Raptor 1 with Hot Methane Thruster upper stage

[rakaydos]

So as a kerbal player, I am interested in exploring the smaller end of the possible Raptor-based architectures. I have no target market, this is just exploring capability.

This "Raptor 1" concept I am exploring is a look back at the old Falcon 1, updated with the Raptor's more advanced capabilities.

The Raptor intended for the ITS uses hot methane and hot oxygen from cooling the engine bell to both pressurize the fuel tank, and to work "10 ton thrust hot methane RCS thrusters" for terminal landing guidace for precice cradle landing.

This theoretical build is a single Sea Level Raptor engine, methane and oxidiser tanks, and the secondary Hot Pressurant tanks. The second stage is 1 or more Hot Methane Thrusters, lifting the primary Hot Pressurant tanks.

The premice is that the Raptor runs hot enough to produce more pressurant than is needed to pressurize the 1st stage tanks. This extra pressurant builds up metephorical steam in the second stage tank, until most of the first stage propellant is spent. Then, the hot gas thrusters take the upper stage up to, hopefully, orbit. (I do not have numbers to support that assertion)

A single raptor has far too much thrust to reliably hoverslam a solid deck- the Z accuracy is too difficult. But it can handle boostback, entry and final guidance burns to accurately land in a net, pool, bouncy castle, or the reach of a parachute-grabbing drone.

According to wikipedia, Raptor has a thrust of 3,050 kN, and  road transportability allows a stage diameter up to as large as the Falcon 9, so there should be volume for an optimized methalox tank for 1 raptor- almost certifiably far shorter than the falcon 9, but probably carrying more deltaV in comparison. What's a good tank mass ratio, 5%?

Hot gas expands, and therefore like hydrogen as a fuel, sacrifices fuel fraction for a given tank for ISP. (that could probably be said better) How much Dv could this approach reasonably expect out of 1 or more HMTs, assuming fully pressurized tanks and Vac optimised HMT nozzles? (assuming the 10 ton rating is for sea level nozzles, given their use for BFR) How much pressurization could the 1st stage raptor reasonably provide to this setup?

I have a great many questions about this setup and it's theoretical feasability. If it doesnt work, great, but I'm interested in the detailed breakdown of WHY it doesnt work. Please do not dismiss this without helping me through the math.

[Jim]

The premice is that the Raptor runs hot enough to produce more pressurant than is needed to pressurize the 1st stage tanks. This extra pressurant builds up metephorical steam in the second stage tank, until most of the first stage propellant is spent. Then, the hot gas thrusters take the upper stage up to, hopefully, orbit. (I do not have numbers to support that assertion)

The premise is wrong.  The thrusters are not part of the tank pressurization system and "extra" pressure is not added to the main tanks.

[rakaydos]

The premice is that the Raptor runs hot enough to produce more pressurant than is needed to pressurize the 1st stage tanks. This extra pressurant builds up metephorical steam in the second stage tank, until most of the first stage propellant is spent. Then, the hot gas thrusters take the upper stage up to, hopefully, orbit. (I do not have numbers to support that assertion)

The premise is wrong.  The thrusters are not part of the tank pressurization system and "extra" pressure is not added to the main tanks.

My understanding of the ITS design is that there are separate pressurant tanks filled by propellant diverted through the raptor bell, and that this hot pressurant is tapped in a controlled fashon to pressurize the fuel tanks, and is also tapped to power the Hot Methane Thrusters. Is this wrong?

[envy887]

The premice is that the Raptor runs hot enough to produce more pressurant than is needed to pressurize the 1st stage tanks. This extra pressurant builds up metephorical steam in the second stage tank, until most of the first stage propellant is spent. Then, the hot gas thrusters take the upper stage up to, hopefully, orbit. (I do not have numbers to support that assertion)

The premise is wrong.  The thrusters are not part of the tank pressurization system and "extra" pressure is not added to the main tanks.

My understanding of the ITS design is that there are separate pressurant tanks filled by propellant diverted through the raptor bell, and that this hot pressurant is tapped in a controlled fashon to pressurize the fuel tanks, and is also tapped to power the Hot Methane Thrusters. Is this wrong?

More or less. The main tanks have to operate at constant pressure (on the order of 50 PSI), there will be a valve system designed to ensure this pressure is maintained. I doubt there will be any seperate tanks for storing pressurant - the valves will tap directly to the feed from the Raptors.

The gaseous methalox RCS thrusters will likely have their own storage tanks, but these will likely be at higher pressure, 100 to 1000 PSI. They will probably start full of GOX and GCH4 at launch, and if they run low be repressurized by electric pumps.

There's no reason a gaseous methalox RCS thruster couldn't be adapted to power an upper stage, but "stored" pressurant won't be involved. Just use a heat exchanger to heat some fuel and LOX and autogenously pressurize in flight like Raptor. If the thrusters are regen cooled (they almost certainly will be) just tap the coolant flow for some hot pressurized gas.

[rakaydos]

The premice is that the Raptor runs hot enough to produce more pressurant than is needed to pressurize the 1st stage tanks. This extra pressurant builds up metephorical steam in the second stage tank, until most of the first stage propellant is spent. Then, the hot gas thrusters take the upper stage up to, hopefully, orbit. (I do not have numbers to support that assertion)

The premise is wrong.  The thrusters are not part of the tank pressurization system and "extra" pressure is not added to the main tanks.

My understanding of the ITS design is that there are separate pressurant tanks filled by propellant diverted through the raptor bell, and that this hot pressurant is tapped in a controlled fashon to pressurize the fuel tanks, and is also tapped to power the Hot Methane Thrusters. Is this wrong?

More or less. The main tanks have to operate at constant pressure (on the order of 50 PSI), there will be a valve system designed to ensure this pressure is maintained. I doubt there will be any seperate tanks for storing pressurant - the valves will tap directly to the feed from the Raptors.

The gaseous methalox RCS thrusters will likely have their own storage tanks, but these will likely be at higher pressure, 100 to 1000 PSI. They will probably start full of GOX and GCH4 at launch, and if they run low be repressurized by electric pumps.

There's no reason a gaseous methalox RCS thruster couldn't be adapted to power an upper stage, but "stored" pressurant won't be involved. Just use a heat exchanger to heat some fuel and LOX and autogenously pressurize in flight like Raptor. If the thrusters are regen cooled (they almost certainly will be) just tap the coolant flow for some hot pressurized gas.

The idea was that the 1st stage raptor's regenerative cooling would be the heat source for the 2nd stage's hot methane being hot. My envisioning was that the hot gasses themselves would be compressed into the upper stage in flight, but if a heat pump works better...

[Jim]

The idea was that the 1st stage raptor's regenerative cooling would be the heat source for the 2nd stage's hot methane being hot. My envisioning was that the hot gasses themselves would be compressed into the upper stage in flight, but if a heat pump works better...

1.  it isn't hot gas

2.  it isn't going across stages.  Each stage uses its own engines and tanks for autogenous pressurizations

3.  The gases are not compressed, they are tapped off the engine pumps