Comparison of Rocket Payload Fractions

[LostCosmonaut]

I have compiled some data on the payload fraction (payload to LEO / Gross mass) of various rocket systems;

 

 

 

From this, several thing can be seen;

 

• Solid rocket boosters utterly ruin your payload fraction.
• Despite having a significantly higher specific impulse than other engines (365 seconds for the RS-68 vs. 285 seconds for the RD-275), hydrogen-fueled launch systems only have a slightly better payload fraction than hypergolic systems, or are even significantly worse.
• Larger rockets generally have a larger payload fraction (Saturn I vs. Saturn V, Falcon 9 vs. Falcon Heavy). Titan II and Titan IV are not entirely comparable.
• STS is a stupid pile of trash.
• Kerolox first stage provide significantly better payload fractions in almost all cases, while avoiding the difficulties associated with liquid hydrogen. Hypergolics generally have inferior performance to both, but are significantly easier to handle, and the difference is not extreme.

 

Data via wiki, except where noted (the gross weights for Delta IV Heavy and Atlas V 551 were horribly off, especially for the latter). Encylopedia Astronautica data mostly agreed, but that site is severely lacking in info on the Falcon family.

@Sturgeon@Collimatrix@T___A

[Alzoc]

Thanks for the info.

I did the same thing for some rockets of the Ariane family, trying to use the same methodology than you did and I get significantly worse payload to mass fraction.

I don't know too much about rockets so maybe I am comparing apples to oranges here.

 

 

(Sorry for the low res)

Could you please cross-check if you get the same results, and tell me if you have any insight on why the difference is so great (or if I am simply indeed comparing things that cannot be compared).

[LostCosmonaut]

11 hours ago, Alzoc said:

Thanks for the info.

I did the same thing for some rockets of the Ariane family, trying to use the same methodology than you did and I get significantly worse payload to mass fraction.

I don't know too much about rockets so maybe I am comparing apples to oranges here.

 

 

(Sorry for the low res)

Could you please cross-check if you get the same results, and tell me if you have any insight on why the difference is so great (or if I am simply indeed comparing things that cannot be compared).

 

For whatever reason Encylopedia Astronautica and wiki give the Ariane 1 a lower value for payload to LEO vs. GTO (1400 vs. 1850), which is highly odd. I can't think of any reason why that would be the case (GTO requires much more delta v than LEO).  Going by the payload fraction to GTO, I get 1850/207200 = .0089, vs. 6300/705000 = .0089 for Proton-M, and 14220/738960 = .0192 for Delta IV Heavy. I think this is explainable by Ariane 1 being an older rocket with somewhat less efficient engines (the Viking-2s on Ariane 1 have 248 sec. sea level isp vs. 285 on the RD-253 and 365 on the RS-68).

 

EA gives the LEO performance for Ariane 5 as 16000 kg, but to a 51.5 degree inclination orbit, which is going to cut into the performance a lot. Arianespace's site gives a payload to LEO of 20 tons, which gives a payload/mass fraction of .0257, which is better but still not great. Payload to GTO is 10000/777000, which gives a mass fraction of.0129. Also, Ariane 5 does a bit of the same thing STS does and uses a hydrolox first stage with solid boosters. All the Ariane variants look like they are optimized for launches to GTO/GEO, so their payload fraction to LEO is going to suffer a bit. This explains why Ariane 4 and 5 have lower payload fractions to LEO (as far as I can tell your Ariane 4 numbers are correct).

 

 

***

 

To build on my previous calcs;

 

STS gets hurt a bit in the payload/mass calcs since it reuses so much of what it lifts to orbit. A better comparison would be Shuttle C, which likewises uses hydrolox/solids, and has a payload/mass fraction of 77000/1966675 = .0391, which is comparable to Delta IV Heavy.

 

Atlas V 552 (actually any Atlas variant without "0" in the middle digit) has solids in the first stage, which, if my thesis is correct, will lower the payload fraction. Theoretically the best Atlas V config for payload/mass fraction would be the Atlas V 402. Performance data for it can be found in the archived Atlas V Handbook.

