Best thrust to weight ratio propellant

[Spacescifi]

 

Let's say all you care about for a spacecraft is maximum thrust to weight ratio. You need not be concerned about good delta v, all you need is something that will store for months on end and when high thrust is required, it can deliver. This is essentially an orbital interceptor spacecraft and is already in orbit.

What propellant gives the best the best thrust to weight ratio?

LH/LOX is out, as it won't store well. Liquid methane requires cryogenics that will cut into the mass fraction that effects what little delta v you have.

Hypergolics might work, even though toxic, and monopropellants also sound good. Solid propellant could work too, although it makes pulse firing either not possible due to one long burn out, or harder to design for.

Know what the funny thing is? Chemical rockets have better thrust to weight ratios than NTR, except for the variations that tend to be unworkable due to operating at the edge of material thermal limits.

NTR's have lower TWR, but make great for interplanetary because of better delta v.

But an orbital interceptor only cares about one thing. Interception via docking with orbital spacecraft.

Thoughts?

 

 

 

Edited September 22, 2019 by Spacescifi

[Rakaydos]

Thing is, you aren't asking about thrust per engine mass, you are asking how much thrust you can get out of each unit mass of propellant. Or, for chemical propulsion, how much energy you can extract from a given fuel, by mass.

Which correlates to ISP, a rocket engine's fuel efficient. Hydrogen is the current clear winner. Antimatter is the theoretical clear winner.

[YNM]

TWR is also dependant on how much did you carry - if your tank is small yet the engine is huge and powerful it'd have large TWR, regardless of fuel type. You'll just run out of fuel really quickly, that's all.

But yeah. I think you just end up asking about ISP really - how much energy can a fuel release. In which case the answer is LH2/LOX - there's a reason such upper stages are referred as "high-energy".

Edited September 22, 2019 by YNM

[Spacescifi]

28 minutes ago, YNM said:

TWR is also dependant on how much did you carry - if your tank is small yet the engine is huge and powerful it'd have large TWR, regardless of fuel type. You'll just run out of fuel really quickly, that's all.

But yeah. I think you just end up asking about ISP really - how much energy can a fuel release. In which case the answer is LH2/LOX - there's a reason such upper stages are referred as "high-energy".

 

LH2/LOX will evaporate long before even a month passes is my guess.

The spacecraft is already in orbit and has smallish fuel tank.

58 minutes ago, Rakaydos said:

Thing is, you aren't asking about thrust per engine mass, you are asking how much thrust you can get out of each unit mass of propellant. Or, for chemical propulsion, how much energy you can extract from a given fuel, by mass.

Which correlates to ISP, a rocket engine's fuel efficient. Hydrogen is the current clear winner. Antimatter is the theoretical clear winner.

 

Antimatter is out for this discussion, as well as theoretical metallic hydrogen. 

Only modern fuels.

[Bill Phil]

1 hour ago, Rakaydos said:

Thing is, you aren't asking about thrust per engine mass, you are asking how much thrust you can get out of each unit mass of propellant. Or, for chemical propulsion, how much energy you can extract from a given fuel, by mass.

Which correlates to ISP, a rocket engine's fuel efficient. Hydrogen is the current clear winner. Antimatter is the theoretical clear winner.

Actually that's impulse, a change in momentum. Thrust per mass of propellant isn't really measurable... thrust per unit mass per second is possible though. Of course that's just specific impulse again...

OP: Thrust is determined by exhaust velocity and mass flow. But there's a tradeoff - usually when you have a higher exhaust velocity you have a lower mass flow. So to maximize thrust you'll need to look at high thrust low specific impulse propellants - such as solid rockets.

[Spacescifi]

1 hour ago, Bill Phil said:

Actually that's impulse, a change in momentum. Thrust per mass of propellant isn't really measurable... thrust per unit mass per second is possible though. Of course that's just specific impulse again...

OP: Thrust is determined by exhaust velocity and mass flow. But there's a tradeoff - usually when you have a higher exhaust velocity you have a lower mass flow. So to maximize thrust you'll need to look at high thrust low specific impulse propellants - such as solid rockets.

