Question about rockets & thrust

Palomar · 2004-03-18 09:00:45

*Okay...as most folks here know, this isn't a "strong suit" topic for me. :-\

I'm still not clear on how much energy is needed to (for instance) lift 1 pound of material up into space.

I suppose it varies for **type** of rocket propellant, etc., as well (?).

Let's use my favorite as an example: How much energy was needed to lift 1 pound's worth of weight in the Saturn V rocket?

--Cindy

Yang Liwei Rocket · 2004-03-18 09:40:10

To see a mass ratio of 27 is possible, but what about the weight of the rocket itself? So even to get 1 Kg off the ground you would need to use a very heavy weight of fuel and also the weight of the rocket. However a rocket is a variable mass body, meaning that mass changes with respect to the material that it is burning. Saturn V was a real ground launcher rocket, it produced its most power thrust at sea levels, while the Russian version didn't kick in to full power until about a minute into the flight.
I think it takes about 900 tonn of fuel to get an object off into space, can't be sure how much power, thrust, force, or weight is involved but I'll try and check it out.

Palomar · 2004-03-18 10:05:38

To see a mass ratio of 27 is possible, but what about the weight of the rocket itself? So even to get 1 Kg off the ground you would need to use a very heavy weight of fuel and also the weight of the rocket. However a rocket is a variable mass body, meaning that mass changes with respect to the material that it is burning. Saturn V was a real ground launcher rocket, it produced its most power thrust at sea levels, while the Russian version didn't kick in to full power until about a minute into the flight.
I think it takes about 900 tonn of fuel to get an object off into space, can't be sure how much power, thrust, force, or weight is involved but I'll try and check it out.

*Thanks. It seems there are -many- variables at play, then. (I know that probably sounds like an "obvious" statement, but I'm trying to understand the basics here).

Thanks also to Byron and SBird for answering my question in the "Hybrid Rockets" thread yesterday.

--Cindy

SBird · 2004-03-18 10:26:36

Tht total amount of energy to loft an object to LEO doesn't change too much for different launch configurations. Primarily, it's matter of how efficient your launch system is. In the case of a Saturn V, I don't have figures but it was quite inefficient since most of the energy expended is put into lifting fuel that goes to lifting fuel that goes to lifting the fuel that puts the capsule in orbit.

As long as you are working with chemical propellants, you're stuck with a pretty high mass ratio. Using a rocket with a higher Isp allows you to get away with a better mass ratio. Unfortunately, all of our high Isp engines don't have enough thrust to get off the ground. Your two main options are chemical and nuclear thermal. (there's a couple of other alternatives but they aren't ready for primetime yet) The relationship is an exponential one, meaning that as Isp goes up, the mass ratio gets much better.

IIRC, the mass ratio for the Saturn V was something like 30:1, meaning that about 3% of the rocket makes it to orbit. Partly, this was because of the use of O2/kerosene on the 1st stage shich has a lower IsP than O2/H2. I don't know why the Saturn V 1st stage used kerosene but I'm guessing that is was a technical issue with handling those quantities of liquid H2 back in the 60's.

Today, the Shuttle uses O2/H2 and hasa higher Isp but ends up wasting most of the additional mass ratio on stuff like wings and the like. As a result, the total cargo to LEO mass ratio of the space shuttle is something moronic like 1%.

NTR rockets that use a nuclear reactor to heat up a fluid are promising but the problem is that there is considerable inertia to going to a nuclear ground lifting system. However, we aren't missing out on too much additional performance by not using NTR because of the high weight of the engines. Much of the additional mass ratio would get eaten up by the reactor mass. Overal, it is a much superior design to chemical rockets but I don't see it happening soon for ground launches.

The alternates are:

Hypersonic air breather launchers. Jet engines get a much higher effective Isp because they don't have to carry oxidizers - they get it from the atmosphere. A hypersonic plane that can use a scramjet engine up to mach 10 would be able to get to orbit with a much larger mass ratio. The problem, though is that the total cargo is small since, as with the Shuttle, most of the spacecraft mass is taken up with things like heat shielding and wings.

Beamcraft: it is possible to use a big laser or microwave antenna array to push a spacecraft into orbit. Right now, most work has been with lasers which doesn't make much sense to me. Lasers don't scale up very well to larger sizes and even the big military megawatt models can push a 'spacecraft the size of a cat to about 200 feet. Whoopie.

Microwave arrays, however, are already powerful enough to do the trick and scale up in size nicely. Basically, you'd beam microwaves to a spacecraft which has a big absorbtion panel on the side. The panel heats up and transfers the heat to the propellant. The total performance is slightly better than an NTR which the design is fairly similar to.

Finally, if someone can actually get it to work, a space elevator is about the cheapest way into space. In this case, you just climb a rope to orbit. It's a slow process but if you beam energy to the elevator car, you basically get whatever mass ratio you want. Furthermore, it's about as efficient as you an get for transferring mass to orbit. The only problem is that it's not entirely clear that building one of these things is even possible.

sab39 · 2004-03-18 11:18:25

This is something I've been wondering as well, in fact some of my recent "Lots of science questions" post was dedicated to this topic.

