Quote from: Joffan on 12/12/2013 08:56 pmYes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.But wouldn't that also increase the g forces on the payload?
Yes, it does make a difference. The reason is that with higher thrust you get a quicker ascent and reduced gravity losses. So the answer is that they could increase the maximum payload even without changing the amount of propellant.
Hi everyone,this question maybe obvious for many of you - please excuse me if this is the case but I'm just in learning mode.As well known, the Merlin 1D engine can be throttled down from 100% to 60%.The vacuum thrust of a rocket engine is proportional to the exhaust velocity and to the mass flow rate of the gas exhaust: dm/dt * V_exh.In vacuum and if also the gravity drag is negligible, when a Merlin 1D engine is throttled down, is dm/dt only changing, OR also V_exh (aka Isp_vacuum) is affected someway, with a consequent reduction in the engine efficiency in using the available propellant?If the second hypothesis is right (as I would say by looking at the physical principles the engine is based on), does someone know a reasonable way to write the relation Thrust(%) = f(%,dm/dt, V_exh)?Thanks for any hint,
Quote from: pagheca on 12/12/2013 01:41 pmHi everyone,this question maybe obvious for many of you - please excuse me if this is the case but I'm just in learning mode.As well known, the Merlin 1D engine can be throttled down from 100% to 60%.The vacuum thrust of a rocket engine is proportional to the exhaust velocity and to the mass flow rate of the gas exhaust: dm/dt * V_exh.In vacuum and if also the gravity drag is negligible, when a Merlin 1D engine is throttled down, is dm/dt only changing, OR also V_exh (aka Isp_vacuum) is affected someway, with a consequent reduction in the engine efficiency in using the available propellant?If the second hypothesis is right (as I would say by looking at the physical principles the engine is based on), does someone know a reasonable way to write the relation Thrust(%) = f(%,dm/dt, V_exh)?Thanks for any hint,V_exh is not affected by thrust directly. There is a second order effect because you slightly lower the chamber pressure. This is my impression at least. I may be wrong.
I have a question for you. If SpaceX were to try the D+, what would they do for propellant? Mass flow goes from 236 to 262 so the burns would be shorter on both stages. Sure, the Isp is up a little but in the wash, it really doesn't make much difference to payload capability. With more powerful engines, will they extend the length of the tanks by 10 % (a guess) to keep the lift-off T/W the same or will they just drive the engines harder for little benefit?
This additional loaded mass of 16,100 lb represented on average an 8.9-percent increase in onboard LO2 propellant. Test results also confirmed the presence of thermally stratified oxygen layers inside the tank. These layers varied in the vertical direction from 122 °R for the colder, denser fluid at the bottom to 166 °R for the warmer, less dense LO2 near the top outlet of the STA tank.
We could always subcool the kerosene...
I've heard of Rankine! It's like the imperial version of Kelvin. Also, if you're going to subcool the LOx... We could always subcool the kerosene...
V_exh is not affected by thrust directly. There is a second order effect because you slightly lower the chamber pressure. This is my impression at least. I may be wrong.
Do the first stage Merlin D engines, as built, throttle?
On photos of various Merlin 1Ds, I've noticed two different turbopump exhausts......(snip)
They are throttled down during a typical launch as the first stage approaches empty in order to limit acceleration.
Quote from: Owlon on 12/14/2013 03:26 amThey are throttled down during a typical launch as the first stage approaches empty in order to limit acceleration. Do you have any reference for this? thanks