 

Here's an idea:

Take a solid propellant and forget about the solid oxidizer powder.

Instead use the heat of a nuclear reactor to burn the solid propellant, which is poured into the reaction chamber as needed.

Has this ever been theorized, or is it just a bad idea for reasons I am not aware of, but will likely learn from you or another?

Thanks.

[YNM]

4 hours ago, Spacescifi said:

LH2/LOX will evaporate long before even a month passes is my guess.

The spacecraft is already in orbit and has smallish fuel tank.

Then you want a large engine with it ?

 

Small Delta-V.

 

Actually, if you want, you can summon Orion...

[Guest]

For maximum TWR? Mercury.  It's very dense and has a high molecular weight. Isp will suck, but density impulse and thrust will go through the roof. It's also hideously toxic, but that didn't stop NOTS from testing a tripropellant motor using it.  

[Thor Wotansen]

Yeah, the term you're all looking for is specific thrust.  The Ignition book has a bit about developing a fuel for SAM missiles that used mercury for this reason.

[YNM]

32 minutes ago, Thor Wotansen said:

specific thrust

Hmm... I only found that in airplanes (jet engines) so far... interesting.

[magnemoe]

35 minutes ago, Thor Wotansen said:

Yeah, the term you're all looking for is specific thrust.  The Ignition book has a bit about developing a fuel for SAM missiles that used mercury for this reason.

Not very efficient for its weight but dense. This matter for an missile especially on an ship there space is limited. 

[Starman4308]

If you're looking for thrust in a liquid motor, the two key variables are going to be specific impulse and propellant density. Specific impulse counts because you get more impulse per kilogram of propellant moved into the combustion chamber. Density counts because you can get more kilograms of propellant into the combustion chamber.

The practical propellants with decent Isp and high density are either solids (primarily ammonium perchlorate composite propellant), or the major hypergols (hydrazine, MMH, or UDHM as a fuel, dinitrogen tetroxide (NTO), mixed oxides of nitrogen (MON), or inhibited red fuming nitric acid as the oxidizer).

The impractical king of this is the aforementioned mercury-based rocket motor, which has terrible Isp but makes up for it in overwhelming density.

As to the original question, of what you want in an on-orbit interceptor, that would basically mean MMH/NTO or MMH/MON. Reasonable specific impulse, storable without complicated insulation/refrigeration equipment, does not have tons of mercury, and there already exists a wide variety of RCS motors for the stuff, etc. The primary issue is that it's an on-orbit interceptor, meaning you're locked into a specific orbital plane. This dramatically reduces your options.

[kerbiloid]

One should not ignore xenon hexafluoride.

[Xd the great]

Hmm. I am thinking about solid motors. One with a large burning surface area and a small throat region.

Edited September 23, 2019 by Xd the great

[Ultimate Steve]

If you want maximum TWR with curre t technology, detonate a fusion bomb behind your ship.

[farmerben]

On 9/21/2019 at 11:55 PM, Spacescifi said:

 

Here's an idea:

Take a solid propellant and forget about the solid oxidizer powder.

Instead use the heat of a nuclear reactor to burn the solid propellant, which is poured into the reaction chamber as needed.

Has this ever been theorized, or is it just a bad idea for reasons I am not aware of, but will likely learn from you or another?

Thanks.

good idea

https://en.wikipedia.org/wiki/Pentaborane(9)

[magnemoe]

Why the focus on TWR, you are trying to dock after all. And even if an killsat, TWR is only relevant if your target can dodge well. Well nothing can do that outside an dragon 2 capsule who use their superdrako engines for an emergency burn. 
Yes you can argue that you want an fast intercept but even here more dV win almost all the time. 
So I would simply go hypergolics  but obviously vacuum engines, you could even use stuff like an proton first stage engine and add an vacuum nozzle. 
Now for plane change, one option here would be to use the atmosphere if you have wings, yes you get drag but cheaper than an plane change unless you raise Ap high who take time. 
 

[kerbiloid]

(Somebody should post it in TWR thread)

Spoiler