SpaceX Falcon Heavy vs. Delta IV Heavy

[MrZayas1]

Whilst taking a look on SpaceX's website for the Falcon Heavy rocket, I noticed the comparison between the Falcon Heavy and the Delta IV. The Delta IV is a bit larger in size but has a bit more than half of the payload capacity of the projected payload tonnage of the Falcon Heavy. This may have been discussed before, I'm not sure. My pondering is, why does the Falcon Heavy, despite it's size and similar staging, have more payload capacity than the Delta IV heavy?

Is it because the Falcon Heavy uses RP-1/LOX as its fuel as compared to the LOX/LH fuel mixture of the Delta IV? If so then what is the purpose of using the LOX/LH fuel mixture in the Delta IV when you can seemingly get more payload capacity from using RP-1?

I think this topic is pretty good for discussion, and I'm curious as to why scientists would engineer a vehicle with such a fuel mixture when you can get more payload capacity from using RP-1. Maybe its just me thanks in advance!

[Kryten]

Part of it is that the payload comparison is quite misleading; it gives LEO numbers, while neither Delta IV heavy nor Falcon Heavy are primarily designed for LEO missions. Payload to GTO (where Falcon Heavy is likely to put most of it's payloads) gives the ratio as about 3:4, and payload to direct GSO (as with most Delta IVH missions) is roughly equal. The other part is that Delta IVH doesn't make terribly good use of it's LOX/LH2 booster stage; the engines were designed for simplicity and low cost rather than performance, and only use hydrolox because that's where most of the US rocket engine expertise was in the post-shuttle era.

[Streetwind]

Is it because the Falcon Heavy uses RP-1/LOX as its fuel as compared to the LOX/LH fuel mixture of the Delta IV? If so then what is the purpose of using the LOX/LH fuel mixture in the Delta IV when you can seemingly get more payload capacity from using RP-1?

How do you define "get more from using RP-1"? This isn't simply an issue of filling a different fluid into the tanks, you know.

The rocket equation doesn't care about "amounts" of something. It cares only about mass fractions. Liquid hydrogen has an extremely low density. That means in order to get a given mass of fuel, you need a huge volume to hold it, and therefore huge tanks. That's why the Delta appears so large.

The Falcon Heavy uses a propellant that's much more dense. That means it needs less tank volume to hold a given mass of fuel. In fact, due to being of similar size to the Delta, it manages to hold significantly more fuel by mass. If you compared the weight of a fully fueled Delta IV Heavy and a fully fuelled Falcon Heavy, the latter is much more massive. Wikipedia says 733,000 kg (Delta) versus 1,462,836 kg (FH). The Falcon is literally twice as heavy.

Because it has so much more propellant mass, the Falcon Heavy can lift a greater payload into low Earth orbit.

But there's a reason the Delta IV uses liquid hydrogen: it's a much better fuel. LH2/LO2 combustion (also called hydrolox for a shorthand) is by far the best (sane) rocket fuel that exists in the world, by efficiency. The RS-68A engines on the Delta get an Isp of 365s (ASL) - 414s (vac)... and that's for a fairly cheap, low-effort engine. If you really put engineering effort to it, you get something like the Space Shuttle's engine, the RS-25: Isp 366s (ASL) - 453s (vac). Now compare the Falcon Heavy that must make do with kerosene-burning Merlin engines... no matter how well you build those, they just can't compare. The Merlin 1D manages an Isp of 282s (ASL) - 311s (vac).

This is also the reason why the Falcon Heavy's performance takes such a sharp nosedive as soon as you try to go beyond low Earth orbit. The extreme Isp advantage and low weight of the hydrogen-fueled DCSS (Delta Cryogenic Second Stage) just walks all over the heavy, kerosene-fueled Falcon upper stages in space. SpaceX took a Merlin and bolted a large vacuum bell onto it, calling it Merlin 1D Vacuum ("MVac"), but that still only improves the Isp to 340s... simply not competitive with dedicated hydrogen vacuum engines which can get as high as 465s.

