Does weight matter for spaceship in space?

[Orion Pax]

Hello

I'm new here and I'm a newbie to physics, my question is does spaceship's weight matter in space? is that a consideration when designing spaceships/space probes? (I know that it does matter because the more the object weights the more power the rocket carrying the object needs to be to get it to space) but once the object is in space does weight matter there?

suppose we have 5K tons spaceship and 10K tons spaceship travelling at the same speed flying from earth to Jupiter and are now away from earth's gravity, which one will get there first and why?

*I know that weight is different from one planet to another, the weights provided above are in metric tons on earth for the sake of simplicity (If I didn't mess the weight/mass )

And excuse any mistake I have made I'm still learning and all this is new to me.

Thanks!   

[peadar1987]

The answer is, weight* does and doesn't matter!

Spacecraft propel themselves by throwing mass out of the engine. The lighter the spacecraft, the less mass is needed to change its speed.

However, in your example, we've forgotten about how the spacecraft got out of earth's gravity. In that case, the orbit of a spacecraft is completely independent of its mass**. Your 5 tonne spacecraft is going to follow exactly the same path as your 10 tonne one until you get to Jupiter.

Now you're at Jupiter, you need to slow down to enter orbit (you will gain enough energy as you fall down the gravity well to just shoot out the other side of it). Now the mass of the spacecraft does matter. Your 10 tonne spacecraft will have twice as much kinetic energy as your 5 tonne one. This means you need twice as much fuel to slow it down. But if you're carrying more fuel, you increase the mass even more, and need even more fuel to slow that extra fuel down, and then you need more fuel to slow down that fuel, and so on. Weight is absolutely key for things like this, and it is why spacecraft try and shave off every gram of mass they can.

 

*So weight and mass are two different things. Mass is a property of an object. It's basically how hard it is to accelerate. 10kg of lead is far harder to accelerate than 1kg of lead, even in deep space. Weight is the force caused by gravity, which can vary. On earth, gravity causes a force of 10 Newtons for every kg of mass. In deep space, 1kg of mass isn't being pulled upon by any gravity, so it weighs nothing.

**This isn't strictly true, but for anything smaller than a decent sized moon, the mass of the orbiting body is irrelevant compared to the mass of the body it is orbiting. Basically, the heavier your spacecraft is, the more it and the sun "pull" on each other due to gravity. But also, because it is heavier, it is harder for the sun's gravity to accelerate. Turns out these two effects cancel each other out completely and the orbit stays the same.

[Green Baron]

Mass, yes. Mass is a property of matter. Mass matters sotosay :-)

Weight comes if an acceleration is applied to the mass and something works against the acceleration. As long as the ships coast they have a mass but no weight. As you point out: different planets, different gravity -> different acceleration. The weight comes into pley as soon a force works against the acceleration (like a scale on the ground).

In your example the ships (suppose they are flying close together and are only exposed to the same overall gravity) arrive at the same time cause nothing works against the gravity induced acceleration.

 

Edited July 1, 2016 by Green Baron

[YNM]

tl;dr It does - just not in the way you've thought, unlike things that move on the Earth's surface with all the friction. Space trajectories (um, paths) is about getting something moves to a certain velocity (speed and direction) for a certain behavior, every now and then. Only during those times mass (not weight, really) matters. Problems are those mass can't increase again - it has to decrease.

Throwing a 100 g rock is easier than throwing 1000 g rock to the same goals (say, hit a particular target). Same goes in space.

Edited July 1, 2016 by YNM

[PB666]

Weight obviously matters otherwise NASA would not have its astronauts on diets. (heh-heh)

[Camacha]

3 hours ago, Orion Pax said:

suppose we have 5K tons spaceship and 10K tons spaceship travelling at the same speed flying from earth to Jupiter and are now away from earth's gravity, which one will get there first and why?

Assuming all other circumstances are the same, they will arrive at pretty much exactly the same time. I say pretty much, because the slight gravitational tug from surrounding bodies will have different effects in minor ways. However, if you arrive there, you are likely going to need to change the speed or orientation of the craft to enter a orbit. This time, mass is very important, as the energy required to move the craft will fully depend on the mass times the speed. A heavier spacecraft will require much more energy to be moved, sped up or slowed down. Conversely, a 10000 metric tonne spacecraft will need a much larger rocket to put it on the same trajectory when compared to a 5000 metric tonne spacecraft.

Also note that 10000 metric tonne is a very high mass spacecraft. 10 metric tonne is much more reasonable. For instance, the full ISS has a mass of 419,455 kg, which means it is 419 metric tonne. Your proposed light spacecraft has a little over 10 that mass and your heavy spacecraft is close to 25 times.

Edited July 1, 2016 by Camacha

[Bill Phil]

Mass matters, yes. The 10k tonne vessel in your scenario has more momentum, more inertia, more energy, and so to slow it down you need to spend more energy. Here's the thing, though: if your spaceship drive is extremely advanced, then it doesn't matter as much.

However, the actual amount of matter (mass) doesn't matter as much as the mass ratio. The mass ratio and the exhaust velocity determine the DeltaV that the ship has. With a higher exhaust velocity, you need much less fuel. Much, much less. Exhaust velocity is dependent on the propulsion system.

What's neat about space is that you can raise the mass ratio in pretty easy ways. Your ship most likely won't ever encounter 1g of gravity (if built in space) and thus you can use weaker designs, and those designs could potentially be lighter.

Edited July 2, 2016 by Bill Phil

[Orion Pax]

Thanks guys, much appreciated! 

