alex_greene
Guest
I hate the sensation of tea shooting up my nose.rinku said:
Atmospheric operation of small craft, incorrect assumptions
- Thread starter DFW
- Start date
Ishmael said:
rinku said:
If I had been given numbers to crunch as I had asked for, I would have.
In fact, I've done so in the past...use the search function.
Nobody here has yet backed themselves up with any figures that mean much in terms of aerodynamics to begin with. Just speculation.
If the requested information for the grav vehicle in question were given, I would have provided a way of determining an approximation for the grav plane's Drag coefficient and thus its top speed. But no, the request for the information was ignored, thus given the impression that a discussion of real aerodynamics is not the purpose of this thread.
I see your point, and I attempted to come up with some data to work with,Ishmael said:
but in the end I would have had to take them from thin air.
While the surface area of the vehicle's front area could be estimated, for
example a Traveller cutter is about 7.5 m high and the front end could
probably be treated as a hemisphere with that diameter, there are no
real world counterparts that would allow me to estimate the mass of any
Traveller small craft's power plant and drive, it would remain pure specu-
lation.
Since no useful real world data for a grav vehicle's mass are available,
I turned to the best speculative vehicle design system I know, the one
of GURPS Vehicles, which produces acceptable results when trying to re-
tro-engineer real world vehicles.
As mentioned in my post above, this system even gives aerodynamic
drag and maximum atmospheric speed for the Traveller small craft, ba-
sed upon the system's assumptions concerning a grav vehicle's mass
- but these assumptions still are no more than the author's educated
guess, although this author usually guesses quite well.
thank you , rust
so the x-sectional area is ~44.17 m^2 (wow!...an F-16's wing area is 27.8m^2...a whole lotta drag going on )
The fineness ratio is needed to get a ballpark figure for Cd
I only need the mass to figure out thrust ( thrust ratio of 6 ), so if a number for thrust is available, that'd be good. Newtons is a preferred unit of measure.
I'll assume a standard atmosphere with a density of 1.025 kg/m^3
Something that no one's mentioned so far is that the grav vehicle's thrust, coming from maneuver drive requires the vehicle to change its attitude to use that thrust to change its vector. Any attitude change except along the roll axis will change that vehicle's profile and apparent cross-section with respect to the airstream causing all sorts of troubles.
Rinku: the space shuttle's thrust ratio during reentry, when it does some sort of actual maneuvering, is 0
And if we can consider lift to be a thrust agency acting on the vehicle's mass, a jet fighter's 'thrust' ratio along the direction of lift can momentarily be 12 and perhaps higher as tech advances. The limitation is that it can't maintain it due to drag.
That's where energy management comes in.
so the x-sectional area is ~44.17 m^2 (wow!...an F-16's wing area is 27.8m^2...a whole lotta drag going on )
The fineness ratio is needed to get a ballpark figure for Cd
I only need the mass to figure out thrust ( thrust ratio of 6 ), so if a number for thrust is available, that'd be good. Newtons is a preferred unit of measure.
I'll assume a standard atmosphere with a density of 1.025 kg/m^3
Something that no one's mentioned so far is that the grav vehicle's thrust, coming from maneuver drive requires the vehicle to change its attitude to use that thrust to change its vector. Any attitude change except along the roll axis will change that vehicle's profile and apparent cross-section with respect to the airstream causing all sorts of troubles.
Rinku: the space shuttle's thrust ratio during reentry, when it does some sort of actual maneuvering, is 0
And if we can consider lift to be a thrust agency acting on the vehicle's mass, a jet fighter's 'thrust' ratio along the direction of lift can momentarily be 12 and perhaps higher as tech advances. The limitation is that it can't maintain it due to drag.
That's where energy management comes in.
I have no idea whether this helps, but a 50 dton cutter capable of 6 G
designed with GURPS Traveller would have a mass of 39.15 tons and
a maneuver drive with a thrust of 300 tons (which would actually give
the cutter a higher G-rating, but the modular design system does not
include partial modules - you could raise the weight to 50 tons loaded
weight to compensate for this).
The fineness ratio ... no idea how to determine that, really.
The cutter is basically a cylinder with a length of ca. 30 meters and a
diameter of ca. 7.5 meters, but on most illustrations the diameter is
not exactly the same over the entire length, and the cylindrical hull
becomes a more conical one around the drive.
designed with GURPS Traveller would have a mass of 39.15 tons and
a maneuver drive with a thrust of 300 tons (which would actually give
the cutter a higher G-rating, but the modular design system does not
include partial modules - you could raise the weight to 50 tons loaded
weight to compensate for this).
The fineness ratio ... no idea how to determine that, really.