As I understand it so far, if you know the specific impulse (Isp) of your fuel and the Delta-V you want to achieve, you can calculate the mass ratio. The mass ratio tells what mass of fuel you must carry for a given dry mass. That dry mass needs to include the mass of the fuel tanks as well as the payload mass, so you plug in a formula that will give you the fuel tank mass as a function of fuel mass and you can solve the whole thing to find the total fuel mass necessary for your payload. That's how I understand it after reading The Case For Mars, anyway. Furthermore, it sounds like the cost of the launcher is pretty much linearly related to the total launch mass, including fuel. (Someone please stop me if I say something totally wrong...)

I'm pretty sure that doesn't tell the whole story though and I'm trying to figure out the rest. First of all, I don't know the actual formulas that are involved in the previous paragraph, but more than that, I think it's actually missing some important steps.

There's obviously an a significant thrust necessary even to stand still (zero Delta-V) in a gravitational field, so a calculation based only on Delta-V doesn't actually seem to include overcoming gravity for the duration of the flight - unless an adjustment is being applied to the Delta-V figure to compensate for that.

Furthermore, I'd be amazed if you can simply ignore atmospheric drag in this calculation, but I don't know how it gets included.

Finally, I don't know what different kinds of orbit exist and what the relationship between the Delta-V's necessary to reach them are. There's obviously LEO and Geosynchronous, then there are things that aren't strictly orbits like escape velocity and X-prize altitude. But I'm not sure what other significant milestones exist, or how big the Delta-Vs are compared to each other. Or which of them are commonly used (eg for ISS, commercial satellites, GPS, shuttle...)

Hopefully I'm adding at least a little bit of useful information here instead of just more questions, but I'd still be extremely grateful if someone could help me fill in the big gaps remaining in my own knowledge...

Palomar · 2004-03-18 11:57:14

IIRC, the mass ratio for the Saturn V was something like 30:1, meaning that about 3% of the rocket makes it to orbit. Partly, this was because of the use of O2/kerosene on the 1st stage which has a lower IsP than O2/H2. I don't know why the Saturn V 1st stage used kerosene but I'm guessing that is was a technical issue with handling those quantities of liquid H2 back in the 60's.

*Thanks for the feedback, everyone.

So the Saturn V -did- use kerosene. I thought I'd read that some time ago, but couldn't find further references to it (I'd begun to think I'd dreamed it or something...).

Another "obvious" question: Is this the same kerosene as used in old-fashioned lamps? I suppose so, but don't like to make assumptions.

I Googled and found this:

"The final step in the Saturn program was the construction of the giant Moon rocket, the Saturn V. At the base of the Saturn V was the 32-foot (10 m) wide first stage or S-IC that was powered by five F-1 rocket engines generating 1.5 million pounds (680,000 kg) of thrust each by burning kerosene and oxygen. Above this was the S-II, which carried liquid hydrogen and oxygen propellants and was powered by five of the J-2 engines from the S-IVB. The S-IVB was used as a third stage. Above the S-IVB was the Instrument Unit, which contained the computer systems that controlled the Saturn V during launch. Atop this was the Apollo spacecraft and its emergency escape rocket. The whole assembly was 363 feet (111 meters) tall and weighed 6.4 million pounds (2.9 million kg)."

Okay, so it was kerosene and oxygen in Stage 1. Were these two elements mixed together? Or housed separately and released at the same time? The Sun-bright blazing streams of fire on lift-off were the burning of the kerosene and oxygen...I'm looking at my poster of Apollo 11 lift-off; those huge streams of fire look like liquid gold.

--Cindy

[http://mars.caltech.edu/launch/saturn.html]Entire article quoted above

SBird · 2004-03-18 14:42:58

The O2 and kerosene were stored in seperate tanks like O2 and H2. A premixed mixture of oxidizer and fuel would be insanely dangerous. A mixture of fertilizer and fuel oil is a much less energetic fuel/oxidier mix and we all got to see what even that did in OK city. Plus, kerosine would probably solidify at liquid O2 temperatures. The two were mixed either right before or in the engine combustion chamber. IIRC, the liquid O2 was sent through tubes in the engine cones to keep them from melting and to preheat and pressurize the O2.

The golden color most definately from the kerosene. (most likely more highly purified than camp stove stuff but basically the same) O2/H2 burns hotter and cleaner so it gives this tiny little blue flame. The Shuttle main engines generate thrust levels comparable to the F-1s, IIRC but you can barely see the flame. O2/H2 is a much better fuel but you're right, the gold kerosene glow was much better looking.

I know that at least one X-prize competitor is using O2/kerosene since kerosene is much cheaper and easier to handle than H2.