In other words, if you really wanted to take full advantage of the Falcon Heavy's 50 ton to LEO capability, you'd want it to lift a 50-ton hydrogen fueled spacecraft instead of trying to throw something smaller by itself.

[Kartoffelkuchen]

Well..SpaceX will probably lower the performance data of their Falcon Heavy anyway, they did that with every of their rockets.

[SciMan]

Agreed, the Falcon Heavy is great for lower-altitude uses, but starts running out of performance when trying to push "inert" mass (satellites, not other rocket stages) to high orbits.

A Kerolox rocket is great for getting out of the atmosphere, as it's quite a dense fuel which avoids aerodynamic drag losses.

Meanwhile, Hydrolox is much better suited for upper stage use, because once you're out of the atmosphere, the only things that matter are thrust, mass, and specific impulse.

Extrapolating from that ends up explaining why SRBs are usually only found on the lowest stages of a rocket, and why ion engines are used on deep space probes.

Hmm, wonder what would happen if SpaceX decided to switch to a Methalox upper stage of the same mass as the Kerolox upper stage currently on the F-H.

Methalox has a lot better specific impulse compared to Kerolox, without the severe cryogenic problems (boil-off, etc) or very low density of Hydrolox.

I'd bet it would improve the payload capacity to GTO, not sure about LEO performance but it would probably not improve quite as much, instead just end up not needing as much propellant mass.

[MrZayas1]

So, is there a real reason that SpaceX didn't change to use hydrolox in their upper stage to gain more DeltaV? I'm guessing it's that the tanks take up too much space for it to work well, trying to learn about these things is fun

[SargeRho]

Because they would have to develop a new engine for that. The Vacuum Merlin is just a slightly modified Merlin engine with a vacuum optimized nozzle.

[sojourner]

They'd have to develop a hydrolox engine and they'd have to redesign the tankage for the upper stage. Both of which would work against the common tooling approach they use for keeping manufacturing costs down.

[SciMan]

As I mentioned above, I think Methalox would be a better choice than Hydrolox. Methane isn't even close to as hard to keep stored as Hydrogen, is much denser, and can still produce a much higher ISP than Kerosene.

And I recall that SpaceX has already been working on a Methalox engine, IIRC it's called "Raptor".

If they had to redesign the tankage for the upper stage, it would most likely involve a tank stretch, not a move to a new tank diameter. The internal bracing probably wouldn't change all that much, and the plumbing would be nearly identical.

Of course I don't know what thrust scale that engine is intended to have, but if it's too big, surely the experience gained in making a big rocket engine scales down to making a smaller one, right?

I know that problems happen when attempting to scale UP a rocket engine (combustion instabilities, POGO) but if they already took care of that on a large engine the smaller one should be easy, right?

Edited June 22, 2015 by SciMan

[Streetwind]

Also simplicity. Hydrogen is a pain in the bum to handle. It embrittles most known metals, drifts through solid container walls, needs to be kept at an absurdly low temperature to stay liquid, and as mentioned before, even then has a very low density. Using it requires you to jump through many, many hoops.

Meanwhile, RP-1 is just a highly refined form of kerosene... in other words, you can just carry it around in a bucket if necessary. Stinks a bit, though

@SciMan: theoretically, yes. Practically, no such smaller engine development program is known to be in progress. The Raptor will, according to latest estimates, have about three times the thrust of a Merlin 1D. But those estimates have changed a lot over time. SpaceX is searching for the configuration with the highest engine TWR, and it will have whatever thrust that point happens to offer.

Edited June 22, 2015 by Streetwind

[Rakaydos]

@SciMan: theoretically, yes. Practically, no such smaller engine development program is known to be in progress. The Raptor will, according to latest estimates, have about three times the thrust of a Merlin 1D. But those estimates have changed a lot over time. SpaceX is searching for the configuration with the highest engine TWR, and it will have whatever thrust that point happens to offer.

...and then clustering it like pre-nerf Rocketmax 48s on an eve lander.