals 463 KTAS, which equals 54 KEAS at 100,000 ft.
242 KCAS at FL450 equals 508 KTAS, which equals 60 KEAS at 100,000 ft.
This still isn't anywhere close to a practical envelope for the orbiter.
At 100,000 ft, those same true airspeed values work out to 56 KCAS to 68
KCAS. The equivalent airspeed, which is what the wing sees, is a bit
lower at 52 KEAS to 61 KEAS. That means the orbiter needs to have a
very, very low wing loading, which doesn't seem compatible with a
re-entry. And it has to support the weight of the tether too. I'm not
sure this idea will work, unless we can get a much faster tow aircraft.
Detachable 200' U-2 type Glider Wings? ;-) Well U-2's fly at low TAS
speeds up there don't they? Hence my detachable 200 ft glider wing idea.
Let's see if I understand Tim Ward's logic in getting the vehicle to
100,000 ft. I might be wrong, but isn't the purpose of this to just get
the vehicle out of the "Max Q" area of the atmosphere? Building up to
"Max Q" is were space vehicles burn most of their propellant from the
first stage isn't it? Do we really care what speed it's at as long as we
can initiate Orbiter burn sequence up high? (I agree, high altitude AND
high speed would be better.)
Well, the orbiter needs to have enough speed to be above stall speed. The
drag will be very high if it is too close to stall. So 100,000 ft is way
too high to tow the orbiter, if we have a subsonic tow aircraft.
Now if you want high tow plane ground speed so that it takes less "Orbit
One"-fuel for orbital insertion, the old jet adage is: "You stay down low
if you want to go fast." Power, wing performance, and curvature of the
earth is "more better." If we stayed down at FL 350 we'd have to drag
another couple of miles of tether cable but: we'd be able to fly at a
550,000 lb weight AND do a 45 degree bank (1.41g.) Low speed stall buffet
would start below 249kts ind/cas . High speed mach buffet would not
happen until the absolute top certified speed of the aircraft .92 Mach (I
think that's just about 305kts IAS/CAS in that bank/alt by interpolation;
chart only says VMO.)
I don't know how position error is handled in the 747 Classic. M0.92 true
works out to 530 KTAS and 318 KCAS at FL350. But there may be a bit of
position error in the machmeter, so your 305 KCAS could be right too.
If you held a 45 deg bank turn, I think your turn radius would be just
under 25,000 ft (if I didn't screw this up - I've just had a couple of
beer). So, if you had a 20 km tether, and the orbiter was at 75,000 ft, I
figure his radius of turn would be about 77,000 ft, and he would be going
about 3.1 times as fast as you, or a TAS of about 1640 KTAS or about 350
KEAS. This might be achievable.
A 90-year-old woman could hold this range no sweat
in the very forgiving 747. So is that about 518 kts IAS/CAS? (somebody
check my math please.) But our ground speed will be higher down low,
right? Isn't that what we are really after for orbit insertion: high
tow-plane ground speed relative to the earths surface? Having trouble
with this cuz computers figure this out for us all the time now!
So I guess a trade off is going to emerge between tow plane ground
speed/bank advantage at lower altitudes and extra cable length drag. Now
that I've reviewed the charts; it seems like at FL 450 we're at the
extreme limits of the stock 747-200F with P&W Q powered engines. It
appears to me that maybe we need a lot more thrust if we're going to do
this on a routine basis at FL450. Someone mentioned using GE C-90's, of
which one was hung for testing on the 74 already. Four of those engines
would make getting up to .92 mach easy, but I'm sure they ain't cheap (10
mil ea?). Maybe Burt would do it anyway and just fly it under
experimental rules since no paying passengers are aboard that vehicle?
This would save tens of millions in re-engine certification costs.
The reason for me obsessing with the 747 is two-fold. The first is low
cost (I want to say about $10,000 an hour is typical op cost in some
fleets. Does that sound right Kevin?)
No idea. I work in the flight test world, so I don't have to bother
myself with the bean counters (no FAs or pax complaining if I go over 30
deg of bank either). Fuel cost is fairly easy to calculate, but I don't
know what maintenance and other costs are.
--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com