Hi,
The F15 is a very large aircraft---thus it can carry that many missiles---. The JF17 is a small aircraft---there is limited space---therefore its utility is designed s such---.
Double racks are not a simple solution---because sometimes once the weapons are launched---they get entangled with the aircraft as there is very little space from the launch position and the weapon---.
Now if the missile rack has ejector like mounts like those on the F35 / F22---where the missile is actually pushed out hard to move it away from the body---that would possible work for he JF17---or the aircraft get the conformal fuel cells mounted on the aircraft and then there would be more BVR missiles---.
That entanglement phenomenon is not restricted to MERs and can happen with regular single weapon pylons too. Though it is almost always the case with bombs or at least I have never heard or read about it happening in terms of missiles. Someone else can explain the physics behind it, but I am guessing its something to do with the airflow and bombs acting as a lifting body. Missiles however are not free falling and have a rocket motor propelling them forward at much greater speeds than the launch aircraft. Eject launchers make sense when the missile is carried either internally or in a semi-recessed form, such as on Typhoons, Tornados, or the upcoming Korean KFX. Carrying missiles under the wing on a hardpoint removes that complication.
Overall, the technology for carrying multiple missiles has been around for decades and in service with a lot of aircraft. Technically, all attack helicopters carry multiple missile racks as well for anti-tank guided missiles. We should certainly look to develop this technology in house, especially if we are going to have our own weapons down the line.
I think it would be beneficial to see what types of missile racks are out there that we can get ideas for the JF-17s future development. Even the dual-rack used for bombs on the JF-17 can be refined to carry more bombs. There are also a lot of developments going on in this space that are in effect force multipliers on a smaller scale. These are integrating systems into the pylons themselves to make them multifunctional (carrying EW, ECM, Decoys etc) and are really handy for smaller aircraft with limited space.
Air-to-Air
Dual-LAU-127 on F/A-18E/Fs and F-15s
By Marvin Engineering Company
http://marvineng.com/product_category/launch-systems/
Air-to-Ground
Cobham AMGL III rack for Brimstone AGM:
Carried by Typhoon, Tornado, F/A-18, Harrier, Apache, & UAVs. India is interested in acquiring them for their Su-30s
https://www.cobham.com/mission-syst...nd-rail-launchers/agml-iii-datasheet/docview/
Electronic Counter Measures
Terma PIDS & PIDS+ (Pylon Integrated Dispensing System)
F-16s and T-50s so far.
https://www.airforce-technology.com/news/terma-modify-belgian-defences-f-16-pylons/
https://www.terma.com/media/105019/two-pager_defense_air_pylon-based_ew_solutions.pdf
There was a discussion on PDF before but it seems to have died down.
https://defence.pk/pdf/threads/term...ronic-combat-integrated-pylons-system.612429/