What percent of a space shuttles mass is fuel used for getting to orbit? (On average)

[Sigma117]

Pretty much the title, just something that came up in conversation, but I can't recall the value.

[PakledHostage]

I did a rough calculation of the shuttle's delta-V expenditure a while back. I used information that I was able to find online. I summarized the results in the table below. The numbers appear to be in the right ballpark, but others on this forum may be able to provide more accurate numbers:

The same analysis can be used to figure out the percentage of fuel burned, by mass. According to the above table, the stack has a mass of 2000 tonnes on the pad. At MECO1, the shuttle and external tank have a mass of 126 tonnes. During the climb to orbit, two spent SRBs are jettisoned. The combined empty mass of the SRBs is 175 tonnes.

2000 tonnes (initial mass) - 126 tonnes (orbiter plus external tank) - 175 tonnes (two empty SRBs) = 1699 tonnes fuel burned

Fuel burned is, therefore, about 85% by mass.

The shuttle obviously isn't in orbit at MECO1 but it only requires a couple of hundred m/s of delta-V to circularise at that point, and the amount of fuel burned to achieve orbit is relatively small by comparison to what's required during the climb to MECO1.

Edited December 8, 2013 by PakledHostage

[SFJackBauer]

That's correct, assuming a fully-loaded shuttle its liftoff mass was 109 tons, of which 30 tons was the maximum payload, 57 tons its dry mass and 21.5 tons of OMS fuel.

At MECO, a shuttle OMS with max payload would have 300m/s dV, from what I've seen the circularization burn spent 100m/s so there goes roughly 7 tons of MMH/NTO.

[Sigma117]

Oh, I wasn't expecting that high a number. Knew it was over half, but not by that much, thanks ^^

[K^2]

There was an article that talked about comparison of a rocket to a molotov cocktail, which pointed out that a typical rocket, has a higher fraction of fuel to total mass than the molotov. Gives you an idea.

[KvickFlygarn87]

Pakled: "2 SRBs at full thrust plus SSMEs at 104% throttle"

Looks kerbal to me!

[rdfox]

85% is pretty typical of most vehicles designed for low Earth orbit; it varies some depending on what propellant mixture you use, the Isp. of your engines, and the vehicle TWR, but it's generally in that neighborhood. You just need that much reactant mass to get up to orbital velocity and altitude...

As for 104% throttle, that's a longstanding, common question. Short version: when the SSMEs were being developed in the 70s, it was discovered that they could run reliably at above the originally planned thrust rating. Since a lot of paperwork and tests had been completed with the originally planned rating being listed as 100% thrust, this meant that if they rerated the engines to higher thrust, they'd either have to go back and change all the paperwork to adjust it to the new 100% rating, or they'd have lots of confusion as to which "100%" they meant in any given situation. NASA's solution was exactly the sort of "avoid the problem altogether" one you'd expect of engineers--they merely redefined the available range of values so that 100% was no longer the absolute maximum power possible. Instead, it was just a reference point, and they would refer to the engine thrust in terms of a percentage of that nominal value. Hence why, very early in the program, they switched to running the engines at 104% power for most of the ascent; they'd found that they could run them at 104% with no loss of reliability, and up to 109% without them failing, albeit at the cost of increased wear. Thus, 104% became normal "full thrust," and in an abort scenario, they would throttle up to 109% for that little extra shove that might be the difference between life and death.

In the 90s, improved metallurgy and a larger nozzle throat allowed SOME of the engines to be refurbished to run at 109% under normal conditions and up to 114% in an emergency, and this became standard on the flights to Mir and the ISS, to increase payload capacity on those flights. (The engine controllers knew which model each engine was, and would operate their throttles independently, allowing the older engines to remain in the flight rotation right up until the end of the program; they could be mixed and matched with no issues, though NASA was careful to make sure there was sufficient TWR for the required payload on every flight anyway and, on a particularly heavy flight, might make sure to have three wide-throat engines grouped together.) This is also why NASA started using the OMS engines during SSME burn, after SRB burnout, as a sort of "afterburner" to give the vehicle a little more payload capacity on those high-inclination flights...

[Kilmeister]

There was an article that talked about comparison of a rocket to a molotov cocktail, which pointed out that a typical rocket, has a higher fraction of fuel to total mass than the molotov. Gives you an idea.

Interesting, but it makes sense. A glass bottle isn't exactly the lightest storage container.