Does core size effect fairing size

[alexterrell]

One of the alleged advantages of Liquid Hydrogen rockets is a larger core size, which allows a larger fairing size. A disadvantage of Falcon Heavy for example, is that a 3.66m core diameter limits the fairing size, compared to the 8.4m core of a Jupiter.

To what extent is this really true?

I would have thought that the fairing size is more influenced by:
- Total mass
- Length from engine to centre of mass
- Thrust

If a kerosene rocket has the same lift capability as a hydrogen rocket, but greater mass and hence thrust, why is the fairing size smaller?

The only issue I can think of is differential wind sheer, which should be solvable (if an issue) by putting fins at the rocket end.

Falcon Heavy already has the issue the the cross section at payload is much less than at core, in one direction. Could it have a 12m wide, 3.66m deep fairing?

[pippin]

To what extent is this really true?

It's complete nonsense.
You could always build a shorter, wider rocket.
It's like stating that it's good that it's more expensive because that way the price difference to the payload is not as big which justifies the use of a more expensive payload.

[kevin-rf]

Remember the Falcon 9 core is sized to be transport by tractor trailer. It is thus taller and thinner than one of an ideal mass fraction. Meaning the first stage is not the optional shape to carry the amount of fuel and oxidizer it needs to carry. The first stage airframe is heavier than ideal in order to make transport easier.

It is always a compromise

There examples of rockets with "hammer head" fairings, Delta III and Scout come to mind.

[Michael Bloxham]

... A disadvantage of Falcon Heavy for example, is that a 3.66m core diameter limits the fairing size, compared to the 8.4m core of a Jupiter.

To what extent is this really true?

I would also very much like to know the answer to this question!

Now that Falcon Heavy is aiming for 53 tonne payloads, it seems to me that its current PLF size of only 4.6m internal diameter would seem very restrictive. I wonder how much potential there is for SpaceX to offer a much larger diameter payload fairing for customers like the DoD (to allow spy sats with larger diameter primary mirrors, for example)?

So really there are three questions that might be asked:

1. Would market demand (e.g. DoD) justify the development cost of a larger PLF (diameter)?

2. What diameter might be expected?

3. How much of an engineering challenge would it be?

And perhaps also a fourth (just for kicks):

4. How big could you go, theoretically?

Also, for the sake of visualization, I made a model of Falcon Heavy with a monster 8m diameter PLF in SketchUp (image attached).

- Mike

[pippin]

4. Until the added structural weight eats up all your payload.

[Zed_Noir]

Does the fairing have to be cylindrical with a round cross-section?

[pippin]

no, but anything else will be structurally less efficient, that means more heavy.

[joek]

Does the fairing have to be cylindrical with a round cross-section?

No, but optimum shape depends on many factors; e.g., see:
Design and Testing of a Large Composite Asymmetric Payload Fairing
http://www.esm.vt.edu/ESM100_Presentations/ESM100_Ochinero.pdf (presentation)
http://www.ata-e.com/uploads/2009_9.pdf (paper on which presentation is based)

[Antares]

It pains me that no one has mentioned aerodynamics and center of pressure thus far on this thread.  That's the crux, kids.  I have something to say about transonics, but I don't know if the Iranians and Chinese are listening.

[joek]

I believe that is addressed by the paper in previous post... lots of factors to be considered

[kevin-rf]

It pains me that no one has mentioned aerodynamics and center of pressure thus far on this thread.  That's the crux, kids.  I have something to say about transonics, but I don't know if the Iranians and Chinese are listening.

So we should highly suggest such a Hammer head fairing to them

In all seriousness, most war shots do not need a fairing and are fairly compact.

[alexterrell]

It pains me that no one has mentioned aerodynamics and center of pressure thus far on this thread.  That's the crux, kids.  I have something to say about transonics, but I don't know if the Iranians and Chinese are listening.

I think Aerodynamics is self evident.

Centre of gravity is going to be a bit lower on a kerosene rocket, with no light H2 tank at the bottom.

How does a narrower fairing effect the centre of pressure?

[Patchouli]

Falcon heavy will most likely fly with a hammer head fairing.

As for stability control authority also has a play in it.

A rocket with lots of power on it's control systems can deal with the center of pressure being too far forwards better then one without.

Spacex also could just add fins if needed.

Having a fairing wider then the rest of the rocket does not necessarily mean it'll be unstable as seen with some model rockets that carry up an egg as the payload.

[kevin-rf]

Having a fairing wider then the rest of the rocket does not necessarily mean it'll be unstable as seen with some model rockets that carry up an egg as the payload.

Umm... It gets how fast again, Has burn time of how many seconds? That is like comparing an apple to a plastic orange and saying they are the same.

[baldusi]

The big problem with the Falcon Heavy is that the Merlin 1C can't throttle. So I assume the Merlin 1D won't either. A bigger fairing means not only greater drag, but a lot of more compression efforts at MaxQ. This would make a bigger fairing really heavy. The true problem is the rest of the thrust structure upto the engines. I really don't know if there's enough margin there to push twice the stress.

[alexterrell]

Just to sum up my take on fairing size:

A Kerosene / LOX rocket CAN have a fairing of the same size as a similar sized LH2 rocket.

However, there is a performance hit to having a larger payload fairings. A Kerolox rocket already has the performance gain of a smaller fairing (and core diameter) built into it's performance figures - one of the advantages of Kerolox.

The performance hit for Falcon Heavy, for say a 6m or 8m fairing has not been quantified.

I assume it would be possible to construct a graph of payload vs fairing diameter, and this will slope negatively towards zero.

[baldusi]

There's the small issue, that the lower density of H2 gets better performance from a more efficient tank. In other words, a shorter and wider tank is more weight efficient for a given volume. Of course a wider tank needs higher structural strength because of the MaxQ Pressure. But just compare the Delta IV and Atlas V to see that the Kerolox gets away with thinner core. That does gives it a slightly better performance, if you also have a smaller fairing.

[RocketmanUS]

For each of these tanks what is the maximum diameter of a fairing that they could handle ( standard cylindrical looking fairing flared out at the base )( Not looking at any particular launcher )?

3.66m diameter tank

5m diameter tank

[baldusi]

For each of these tanks what is the maximum diameter of a fairing that they could handle ( standard cylindrical looking fairing flared out at the base )( Not looking at any particular launcher )?

There's some discussion if it's 1.5x, 1.67x (as Titan IV did) or 2.0x (stated by many, never actually done. Also, length of the fairing affects. For the Atlas V, for example, the Long 5m fairing is so long (45% of total length) that it's questionable if the core's width matters that much.

3.66m diameter tank

That would be between 5.5m and 6.1m (1.5x and 1.67). But you should take about a 10% for the internal size (about 5m and 5.5m internal width)

5m diameter tank

That would be 7.5m and 8.35m. Again, that's abut 6.7m and 7.4m internal space.