NASA has this week submitted its first proposal for the heavy-lift rocket it’ll build to succeed the Space Shuttle. Among the science and political constraints, it sounds extremely sensible, with one hitch: NASA says it can’t afford it.

Over the last year or so we’ve been excited by the initial successes of the Project Constellation space program–destined to follow the Space Shuttle–confused by various changes of heart by the government, and then intrigued by the promise of $6 billion in funds for a new heavy lift rocket. This week came NASA’s decisive response to the future of its manned space program, and an associated heavy-lift rocket to take over the Space Shuttle’s large satellite launch powers when the Shuttle retires. The initial proposal seems very smart, given the mess it could’ve become.

Explaining how its Space Launch System and Multi-Purpose Crew Vehicle program, will proceed, NASA notes “With the President’s signing of the NASA Authorization Act of 2010 on October 11, 2010, NASA has a clear direction and is making plans for moving the Agency forward.” Conscious of the political efforts needed to get this far, the Agency also notes “this is a time of opportunity for NASA to shape a promising future for the Nation’s space program. Today it is no longer a question of IF we will explore, but how.”

Then there are some 20 pages of technical and financial plans. Here’s how NASA will build its new rocket: It’ll take the core technology of the Space Shuttle’s launch systems, combine them with advances in solid- and liquid-fueled engine tech that have been made during the early part of the now-cancelled Project Constellation, and add in some clever thinking. Essentially, NASA’s making a patchwork effort so that no expertise that’s been gained during the 40-ish years of Space Shuttle research (and since) is wasted.

The new rocket’s main stage will have a fuel tank structure based on the big orange tank familiar from the Space Shuttle

The Initial thrust will come from five Space Shuttle Main Engines, with later rocket versions using uprated designs

Two Solid-Rocket Boosters will sit either side, just like the Space Shuttle’s–but they’ll be higher-thrust five-segment versions, which have already been in advanced testing for the now-abandoned Ares-1 rocket.

The second stage will use a J2-X engine, new but based on Apollo program Saturn V rocket tech, and already in advanced development for Constellation

The Multi-Purpose Crew Vehicle will be based on the Orion crew capsule, which was the only part of Project Constellation that remained funded post-cancellation, and is highly developed

It all sounds fabulous, and should be capable of launching 70-100 tons of cargo, including a crew capsule, up to low Earth orbit–and it could act as a service/supply vehicle for the International Space Station “in the event that such requirements are not met by available commercial or partner-supplied vehicles” (a back-up for the young commercial space business’s plans). It’ll even use actual Space Shuttle engines at first, around 20 of which are available, before later versions use newer tech to make a rocket that can lift up to 130 tons to space.

But there’s one huge issue: “With […] traditional cost models, development of these systems did not fit within the funding and schedule specified in the Authorization Act of 2010.” This means “while the Authorization Act sets a goal [for test flights] of 2016, a first flight this early does not realistically appear to be possible based on our current cost estimate for the Reference Vehicles.” In other words, NASA doesn’t think its funding will let it develop the rocket on-budget and on-time, even with the huge re-use of existing tech. Though NASA notes it will make use of “current investments and workforce” it needs to design with “innovation, robustness and affordability” in mind, and “employ modern manufacturing and processing techniques, improved insight and oversight practices” to keep things on track. Costs will be minimized by streamlining “infrastructure requirements.”