SLS vs. Falcon Heavy Raptor: Hypothetical Comparison

[DnA915]

I am not trying to start a debate again about if SpaceX should retroactively apply Raptor to existing rockets, but I am curious about what a back of the envelope payload calculation would be for Falcon Heavy Raptor if they did it with cross-feed. How would it stack up to the SLS block slated for mars explorer when being used in a fully reusable and expendable configuration?

[Jim]

I am not trying to start a debate again about if SpaceX should retroactively apply Raptor to existing rockets, but I am curious about what a back of the envelope payload calculation would be for Falcon Heavy Raptor if they did it with cross-feed. How would it stack up to the SLS block slated for mars explorer when being used in a fully reusable and expendable configuration?

With Raptor, it would not be able to be reusable. (too high of thrust)
It would have less performance, since methane has less power density

[DnA915]

With Raptor, it would not be able to be reusable. (too high of thrust)
It would have less performance, since methane has less power density

Could you explain what you mean by this? Obviously the ITS booster is reusable and slated for raptor. Why would this not be the case with the 3 smaller cores? Also, why do you say less power density? I do not know the energy in RP1 vs Methane, but Raptor has 4X pressure and therefore would be far more efficient in fuel weight to thrust, would it not?

[awbyrdjr]

With Raptor, it would not be able to be reusable. (too high of thrust)
It would have less performance, since methane has less power density

Could you explain what you mean by this? Obviously the ITS booster is reusable and slated for raptor. Why would this not be the case with the 3 smaller cores? Also, why do you say less power density? I do not know the energy in RP1 vs Methane, but Raptor has 4X pressure and therefore would be far more efficient in fuel wait to thrust, would it not?

When the present Falcon 9 stage is coming back to land, its tanks are mostly empty, and so it doesn't weigh very much.  A single Merlin, throttled down, can slow the stage down gradually enough that it can land safely.  They've tried to do it with three Merlins and had mixed results - once, the stage hit the barge so hard that it punched a hole in the deck.  A single Raptor throttled all the way down would put out even more thrust than three Merlins, and landing it safely would be nigh impossible.

While you're correct that a high-pressure methane engine would be more efficient than a lower-pressure kerosene engine, that's offset by another factor - methane isn't as dense as kerosene.  There's less energy to be had in a liter of liquid methane than in a liter of kerosene.  That means that while a methane engine gives you more impulse per unit mass of propellant, it gives you less impulse per unit volume of propellant.  Falcon 9/Heavy tanks would be too small for a methane engine - you'd run out of fuel too early.  You'd need to widen them - look at the wider tanks for liquid hydrogen rockets, like Delta IV and Ariane V, vs. narrower kerosene tanks on Falcon 9 and Atlas V.

[envy887]

Actually, the combination of lighter propellent, full flow cycle, and MUCH higher chamber pressure more than make up for the bulk density loss. Going to Raptor is about a 10% improvement. However, the biggest difference is a larger upper stage, by far. That gets nearly 25% more payload to orbit. Crossfeed does not do so well in my models.

For a fully expended all Raptor FH with larger upper stage, I get 82 tonnes to LEO with CF and 79 without CF. The same all-Raptor FH with the current sized upper stage gets 60t without CF and 70t with CF.

The best bang for the buck seems to be keeping the Merlins on the core and boosters, but using a larger Raptor upper stage. That gets 71t to LEO without crossfeed. Adding CF only gets about 1 more tonne, which hardly seems worthwhile.

All my numbers assume the boosters same the same size, volume, and weight as Block 5 Falcon 9. Orbit is 100 nm (185km) circular LEO. Everything is expended in these scenarios, but reuse makes crossfeed even more worthless and also drives strongly toward a larger upper stage.

[Jim]

Actually, the combination of lighter propellent, full flow cycle, and MUCH higher chamber pressure more than make up for the bulk density loss. Going to Raptor is about a 10% improvement. However, the biggest difference is a larger upper stage, by far. That gets nearly 25% more payload to orbit. Crossfeed does not do so well in my models.

For a fully expended all Raptor FH with larger upper stage, I get 82 tonnes to LEO with CF and 79 without CF. The same all-Raptor FH with the current sized upper stage gets 60t without CF and 70t with CF.

