Why Was the DCSS/ICPS Designed This Way (launch vehicle)?

[MattRob]

So, recently I've been interested in the SLS - renewed particularly on hearing it'll launch this year for the first time. That's super exciting news.

But looking into it (and building a replica of one in Realism Overhaul of Kerbal Space Program...), I found the ICPS (Interim Cryogenic Propulsion Stage), which is a modified DCSS (Delta Cryogenic Second Stage), and I think it looks incredibly awesome, but I can't help but wonder why they built it like this:

Edit: Very high resolution image.

As can be seen here, this design, awesome as it looks, means that a fairing structure has to reach up to the upper Liquid Hydrogen (LH2) tank.

Typically, launch vehicles will combine the walls of the tanks with the structural walls of the vehicle. But for some reason they opted to make the Liquid Oxygen (LOX) tank significantly thinner than the vehicle so its walls are not structural supports for the rocket until the upper stage engine fires.

It seems very counter-intuitive to do this. It means you need a second structure - you need the structural elements connecting the LOX and LH2 tank as well as the fairing walls that support the LH2 tank during the first stage burn.

It just seems like it adds a lot of completely unnecessary dry mass. So why did they design it this way instead of the more usual way? Does anybody know or have any ideas?

I was going to ask why it was so unoptimized, in terms of being so much smaller than the stage before it/having a much lower mass fraction, but in hindsight it somewhat makes sense - perhaps to drop the first stage just before orbit, then do orbit and orbit adjustment operations (adjustment or placement into GTO) using the tiny thrust, but high-efficiency RL10-B2?

Still doesn't explain the odd design, though.

EDIT: Perhaps it's related, though - that the LOX tank walls would have to be much stronger to take the peak G forces just before first stage cutoff, but this way they can be much lighter since they don't take the pressure from supporting the entire vehicle under high g-forces, and don't need to since the second stage has such low thrust?

 

[tiredofms]

The LH2 tank mass is proportional to the amount of Hydrogen needed for the Second Stage burn. Same as the Oxygen tank which is proportional to the job that needs to be done. The X-Panel design was incorporated to reduce the overall weight of the combined Oxygen tank structure and fairing walls. Each panel can withstand a load of over 98000 pounds. Considering there are 8 panels that's a lot of payload.

 

[trurle]

...

Typically, launch vehicles will combine the walls of the tanks with the structural walls of the vehicle. But for some reason they opted to make the Liquid Oxygen (LOX) tank significantly thinner than the vehicle so its walls are not structural supports for the rocket until the upper stage engine fires.

It seems very counter-intuitive to do this. It means you need a second structure - you need the structural elements connecting the LOX and LH2 tank as well as the fairing walls that support the LH2 tank during the first stage burn.

It just seems like it adds a lot of completely unnecessary dry mass. So why did they design it this way instead of the more usual way? Does anybody know or have any ideas?

I was going to ask why it was so unoptimized, in terms of being so much smaller than the stage before it/having a much lower mass fraction, but in hindsight it somewhat makes sense - perhaps to drop the first stage just before orbit, then do orbit and orbit adjustment operations (adjustment or placement into GTO) using the tiny thrust, but high-efficiency RL10-B2?

Still doesn't explain the odd design, though.

EDIT: Perhaps it's related, though - that the LOX tank walls would have to be much stronger to take the peak G forces just before first stage cutoff, but this way they can be much lighter since they don't take the pressure from supporting the entire vehicle under high g-forces, and don't need to since the second stage has such low thrust?

The design is made mostly to avoid large diameter/height of oxygen fuel tank. Wide and short tanks are problematic from engineering (weight-efficient thin plate may be not available) and propellant management (propellant sloshing become bad, and ullage flows slow) perspectives. If you take into account anti-sloshing vanes and ullage sponge (collectively called PMDs), small diameter oxygen tank may be actually lightest solution.

 

[achilles03]

There are two reasons the LOX tank is designed this way. The first reason is so the upper stage doesn't have to carry much mass into space. A cylinder designed to resist compressive loads has a lot of mass... mass that will just slow the upper stage down. In this case, the compressive loads bypass the LOX tank, which keeps the LOX tank and supporting struts (aka X-braces) much lighter. The heavy cylinder that is carrying the compressive loads (interstage) has to be taller to envelope the LOX tank but it does not stay attached to the upper stage, allowing the upper stage to achieve a higher acceleration. Otherwise you'd have to haul a heavy cylinder with you to orbit. This is also known as a "hung tank" configuration.

The second reason is that making the LOX tank the same diameter as the LH2 tank would make it a less efficient pressure vessel (as mentioned in a prior response). Generally the most efficient dome shapes are between hemispherical and elliptical with major radius sqrt(2) times more than the minor radius. Any dome shorter than that will start incurring a significant weight penalty. You could make a tank that was bigger and had more empty/unused space (aka "ullage") but this too is more mass. Generally, the closer a tank is to spherical, the more efficient it is.