[Teilnehmer]

On 7/2/2016 at 3:18 AM, peadar1987 said:

Now the mass of the spacecraft does matter. Your 10 tonne spacecraft will have twice as much kinetic energy as your 5 tonne one. This means you need twice as much fuel to slow it down. But if you're carrying more fuel, you increase the mass even more, and need even more fuel to slow that extra fuel down, and then you need more fuel to slow down that fuel, and so on.

According to the Tsiolkovsky equation, if both spacecraft have the same I_sp and perform burns with the same Δv, the  the m_wet/m_dry ratio will be the same for them. So the 10 t craft will need twice as much fuel, but not even more.

Or am I mistaking?

[Green Baron]

Tsiolkovsky says (cheat sheet http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet), when Isp is the same for both crafts and the quotient of mass before / mass at end for the burn is the same for both crafts then both have the same dV. Which means: sizing a craft up without changing the mass-ratio and/or specific impulse does not change it's dV. So, yes, your basically right i'd say.

That is why we do staging to get rid of dry mass and use different engines for different jobs :-)

 

[lugge]

On 2.7.2016 at 0:35 AM, Bill Phil said:

However, the actual amount of matter (mass) doesn't matter as much as the mass ratio. The mass ratio and the exhaust velocity determine the DeltaV that the ship has. With a higher exhaust velocity, you need much less fuel. Much, much less. Exhaust velocity is dependent on the propulsion system.

I disagree.

TWR is less important in space. When in a gravity well (like on earth ground level), you need a TWR > 1 to be able to lift the craft.

Once in orbit, TWR only determines the required burn-time to accelerate the craft with a given mount of dv.
A 0.2 TWR craft only needs to burn half the time then a 0.1 TWR craft (assuming the mass is the same).

Possible dv of a craft is determined by the wet/dry-mass ratio and engine ISP. A lower-mass craft has a better wet/dry ratio and therefore more dv.

[peadar1987]

1 hour ago, Teilnehmer said:

According to the Tsiolkovsky equation, if both spacecraft have the same I_sp and perform burns with the same Δv, the  the m_wet/m_dry ratio will be the same for them. So the 10 t craft will need twice as much fuel, but not even more.

Or am I mistaking?

Nope, you're dead right. Amateur mistake from me! 

Doubling fuel does not double delta-V, but it does double payload capacity for a given delta-V.

[Green Baron]

35 minutes ago, peadar1987 said:

Doubling fuel does not double delta-V, but it does double payload capacity for a given delta-V.

errr ... you mean doubling whet mass, same engines ? For a craft on the pad that could (would for my crafts) bring TWR below 1.

For a craft in space where TWR doesn't matter dV stays the same when the dry mass fraction doubles equally. If dry mass stays the same - or only the bigger tank has a higher mass and the rest (payload) remains the same - then dV will be higher, but not double (ln applies). You're right.

If you want more payload and still lift off you need more thrust on the pad. That leads to more fuel. That would be a bigger rocket with higher thrust, same Isp than before and same burntime to keep the dV.

 

Edited July 4, 2016 by Green Baron

[Plusck]

1 hour ago, lugge said:

I disagree.

TWR is less important in space. When in a gravity well (like on earth ground level), you need a TWR > 1 to be able to lift the craft.

Once in orbit, TWR only determines the required burn-time to accelerate the craft with a given mount of dv.
A 0.2 TWR craft only needs to burn half the time then a 0.1 TWR craft (assuming the mass is the same).

Possible dv of a craft is determined by the wet/dry-mass ratio and engine ISP. A lower-mass craft has a better wet/dry ratio and therefore more dv.

You can't really "disagree" with the proposition that exhaust velocity is the only thing that really matters. Because at the end of the day, exhaust velocity is the only thing that matters... For a given propellant, increasing exhaust velocity increases I_sp, which increases dv per mass of propellant. The units that you count mass in don't matter in the slightest.

Therefore, if you have a given I_sp and a given dv, then there is going to be no difference at all in the wet/dry mass ratio between a huge ship and a tiny ship. By definition, the mass ratio must be identical.

[peadar1987]

17 minutes ago, Green Baron said:

errr ... you mean doubling whet mass, same engines ? For a craft on the pad that could (would for my crafts) bring TWR below 1.

For a craft in space where TWR doesn't matter dV stays the same when the dry mass fraction doubles equally. If dry mass stays the same - or only the bigger tank has a higher mass and the rest (payload) remains the same - then dV will be higher, but not double (ln applies). You're right.

If you want more payload and still lift off you need more thrust on the pad. That leads to more fuel. That would be a bigger rocket with higher thrust, same Isp than before and same burntime to keep the dV.

 

Of course. But the OP is assuming you're already in space. If you're on the pad and need a TWR>1, you also need to ramp up your thrust with heavier engines, and deal with drop-offs in ISP because of the density of the atmosphere, which complicates the issue further.

[Bill Phil]

5 hours ago, lugge said:

I disagree.

TWR is less important in space. When in a gravity well (like on earth ground level), you need a TWR > 1 to be able to lift the craft.

Once in orbit, TWR only determines the required burn-time to accelerate the craft with a given mount of dv.
A 0.2 TWR craft only needs to burn half the time then a 0.1 TWR craft (assuming the mass is the same).

Possible dv of a craft is determined by the wet/dry-mass ratio and engine ISP. A lower-mass craft has a better wet/dry ratio and therefore more dv.

That's not disagreeing, that's agreeing.