The best bang for the buck seems to be keeping the Merlins on the core and boosters, but using a larger Raptor upper stage. That gets 71t to LEO without crossfeed. Adding CF only gets about 1 more tonne, which hardly seems worthwhile.

All my numbers assume the boosters same the same size, volume, and weight as Block 5 Falcon 9. Orbit is 100 nm (185km) circular LEO. Everything is expended in these scenarios, but reuse makes crossfeed even more worthless and also drives strongly toward a larger upper stage.

What larger upper stage?  That was never part of parameters.

[envy887]

What larger upper stage?  That was never part of parameters.

It's a hypothetical upper stage for this hypothetical rocket. It's a much higher performance solution than crossfeed for about the same amount of work.

[MATTBLAK]

There was a briefly discussed, but not official idea for a 5.2 meter diameter LOX/CH4 fueled upper stage for Falcons 9 and F.H. that would be powered by a single Raptor engine. The stage would match the diameter of the 5.2 meter payload fairing and would be the same length/height as the standard Falcon second stage. In the case of Falcon 9, the engine may even have been throttled down a bit and the propellant levels short-filled for some scenarios.

But let me stress - this stage is more of an armchair, internet-rocket scientist meme than a real thing. It could be done, but is highly unlikely that it will be done. It would be simpler for Space X to upgrade the Merlin upper stage engine a bit more and stretch the second stage propellant tanks a little more to get most of the same benefit as a LOX-Methane upper stage that would add complications for pre-launch ground operations.

[envy887]

A larger diameter upper stage would be simple compared to converting Falcon to Raptor. But really, none of the above is at all likely to happen.

The answer to the OP is that without orbital refueling, Falcon Heavy will never fully match even Block I SLS.

[TomH]

The answer to the OP is that without orbital refueling, Falcon Heavy will never fully match even Block I SLS.

However two FH launches would still cost less than a single SLS launch. To some, it's the cost that matters, to others it's about politics and pork.

[RotoSequence]

The answer to the OP is that without orbital refueling, Falcon Heavy will never fully match even Block I SLS.

However two FH launches would still cost less than a single SLS launch. To some, it's the cost that matters, to others it's about politics and pork.

Payload capacity to LEO doesn't seem to mean much for most future payloads of interest. Better BEO performance from a higher ISP upper stage would be great for future science missions and human space exploration.

[MATTBLAK]

Borrowing a leaf from the Constellation playbook - don't tune out now - by having dual launches of a Falcon Heavy and a Falcon 9 Block 5; utilizing most or all of it's 22 ton payload potential - launch a 22 ton spacecraft (Command Module or Lunar Landing Module) followed by a 50+ plus ton Earth Departure Stage on a Falcon Heavy. Dock in LEO and send the spacecraft to the Moon, where it could insert itself into High or Low Lunar orbit.

You could be launcher agnostic for some missions to Cislunar space or beyond. But assuming a Falcon dominated launch infrastructure; 1x spacecraft on a Falcon 9 Block 5 and 2x EDS on Falcon Heavies to send more than 20 tons towards Mars. Or 35 ton spacecraft sent up on a reusable Falcon Heavy would need a 3x inline 'train' of Earth Departure Stages for Mars, or 2x for the Moon. Methane is not essential if you are using unaltered Falcons for your mission DRMs - methane will be the modus operandi for ITS...

But the path to ITS could be paved by launching Tanker Modules to refuel Falcon upper stages that had made it into LEO with their tanks depleted - less 'trains' of EDS needed, then.

[rakaydos]

A single raptor is too powerful to land a Falcon 9 sized tank, clearly. But what if the 3 tank set didnt separate? Burn the outer tanks to depletion and inner tank to near depletion, and have the center Raptor of the cente core handle boostback, entry and landing burns for the whole 3 core assembily?

It's more cobbled-together than a proper single-stick with the same performance, but it maintains the highway-transportable tankage of the weaker kerosene rocket.

[guckyfan]

A single raptor is too powerful to land a Falcon 9 sized tank, clearly.

A raptor throttles down to 20%. It seems, landing of a 3 Raptor vehicle may be possible. That does not mean I believe, putting raptors on Falcon cores makes a lot of sense.