SBird · 2004-03-18 14:50:10

Oh yeah, you might also be interested to note that the Saturn V had a larger cargo mass fraction that most modern launch systems despite its lower performance.
For example, the Shuttle has a higher total mass fraction (being defined as the mass of the spacecraft that gets to orbit vs the total mass including fuel that it starts out with) than the Saturn V. However, the Shuttle wastes most of this on things like wings and carrying heavy engines to orbit, etc. As a result, the Shuttle can barely manage a cargo mass fraction of 1.5%. That means that of the total mass of the Shuttle, esternal tank and SRBs ar launch, 1.5% of that is actual satellite cargo. All of the rest of the extra mass is flown back down to Earth - how wasteful!
The Saturn V had a cargo fraction of 3%. This is because it was basically a set of minimal fuel tanks with some cargo strapped to the top - nothing was included in the launcher that wasn't strictly necessary.
This is why I'm a big proponent of scrapping the Shuttle and going back to the old big disposable booster style of doing things. Done properly, it's a lot cheaper.

Bill White · 2004-03-18 15:58:30

This is why I'm a big proponent of scrapping the Shuttle and going back to the old big disposable booster style of doing things. Done properly, it's a lot cheaper.

My favorite topic. Shuttle derived launchers.

GCNRevenger · 2004-03-18 17:46:03

The Saturn-V had a better mass fraction because it had an additional stage...

LOX/Kerosene is fine for liftoff and to get you going, but Hydrogen's much lighter weight gives it a substantial edge as a second stage propellant.

Palomar · 2004-03-18 20:16:34

*Why the use of kerosene?

What special properties does it possess which made it ideal for the Saturn V's 1st stage (along with 02)?

I've Googled, hoping to find an explanation. I get returns of articles which -mention- kerosene in conjunction with the Saturn V, but no *explanation* as to why kerosene was used.

Any answers will be appreciated, of course.

--Cindy

GCNRevenger · 2004-03-18 20:33:31

Kerosene is preferable as a first stage propellant for a number of reasons... primarily though is that at zero altitudes and speeds, thrust is more important than fuel efficency, and since this stage is the largest you want to use the fuel that is easiest to handle and not nessesarrily the most efficent.

It is also much more compact per-mass than other fuels, especially hydrogen, so you can keep the physical size of the stage small, which is especially nice since it must support the weight of the fueled stages above it, even at max thrust.

And lastly its not toxic and its easy to store, it doesn't need cryogenic cooling/insulation, it will keep for long periods in storage, and won't vaporize/boil off as readily as other fuels with its high intermolecular attraction.

Yang Liwei Rocket · 2004-03-18 21:35:06

yes, no matter how many times you watch it, no matter where it launches from Europe, Russia, USA or what type of rocket it is or type of probe.
I think it still is a wonderful site, watching something blast off, pushing out past the Earths air and off into space. There is something special about watching a lift off

Palomar · 2004-03-19 11:57:30

yes, no matter how many times you watch it, no matter where it launches from Europe, Russia, USA or what type of rocket it is or type of probe.
I think it still is a wonderful site, watching something blast off, pushing out past the Earths air and off into space. There is something special about watching a lift off

*I agree. I have a DVD collection of the Saturn I and IB launches ("The Mighty Saturns: I & IB")...every launch is contained on one DVD. It's like watching fireworks, in a way (only better).

And I absolutely can't wait for "The Mighty Saturns: Saturn V" DVD collection, due out this Spring. Nothing like watching the Royalty of Rockets blasting off.

[http://www.scientific.co.uk/aerospace/w … vplans.htm]Surprise...Mars Direct mentioned!

*You'll have to scroll down a ways; is to the right of the screen.

--Cindy

bolbuyk · 2004-04-08 06:19:38

The main reason to use Kerosene on the first stagge of the S aturn V, and essentielly the main reason to use the rather low-ISP propellant for the shuttles SRB´s, is the low kost per kg. For the first stage the prize is rather important. For upper stages one has to realize that all fuel has to be lift up one or morre, so ISP is more important. A first stage with LO2/LH2 is also yet not rational (you can consider the SSME as second stage by considering the SRB´s as the first).

As I thought, depending on different factors, the cargo for LEO inclation max. 30° the ratio is 1.5 to nearly 6 percent.

New Mars Forums

Announcement

#1 2004-03-18 09:00:45

Re: Question about rockets & thrust

#2 2004-03-18 09:40:10

Re: Question about rockets & thrust

#3 2004-03-18 10:05:38

Re: Question about rockets & thrust

#4 2004-03-18 10:26:36

Re: Question about rockets & thrust

#5 2004-03-18 11:18:25

Re: Question about rockets & thrust

#6 2004-03-18 11:57:14

Re: Question about rockets & thrust

#7 2004-03-18 14:42:58

Re: Question about rockets & thrust

#8 2004-03-18 14:50:10

Re: Question about rockets & thrust

#9 2004-03-18 15:58:30

Re: Question about rockets & thrust

#10 2004-03-18 17:46:03

Re: Question about rockets & thrust

#11 2004-03-18 20:16:34

Re: Question about rockets & thrust

#12 2004-03-18 20:33:31

Re: Question about rockets & thrust

#13 2004-03-18 21:35:06

Re: Question about rockets & thrust

#14 2004-03-19 11:57:30

Re: Question about rockets & thrust

#15 2004-04-08 06:19:38

Re: Question about rockets & thrust

Board footer