The cutter is basically a cylinder with a length of ca. 30 meters and a
diameter of ca. 7.5 meters, but on most illustrations the diameter is
not exactly the same over the entire length, and the cylindrical hull
becomes a more conical one around the drive.
again, thanks
300 tons thrust would be about 3e6 newtons of thrust
setting that as the total drag ( thrust = drag in level flight)
44.17 x-sectional area
air density of 1.2kg/m^3
drag=.5*density*x-sect_area*Cd*sqrt(V)
Cd is abase Cd for certain shapes, sphere, hemisphere, etc. and divided by the fineness ratio^.66667
sphere=.2
hemisphere, round front=.5
hemisphere, round aft=.8
flat plate=1.2
Approximations, of course ( they vary with reynolds number ) but good enough
The sonic boom will be pretty powerful
yikes...sorry, I missed the actual dimensions of the cutter (25ft diameter and 1ooft long )
30m/7.5m give a fineness ratio of 4 and thus a Cd of ~ .19 ( round front, blunt tail as normally pictured)
That'd give a top speed down to about ~2800km/hr @ sea level if partially streamlined
~ 3900kph @ 5.6km
~5600 @ 11.2km altitude ( on earth) where the density is about 1/4 what it is at sea level.
corrected a couple of mistakes... sorry for the inconvenience
300 tons thrust would be about 3e6 newtons of thrust
setting that as the total drag ( thrust = drag in level flight)
44.17 x-sectional area
air density of 1.2kg/m^3
drag=.5*density*x-sect_area*Cd*sqrt(V)
Cd is abase Cd for certain shapes, sphere, hemisphere, etc. and divided by the fineness ratio^.66667
sphere=.2
hemisphere, round front=.5
hemisphere, round aft=.8
flat plate=1.2
Approximations, of course ( they vary with reynolds number ) but good enough
The sonic boom will be pretty powerful
yikes...sorry, I missed the actual dimensions of the cutter (25ft diameter and 1ooft long )
30m/7.5m give a fineness ratio of 4 and thus a Cd of ~ .19 ( round front, blunt tail as normally pictured)
That'd give a top speed down to about ~2800km/hr @ sea level if partially streamlined
~ 3900kph @ 5.6km
~5600 @ 11.2km altitude ( on earth) where the density is about 1/4 what it is at sea level.
corrected a couple of mistakes... sorry for the inconvenience
A nice result, according to the official GURPS Traveller data the atmosphe-Ishmael said:
ric top speed would be 2,353 mph (ca. 3,800 km/h), and most probably
not calculated for sea level.
So we get approximately the speed that an YF-12 could achieve with the
technology of 1963.
Of course they are. Your numbers are almost exactly the same that DavidIshmael said:
Pulver came up with, the author of most GURPS technical stuff and of the
Transhuman Space line, and he is famous for his faible to use the most
realistic numbers possible for his science fiction stuff.
People have been suggesting "helicopter" as the best real-world analogy for a craft with gravity-cancelling technology, but really the answer is "zeppelin".
Imagine a zeppelin with all the weight and inertia of a cargo ship, yet still capable of making itself "lighter than air" in terms of defying gravity, and with engines capable of generating enough thrust to push it forward at a handful of g's acceleration, and you've got your non-streamlined grav-equipped ship.
Nothing about that design says it's fundamentally able to change direction faster than an equivalent vehicle with maneuvering surfaces.
The fact is, atmosphere is as much an enabler for maneuvering as it is a limitation. Sure, it resists your movement, produces friction, slows you down - but, if you have maneuvering surfaces, it's also something for your craft to push against, allowing you to transfer momentum and change your direction far faster than thrusters alone. An atmospheric aircraft can achieve levels of delta-V that one with equivalent engine output outside an atmosphere could only dream about.
There are trade-offs, and there are times when having the option to stop without falling is going to be more useful than having the ability to alter your velocity radically with minimal energy investment. But those will be situational. It is certainly not the case that a vehicle which can cancel gravity but can't employ aerodynamic manuevers will always have tactical superiority over one which can move aerodynamically but must maintain a minimum speed.
That being the case, it shouldn't be surprising that people on both sides can throw out examples of situational superiority for one design or the other - both can exist, but neither is proof of overall capability. The question of which is more capable overall depends upon the specific technologies, weaponry and physical limitations involved, and those are not sufficiently clearly-defined to draw any unassailable conclusion either way.
Imagine a zeppelin with all the weight and inertia of a cargo ship, yet still capable of making itself "lighter than air" in terms of defying gravity, and with engines capable of generating enough thrust to push it forward at a handful of g's acceleration, and you've got your non-streamlined grav-equipped ship.
Nothing about that design says it's fundamentally able to change direction faster than an equivalent vehicle with maneuvering surfaces.
The fact is, atmosphere is as much an enabler for maneuvering as it is a limitation. Sure, it resists your movement, produces friction, slows you down - but, if you have maneuvering surfaces, it's also something for your craft to push against, allowing you to transfer momentum and change your direction far faster than thrusters alone. An atmospheric aircraft can achieve levels of delta-V that one with equivalent engine output outside an atmosphere could only dream about.
There are trade-offs, and there are times when having the option to stop without falling is going to be more useful than having the ability to alter your velocity radically with minimal energy investment. But those will be situational. It is certainly not the case that a vehicle which can cancel gravity but can't employ aerodynamic manuevers will always have tactical superiority over one which can move aerodynamically but must maintain a minimum speed.
That being the case, it shouldn't be surprising that people on both sides can throw out examples of situational superiority for one design or the other - both can exist, but neither is proof of overall capability. The question of which is more capable overall depends upon the specific technologies, weaponry and physical limitations involved, and those are not sufficiently clearly-defined to draw any unassailable conclusion either way.
Ishmael said:
Thanks for clarifying Ishmael. I apologise for the tone of my previous post; you have backed yourself well.
I only included the Space shuttle because it's the nearest real world equivalent to a Traveller small craft (being in the same size range and having much the same role), so it does act as a source of comparable data. My comment re "leaving a jet in the dust" was a bit loose - naturally at lower altitudes drag is going to kick in. I guess what I meant is that an interface craft just has to nose up and power climb away from the jet.
