The dual-mode fighter (DMF) or "spaceplane" or "areospace fighter" is a wonderful source of contention for Sci-Fi. It really doesn't matter if they would have a real place in an actual space war - they are "too cool" to leave out.
From an engineering prospective, all fighters would eventually evolve into "spaceplanes" for entirely practical reasons. Where can they go? Where can they land? Where can they resupply? Where can they be repaired?
(FYI: I agree that most of the current depictions of DMFs are ridiculous - airplanes with big engines. I guess artist can easily draw airplanes.)
You believe a pure space-only fighter is better because it doesn't need streamlining. OK that seems reasonable. I might even buy that if I weren't an engineer. The question hinges on the question of "what is streamlining?"
A minimal space fighter is basically an engine (we will just treat fuel as part of the engine system), a cockpit and place to mount the weapons. You make it easy to launch/store/transport by making it either rectilinear (a brick) or round (a long cylinder.) How much streamlining does such a ship need? Does it need just enough to prevent it from burning up in an atmosphere? Basically just a nosecone and a few blisters for the weapon ports - give it a smooth outer skin and your done. Wings? Don't need them. The M-drive thrusters (4G - 6G) and thruster controls will be more than capable of controlling the ship in atmospheric flight. They don't fly using aerodynamic forces to provide lift - they fly using direct lift or vectored thrust. Does it need to be missile shaped to go fast? Not really too much - G6s of thrust is a lot.
Stick a Traveller 6G thruster on the back end of the US Space Shuttle - BAMB - you have a single stage to orbit space ship - and you could land on Mars in 40 Hours. (Well almost - the shuttle is designed to glide to a landing. Once it is on the ground - I don't think it has any way to pull the nose up for a takeoff. We might need a few thrusters around the hull.)
Making your vessel look like a pointy brick is a small price to pay for the added capability to take your mission all the way to the ground.
That is enough about "a lot of useless streamlining". It is not a lot - and it is far from useless. (Even in space the hull form would help bounce tiny space debris away from the ship.)
Lets look at your "an atmospheric fighter that's carrying around a lot of extra environmental support equipment" idea. I can see where you are coming from if your are talking about long endurance (heavy) fighter variants that have poped up in Traveller - the ones that include a small cabin on-board. That is a edge case, and we can skip over it.
We will start with the minimalist fighter (engine, weapons, cockpit, nosecone.) The cockpit is depressurized to avoid explosive decompression should the craft take combat damage. The pilot wears a Vaccsuit that provides his life support. For long missions the host fighter has auxiliary bottled O2 that can be piped to the flightsuit using an unbilical.
For an aircraft to have any chance to effectively fight off a DMF - it would have to intercept at high altitude. A life support system like a vaccsuit will be a requirement for high-altitude flight. That same vaccsuit seems to solve the life support problem. Again a vaccsuit doesn't seem to qualify as "a lot of extra environmental support equipment".
Just in case you don't believe that the life support gear would fit in an aircraft - I point you to this site: http://www.wvi.com/~sr71webmaster/press_suit001.html. We crazy humans have already solved the problem - and we did it in the 60s. Traveller vaccsuits are quite a bit better than the 1960s era S1030 Suit.
The conclusion of "Such a craft will get pasted by a similarly sized specialized fighter" is a bit short of the mark. First off, size is not really a point of comparison. Use cost instead. Traveller is about credits. He who controls the credits controls the ... Anyway. You must consider the broader cost of deployment and support in addition to the actual purchase price of the fighter.
The DMF is cost effective. The DMF can do every job the space-only fighter can do in space. The DMF can do every job the aircraft can do in the air. The DMF can do things that the other two cannot. The DMF can operate in two theaters of combat - space and atmosphere. The two-for-one is really hard to pass up during the budget sessions.
The dual-mode fighter (DMF) has combat-flexibility. It is tactically superior in the near-orbit transition space. The DMF can evade space-only fighters by diving into the atmosphere (that includes non-breathable atmospheres.) It can escape enemy aircraft by exceeding their operational ceiling. The aircraft cannot follow the DMF into space.
The space-only fighter (SOF) will have a little more payload - swaping the streamling for other gear. Does that really make it better. It might have a bit more armor, or an extra weapon, or more maneuver thrusters, or more thrust, or a bit more of everything. So what - it still cannot go where the DMF can go. The paper math says the SOF is more likely to win in one-on-one combat under ideal conditions. (FYI: Never fight in ideal conditions - only fight if you have some advantage.) Even if you build two ships to the exact same specifications, one with streamlining and one without, a victory would still not be guaranteed. As soon as an atmosphere is part of the equasion the DMF has the possibiliy of escape or suprise.
The DMF vs SOF of similar whatever is a kind of sillness. If you need a fighter with a bigger payload - make a fighter with a bigger payload - make a bigger DMF. You can get whatever you want in a design if you are willing to pay for it.
When your spending your credits to build your World Defense Fleet, do you spend the money on a DMF that can protect your ground, sky, and space. Do you buy two vessels, one aircraft and one space-only fighter, to cover the same job? Even better, spend the credits on two vessels - but make them both DMFs. Now you have two fighers that can operate both in the air and in space. Make the right choice and you are safe - make the wrong choice and you get killed.
When building your Invasion Fleet, you spend money on your SOFs that are slightly "better" than the DMFs. So the planet launches a squadron of DMFs at you. Your SOF squadron intercept them and combat ensues. After you win the combat, 20% of your SOFs are still combat capable. Now what? The planet doesn't have to send any more DMFs up into space just so that you can kill them. Your SOFs are not going to burn themselves up in the atmosphere chasing down the DMFs.
What do you do when you want to take the planet? Do you now land troops and then deploy your aircraft against the low flying DMFs defending the planet? You have to get the aircraft down into the atmosphere before they operate. Do you do that with some kind of aircraft carrier spaceship (ACS). As intresting as that sounds it also sounds like a big expense. It also sounds like a bigger risk if the carrier is shot down. (The aircraft cannot escort the ACS across the space-atmo boundary - only a DMF could do that job.)
(Honestly, an aircraft vs spacecraft encounter would be very lopsided. In all likelyhood, once the powerful spacecraft thrusters and vectored thrust controls become available (or even gravitic tech) - aerodynamic flight will probally be entirly replaced - or religated to hobbiest or specality excersizes. Streamling will only be conserned with reducing wind resistance to increase forward speed. Aerodynamic lift generation will no longer be necessary. Aircraft will become more bullet shaped and wings will get much shorter - if they don't vanish completely.)
This is begining to sound like a Trillion Credit Squadron project.
>On Tuesday, October 7, 2014 9:53 AM, Craig Berry <cdberry@gmail.com> wrote:
a>The idea of a dual-mode vacuum and atmospheric fighter has never made
>sense to me. The optimizations for each of them are so different that
>in combining them you end up with the worst of both worlds -- a space
>fighter that's carrying around a lot of useless streamlining, and an
>atmospheric fighter that's carrying around a lot of extra
>environmental support equipment. Such a craft will get pasted by a
>similarly sized specialized fighter.