 

Adding components from page 25 of that doc, we get a gross mass of 331702 kg for Atlas V 402. Wiki says that 402 has a payload to LEO of 12500 kg, which gives a payload/mass fraction of .0377, which is better, but not by much compared to 552. Interesting.

 

 

 

[Alzoc]

8 minutes ago, LostCosmonaut said:

All the Ariane variants look like they are optimized for launches to GTO/GEO, so their payload fraction to LEO is going to suffer a bit.

 

Well that would make sense since they all launch from Kourou which since it is very close to the equator is very suitable for GTO launches.

[Collimatrix]

There are two factors militating against the efficiency of hydrolox first stages:

 

1)  Hydrolox is very bulky, so the first stage will lose more of its total energy budget to atmospheric drag.  This effect will decrease as the rocket is scaled up, and indeed we should expect the mass payload fractions of all launch vehicles to improve as they get larger due to square/cube.

 

2)  Hydrolox engines have inferior thrust to weight ratios, and when I thought more about this I realized that this is unavoidable.  There is an inherent trade-off between thrust to weight ratio and specific impulse in chemical rockets.

The weight of a rocket nozzle is determined by the pressure and temperature it needs to contain (and I guess secondarily by how good your engineers are at designing nozzles, but that's why you feed them lots of amphetamines to keep them productive and happy).  Thrust is essentially a function of how much propellant mass the nozzle can chuck backwards at a given velocity per second.  Specific impulse is essentially a function of how hot the fuel and oxidizer burn and how light the combustion products are.

 

So, for maximum specific impulse you want something that burns extremely hot and leaves extremely light combustion products.  Hydrolox is the best practical combination, since the only combinations that are higher all involve fluorine.  Fluorine is not your friend and should not be invited to parties.

But here's the thing; the Ideal Gas Law does not say shit about mass.  Pressure is a function of how many molecules are bouncing around inside the nozzle at any given instant and how hot they are.  So, for a given temperature and pressure limit a kerolox or hypergolic fuel will beat hydrolox, because for a given number of molecules bouncing around at a given temperature (i.e. a given pressure, more or less), a hydrolox engine will have lower mass flow because its exhaust products are lighter.  Actually, it's worse than that, because hydrolox burns at a higher temperature, which further drives nozzle mass.

 

So the very qualities that make hydrolox have high I_SP also kneecap its thrust to weight ratio, which means that a first stage using hydrolox will also lose more of its total energy budget to gravity drag.

[Collimatrix]

Actually, now that I think about it I'm not sure that second part is right.  Could someone who knows more about rocket theology double-check my mathings?

[LostCosmonaut]

Black Arrow, which used kerosene and hydrogen peroxide (a combination which has recently come into favor among SH members), has the following numbers (per E.A.);

 

mass: 18038 kg

payload: 73 kg

payload/mass ratio: .0040

 

Pretty poor, but bear in mind this was a nation's (UK) first attempt at a satellite launcher, in 1970. A modern rocket using kerosene/peroxide designed for payload capacity and economics would certainly do better.

[Sturgeon]

2 hours ago, LostCosmonaut said:

Black Arrow, which used kerosene and hydrogen peroxide (a combination which has recently come into favor among SH members), has the following numbers (per E.A.);

 

mass: 18038 kg

payload: 73 kg

payload/mass ratio: .0040

 

Pretty poor, but bear in mind this was a nation's (UK) first attempt at a satellite launcher, in 1970. A modern rocket using kerosene/peroxide designed for payload capacity and economics would certainly do better.

 

Also your scaling factors are massively against going small. That's also how BFR can have a payload fraction approaching 0.06.

 

I think Astronautix numbers for the Black Arrow may be wrong, as well. But it wouldn't surprise me if the Brits were just bade at rikkits.

[Xlucine]

Black arrow was launched from australia, which is going to hurt it getting into orbit