Exploring Airframes

[Carlobrand]

In the real world, flying craft use fractional-g engines to remain airborne by relying on lift surfaces - wings, or rotating wings in the case of helicopters. An F4, for example, had a loaded weight of 19 metric tons; its engines delivered 53 kN each normally and 80 kN each in afterburner, or between 0.55 and 0.84g depending on load and whether you're using afterburners - if my math is right. Some sources said its power-to-weight ratio could exceed 1 (in effect generating greater than 1g) if it was loaded light and hitting afterburners. An old Piper Cub had a maximum takeoff weight of 550 kG and a ~48kW engine delivering about 1.7kN, or 0.3g.

In the MT Referee's Manual world, grav vehicles in space apply gravitic thrust directly to generate acceleration, with the available thrust the same regardless of direction (provided you're within 10 planetary diameters) - which is to say you've got # g's available whether your thrusting upward, laterally, or downward.

Grav vehicles in atmosphere remain airborne by applying gravitic thrust to counter gravity directly; anything past that can be used to generate speed. However, per Errata 2.2, "...an airframe generates its own lift in an atmosphere. An airframe craft can glide to a landing without power if necessary."

In the MT COACC world, airframe vehicles apply all their g's to speed; their wings do the lifting.

So, the question: can an airframe grav vehicle apply all its g's to speed while in atmosphere, since lift is being provided by its airframe?

 

[far-trader]

In the MT COACC world, airframe vehicles apply all their g's to speed; their wings do the lifting.

Was it all lift from wings? I seem to recall it being some fraction and dependant on clean weight vs loaded. Though I might be thinking of Fire, Fusion, and Steel (TNE) design rules.

So, the question: can an airframe grav vehicle apply all its g's to speed while in atmosphere, since lift is being provided by its airframe?

Certainly. Again I recall a design example of that somewhere but may be confusing MT sources with TNE sources

 

[Carlobrand]

Was it all lift from wings? I seem to recall it being some fraction and dependant on clean weight vs loaded. Though I might be thinking of Fire, Fusion, and Steel (TNE) design rules...

I don't have TNE. I can't find anything in MT that quantifies the airframe lift effect other than a rule that says if your propulsion goes, you descend at a fixed rate of 10 meters per second until you hit the ground, at which point you roll to see if you can avoid a crash.

 

[sabredog]

I think, as pointed out by Far-Trader, that drag would have to be somehow added into the speed equation. In atmospheres, wing-loading and drag relative to thrust is going to determine a lot of things regardless of the propulsion used to power the craft.

Traveller has always used the anti-gravity loophole to avoid that highly variable (from one world to the next) real life issue. For every atmosphere density, which then changes with temperature and altitude, you would have to redo the calculations for a proper drag coefficient, lift and stall rate, cornering velocity,.....I already have a headache. I'd say just allow for the grav thrust to push the weightless, drag-less wingy thing through the atmosphere and call it good.

 

[Dragoner]

In the op examples given, the F-4 isn't a true lifting body but control surfaces, so that if the propulsion units lost thrust, the aircraft would lose control (most likely tumble). In the second example with the cub, it's airfoil is designed for lift coefficient, thus helping the puller effect of the prop (and giving some glide ability if the engine shut down). The caveat is that the two designs work against each other, lift working against super sonic speeds and angled control surfaces not having enough lift, esp at low speeds.

 

[hiro]

Most grav vehicles are also much heavier than aircraft. I'd seriously doubt the amount of lift generated would be enough to fly without a large negative agility, if at all.

 

[mike wightman]

You are getting confused with terms.

A lifting body uses fuselage shape to provide lift - the wings of a F 4 do provide a lot of lift.

In the event of an engine failure an F 4 would glide, however it's glide ratio is really bad, until it slows down below stall speed at which point it tumbles out of the sky.

Unstable in flight aircraft are a relatively modern concept and require very powerful computers to fly them, constantly adjusting the control surfaces in a way no human pilot could ever cope with. Engine failure and they glide - computer failure they fall out of the sky.

 

[Carlobrand]

...Unstable in flight aircraft are a relatively modern concept and require very powerful computers to fly them, constantly adjusting the control surfaces in a way no human pilot could ever cope with. Engine failure and they glide - computer failure they fall out of the sky.

Which interestingly is consistent with the MT rules. Under MT rules, if propulsion goes out then the flying craft descends at a fixed rate until encountering ground, at which time you make a roll to see if you can avoid a crash landing. However, if the power plant goes out, all the powered goodies go out with it and, "flying vehicles automatically crash: roll 3D on the Mishap table." (Player's Manual, p. 71. Me, I'd opt for a bit of battery back-up; the crew will thank you for it.)

