Where to put the tanks? And maybe the expected mass is too small?

This is the schematic where 2 tanks of the size we talked before would fit in the frame of the spaceship. The black lines between them are the fuel lines up to scale.

The tanks are small in respect to the frame of the ship, very hopelessly so... But keep in mind doubling the size of a tank multiplies by 8 its capacity (i.e 2^3) and we do not have the technology for hybrid air breathing jet-rocket engines that can push 1000+ tons of trust...

And we have a whopping 45 tons of dry mass, including the tanks themselves.

The hydrogen tank can be a scaled up version of the one BMW uses for their Series 7 hydrogen car:
 BMW 7 hydrogen
"The hydrogen fuel is stored in a large, nearly 30-gallon (110 liters),^[5] bi-layered and highly insulated tank that stores the fuel as liquid rather than as compressed gas, which BMW says offers 75% more energy per volume as a liquid than compressed gas at 700 bars of pressure.^[6] The hydrogen tank’s insulation is under high vacuum in order to keep heat transfer to the hydrogen to a bare minimum, and is purportedly equivalent to a 55-foot (17 m) thick wall of polystyrene Styrofoam.^[7]

To stay a liquid, hydrogen must be super-cooled and maintained at cryogenic temperatures of, at warmest, −253 °C (−423.4 °F). When not using fuel, the Hydrogen 7’s hydrogen tank starts to warm and the hydrogen starts to vaporize. Once the tank’s internal pressure reaches 87 PSi, at roughly 17 hours of non-use, the tank will safely vent the building pressure. Over 10-12 days, it will completely lose the contents of the tank because of this"

The space shuttle gives us an idea of how much this can be scaled up:  The liquid hydrogen tank is 331 inches in diameter, 1160 inches long, and has a volume of 53518 cubic feet and a dry weight of 29000 pounds i.e 13 metric tons.

The space shuttle tank is equivalent to a 10 meter in diameter spherical tank. Our requirements call for a 30 % smaller tank (1.1^3).

If we use conservative good techniology I would think a 5 metric tons tank will fit the bill since we are also dealing with a spherical shape, which is way much more efficient.

We can use the same kind of storage materials for LO2 although it is easier to store the later. A 3 metric tons tank will fit the bill.

So we have:

8 tons for the 2 tanks, and they will store 225 tons of propellants.

As for the engine, let's allocate about 12 tons. Skylon's plans call for 10 tons for the engines and we can add an extra 2 for additional components and redundancy.

The engines are the single most critical component of the whole system.

That leaves us with 45 tons - 8 - 12 = 25 tons for the remaining frame etc...

Things are getting tight... I will now cross my fingers we can somehow fit the frame supporting the engines and tanks and the hull in that number, about the mass of an 18 wheeler that needs to expand and fill the size of a 90 meters long ship.

Gotta love extreme challenges in one's lifetime...