[envy887]

Geometry is not ideal for landing on Raptor. The nozzle is about 1.6m across so only two across will fit on a 3.7m core. Three would fit in a triangle, but neither of those configurations can get one engine under the center of mass. A single engine per core doesn't have enough thrust for launch.

Even if the three cores remain together to landing, with 3 engines on the outer boosters and 1 on the center for landing, it would need crossfeed to deplete the center core fast enough.

Probably easier to add terminal landing thrusters, to light up after the Raptors bleed off most of the velocity. Either a few Super Dracos or a couple GOX/GCH4.

[OneSpeed]

Geometry is not ideal for landing on Raptor. The nozzle is about 1.6m across so only two across will fit on a 3.7m core. Three would fit in a triangle, but neither of those configurations can get one engine under the center of mass. A single engine per core doesn't have enough thrust for launch.
...
Probably easier to add terminal landing thrusters, to light up after the Raptors bleed off most of the velocity. Either a few Super Dracos or a couple GOX/GCH4.

Another alternative would be to re-use the existing half scale development Raptor. With nozzle diameter of around 0.9m, at least seven would fit on a 3.7m core, with one in the centre.

[TomH]

The answer to the OP is that without orbital refueling, Falcon Heavy will never fully match even Block I SLS.

However two FH launches would still cost less than a single SLS launch. To some, it's the cost that matters, to others it's about politics and pork.

Payload capacity to LEO doesn't seem to mean much for most future payloads of interest. Better BEO performance from a higher ISP upper stage would be great for future science missions and human space exploration.

One FH takes up the deep space payload, the other FH takes up an Earth Departure Stage, none of whose prop is expended yet. The two dock and the EDS performs deep space injection.

[MATTBLAK]

Yes - a Falcon Heavy reusable can take up a bigger payload than even a Falcon 9 expendable. Then, if you use a Falcon Heavy expendable to send up a 54 ton EDS - most of that 54 tons will be propellant.

[Ultrafamicom]

With Raptor, it would not be able to be reusable. (too high of thrust)
It would have less performance, since methane has less power density

Could you explain what you mean by this? Obviously the ITS booster is reusable and slated for raptor. Why would this not be the case with the 3 smaller cores? Also, why do you say less power density? I do not know the energy in RP1 vs Methane, but Raptor has 4X pressure and therefore would be far more efficient in fuel wait to thrust, would it not?

When the present Falcon 9 stage is coming back to land, its tanks are mostly empty, and so it doesn't weigh very much.  A single Merlin, throttled down, can slow the stage down gradually enough that it can land safely.  They've tried to do it with three Merlins and had mixed results - once, the stage hit the barge so hard that it punched a hole in the deck.  A single Raptor throttled all the way down would put out even more thrust than three Merlins, and landing it safely would be nigh impossible.

While you're correct that a high-pressure methane engine would be more efficient than a lower-pressure kerosene engine, that's offset by another factor - methane isn't as dense as kerosene.  There's less energy to be had in a liter of liquid methane than in a liter of kerosene.  That means that while a methane engine gives you more impulse per unit mass of propellant, it gives you less impulse per unit volume of propellant.  Falcon 9/Heavy tanks would be too small for a methane engine - you'd run out of fuel too early.  You'd need to widen them - look at the wider tanks for liquid hydrogen rockets, like Delta IV and Ariane V, vs. narrower kerosene tanks on Falcon 9 and Atlas V.

Don't forget that the mixture ratio of MLOx  engines is about 50% higher than Kerosene ones(3.82 of raptor vs 2.63 of RD-170).This results in an overall density reduction of less than 10% , or what F9FT gains from subcooled fuels.
BTW:l used to believe what we mean by a"raptor FH" is standard FH S1+ Boosters with an S2 using either a scale-down raptor or full-size one. It's obvious that a full raptor FH is by all means worthless.

[dror]

I am not trying to start a debate again about if SpaceX should retroactively apply Raptor to existing rockets, but I am curious about what a back of the envelope payload calculation would be for Falcon Heavy Raptor if they did it with cross-feed. How would it stack up to the SLS block slated for mars explorer when being used in a fully reusable and expendable configuration?

...
It would have less performance, since methane has less power density

It will have less energy, not necessarily less performance.