That always puzzled me, but it makes more sense in light of your explanation, although I'd say the TL 5-6 barnstormers - and perhaps any craft relying on mechanical rather than electrically powered controls - ought to be excepted from the crash rule and allowed to glide down too.

 

[aramis]

That always puzzled me, but it makes more sense in light of your explanation, although I'd say the TL 5-6 barnstormers - and perhaps any craft relying on mechanical rather than electrically powered controls - ought to be excepted from the crash rule and allowed to glide down too.

Submit that as errata...

 

[sabredog]

Me, I'd opt for a bit of battery back-up; the crew will thank you for it.)

the vast majority of jet aircraft built after the 50's have incorporated Emergency Power Units that are ram-air, prop-driven, or hydrazine turbine units that pop out and provide enough juice for a limited time to power the emergency systems on an aircraft. It will allow for the basics so you can use the control surfaces, radios, and other things - but no engine start. It means the crew can keep the plane level and stable for a while and get it to glide long enough (hopefully) that they can at worst bail out with a better chance of rescue and no injury. There have even been a few instances of pilots returning from missions gone hairy in Vietnam that have landed Phantoms and F-100's on EPU power because they lost so much fuel from holes in the plane they had the engines shut down on approach.

So some sort of back-up system similar to that ought to be incorporated from maybe TL-8+? At higher TL's the emer power system lasts longer?

That always puzzled me, but it makes more sense in light of your explanation, although I'd say the TL 5-6 barnstormers - and perhaps any craft relying on mechanical rather than electrically powered controls - ought to be excepted from the crash rule and allowed to glide down too.

Maybe, but some modern jet aircraft, depending on the TL, will still be mechanically controlled (Me-262, MiG-15), and then some will be mechanically controlled, but with hydraulic boosting (F-4, MiG-29) - and when the power goes down on those they can still function but they will (depending on speed and altitude) feel like what your car's power steering is like without power. True fly-by-wire aircraft (F-16, SU-27+) are the ones that are really unstable without power and those would definitely need an EPU to function at all without power and drop like a rock otherwise.

And then, a lot will realistically depend on design issues: the F-105 had hydraulically-boosted controls and EPU units, but was nicknamed the "Thud" because if the power went out it's design characteristics (the ones that made it so screaming fast and still carry a lot of ordnance) ensured it turned into a Mach+ lawn dart even from altitude.

 

[Timerover51]

Let me complicate things a bit more for you.

In the MegaTraveller Referee's Manual, page 68, it covers radars and radar direction finders. For a Tech Level 6 radar, circa 1950, so figure WW2, it gives the following:

TL6 Radar, Range=5 kilometers (distant), weight of 250 kilograms, volume of 0.5 cubic meters, cost is 250,000 Cr., all-weather radar cost is 375,000 Cr.

The following is the data on the US AN/APS-6 3cm Radar that the US developed in WW2 and had flying in 1944. For full data, I will email you the Pilot Manual for the Radar, or see the following website: http://www.ibiblio.org/hyperwar/USN/ref/NightFighterRadars/index.html

Max. Reliable Range on:
Bombers, at 10,000 yd. 10,000 yd.
Fighters, 8,000 yd. 8,000 yd.
Battleships 35 miles

So depending on the target size somewhere between 7.3 Kilometers and 65 Kilometers.

Weight: 242 pounds less cables, or about 110 kilograms, volume might be about 0.5 cubic meters, if you included the radome.

Power Supply: 780 watts

Unit Cost: $10,938, and if you assume $4 per Imperial Credit, 2734.5 Credits. 2161 were delivered to the US Navy by 1 August 1945.

And that was an airborne radar unit carried by the US Navy night-fighting version of the Hellcat and Corsair, and also by the Avenger. Interesting comparison of the real world to MegaTraveller.

With respect to Radar Direction Finders for TL6, the MegaTraveller specifications are:

2.5 tons, 25 Million Credits, and 5 cubic meter volume.

The Germans were using the Metox detector for British Air to Surface Vessel radar in 1943, which basically was a couple of antennas hooked-up to a radio receiver. The Japanese were picking up our microwave radar transmissions and tracking our surface ships by mid-1943 during the Solomon Islands campaign. Japanese ASW aircrafts were using the US submarine Air Search radar to home in on a surfaced sub. The Germans were using an airborne detector on their nightfighters to home in on the tail warning radar of the RAF bombers, again by 1943/44. None of those units came anywhere remotely close to the MegaTraveller specs, especially with respect to price.

Enjoy.