I don't see a Mk-4 as all funding would be poured into UMLV(which is a good thing).
What is the estimate on how long it will be before UMLV is in place as a working launch system?
UMLV is not a single launcher but a family of launchers (maybe very close to Angara) all whose basis will be the 2MN LOX/RP-1 engine under development. It will be more economical and easier to manufacture.
Yes, I'd seen you all compare UMLV to Angara before, in this forum. Other than Russia's Angara, does anyone else have a comparable system in development? The closest thing I can see is the Falcon family of rockets from budding private launcher SpaceX.
UMLV will be of the payload category of 2-3 tons to how much ever required to LEO. The RLV's capability is predicted to be 1 ton to LEO (TSTO version).
Are all types of payload-weights equally likely? Which are the main payload weight ranges that would be expected to be serviced? I'm assuming that UMLV could be used to service the wider global launch market just as easily as it could be used to serve Indian needs.
There is no RLV yet, so thinking of a manned RLV would be unrealistic. Anyway, it willn't be powerful enough to launch humans. Conventional launches are the way to go until the RLV technology is mastered.
Fair enough, I keep unconsciously thinking of RLV as a smaller version of the US Space Shuttle.
I'm thinking that RLV should only be developed as a scramjet anyway, which implies a narrower fuselage in which it's harder to accommodate larger payload sizes.
I'm thinking it should take off horizontally using rocket engines, then switch to scramjet in upper atmosphere when it reaches sufficient mach number, and then after scramjet acceleration has been maxed out while collecting more oxygen, finally switch back to rockets again for ascent to LEO.
I feel RLV should only evolve from a reusable scramjet vehicle, which itself is a project that should jointly be pursued with another country like Russia, as part of an intercontinental rapid-transportation solution. Even if you can't build a full-sized airliner with scramjet technology, you could perhaps build a 20-passenger vehicle and charge high prices to VIPs.
The eventual plan is to have a family of modular launch vehicles (UMLV) for medium, mid-heavy, heavy and super-heavy lifting and the RLV for light launches. The only exception could be the man-rated GSLV. Human-rated launchers are a huge investment and they tend to be in service for a longer periods of time.
That's why the cost of developing a man-rated SSTO vehicle should be spread to the regular commercial transportation market by way of a RLV-scramjet type of vehicle. The intercontinental rapid-travel market that was serviced by Concorde may face weaker demand than the broader conventional air travel market, but it's still much stronger than the space travel market.
Besides, now that the era of globalization has dawned, there is a greater demand for rapid intercontinental travel.
I sometimes wonder whether very high mach conditions in the upper atmosphere / ionosphere could support MHD/plasma propulsion, with its higher efficiency but lower thrust for longer acceleration period. Also, forming a plasma envelope around the vehicle would radically reduce its aerodynamic drag and thus its thermal heating at high mach conditions, which would be good for a high-mach craft.
The MHD/plasma propulsion would be powered by a small compact particle-bed nuclear reactor.
So, once again, 2 modes -- rocket plus MHD. Rocket motors let you take off from the ground horizontally, and gets you upto high mach conditions in the upper atmosphere/ionosphere, and then MHD takes over to achieve most of the required escape velocity over a longer acceleration period through the upper atmosphere/ionosphere where there is opportunity to collect oxygen. After enough oxygen has been collected, then rockets take over again to let you leave the atmosphere and proceed to LEO.
Furthermore, the technology used to maintain a plasma envelope for drag-reduction purposes in the upper atmosphere/ionosphere could eventually evolve into mechanism for maintaining a space-borne plasma envelope/bubble of very large radius for protecting astronauts and spacecraft against radiation. Just as the ionosphere protects the Earth and its inhabitants against radiation, a spacecraft could carry its own ionosphere in the form of a plasma bubble, as an absorbent barrier against high-energy EM radiation, which would otherwise be impossible to stop.
That way, the solar outbursts that broke down Chandrayaan-1 would not similarly imperil manned missions.