Talk:Propellant mass fraction

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Powered re-entry[edit]

Ok, I yield on the powered re-entry thing. It really was pretty pointless to begin with. --Doradus 01:39, May 7, 2005 (UTC)

When is lower mass fraction better?[edit]

The article used to have this statement:

When applied to a rocket as a whole, a low mass fraction is desirable, since it indicates a greater capability for the rocket to deliver payload to orbit for a given amount of fuel.

It is true that given two vehicles of equal mass, the one with the lower mass fraction gets more vehicle+payload into orbit. This statement has been reversed:

When applied to a rocket as a whole, a higher mass fraction is desirable (everything else being equal), since it gives a higher delta-v.

It is true that given two vehicles of equal specific impulse, the one with the higher mass fraction will achieve higher delta-v.

Neither of these statements is false. I think we just need to be more specific about what we mean by "everything else being equal". --Doradus 19:16, 19 June 2006 (UTC)[reply]

The first statement is not true (not necessarily false, but not true in general). You can trivially get a lower mass fraction by adding lead ballast to the vehicle.

No you can't. If you add ballast to a rocket, you must also add more fuel, or else it won't reach its destination. --Doradus 17:26, 22 August 2007 (UTC)[reply]

Whilst a vehicle that can achieve orbit with a lower mass fraction is desirable, that's a statement more about engines or other aspects of vehicles, than about mass fraction, so probably shouldn't be in this article anyway.WolfKeeper 00:12, 20 June 2006 (UTC)[reply]

On the contrary, a high mass fraction is desirable since you can usually add more payload or reduce the fuel you add to the vehicle, and save costs.WolfKeeper 00:14, 20 June 2006 (UTC)[reply]

I don't understand this claim. If you have a rocket with a given mass ratio, and remove fuel, you must remove a corresponding quantity of payload or else the rocket will not reach its destination.

NASA web sites make it clear that lower mass fractions are desirable: [1], [2]. If you can cite a source for your claim that high mass fractions are better, please do so. Otherwise, let's put this article back in a state that agrees with NASA. --Doradus 17:26, 22 August 2007 (UTC)[reply]

I have made the change. I hope you find this acceptable. --Doradus 17:01, 29 October 2007 (UTC)[reply]

Inconsistent, unclear wording[edit]

The article title is "propellant mass fraction". For an overall vehicle, that's the fraction of propellant mass vs everything else (structure, payload, engines, etc). However the article wording seems rambling and inconsistent. E.g, "...(SSTO) vehicle the mass fraction is simply the fuel mass divided by the mass of the full spaceship, but with a rocket employing staging, which are the only designs to have reached orbit, the mass fraction is higher because parts of the rocket itself are dropped off en route". Shouldn't that be lower? Relative to an SSTO of equal payload, a staged vehicle will have lower propellant mass fraction. Article may be confusing payload mass fraction with propellant mass fraction.

Another example: the table shows the Saturn V mass fraction is 0.957. However Saturn V total vehicle wet mass is about 3,038,500 kg. The total three-stage propellant mass is about 2,708,090 kg, or a propellant mass ratio of 2,708,090 / 3,038,500 = 0.891.

Similarly, the table lists the Space Shuttle mass fraction at 0.935. However the Space Shuttle wet mass is about 2,040,000 kg. The total propellant mass is about 735,601 kg for the ET, and 998,000 kg for the SRBs. That gives a propellant mass fraction of (735,601 + 998,000) / 2,040,000 = 0.850.

You can't use the formula for mass ratio of single stage vehicle (mf / mo), convert it to propellant fraction (prop mass fract = 1 / (1 - mass ratio)) and apply it to a multi-staged launcher. It doesn't yield the overall vehicle propellant mass fraction, which is the article title. Joema (talk) 15:22, 25 March 2008 (UTC)[reply]

I think the article should probably cover:

mass fraction
propellant fraction
payload fraction

They're all different. I agree that the article is currently inconsistent. The table currently covers mass fraction, rather than payload fraction, which is what the article name is.- (User) Wolfkeeper (Talk) 22:18, 26 December 2008 (UTC)[reply]

I concur. The rule of thumb I learned is that propellant mass fraction is typically 0.8 ± 0.1. In the table, the V-2 measure is correct because it is a single-stage vehicle, but the other rockets' Lp are far too high (>>0.9) for a realistic Vp / Vi (propellant to inert mass ratio). Here is another table with Lp estimates, and the entire slideshow is a good intro to the topic. --IanOsgood (talk) 14:40, 9 September 2009 (UTC)[reply]

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