About the SSMEs angle

[evilfer]

While I've always thought the Space Shuttle the greatest engineering marvel, there's a technical issue that bothers me.

I don't know if it's been commented in this forum before, but the fact that the SSME and the SRBs are not aligned means that part of their thrusts cancel each other.

I've been doing some rudimentary numbers, just for the sake of doing numbers. I've considered a 15º angle difference, 1.8 MN of SSME thrust (SL), and about 13*2 MN of SRB thrust. With this data, I got that the SSME and SRB dedicate ~0.5 MN against each other. This means that for the time the SRB are burning, 24% of SSME thrust and 1.7% of SRB thrust do not actually push the shuttle.

In terms of fuel mass, this means 16800 kg of SRB fuel mass, and (assuming constant 104.5% rated thrust, 500s SSME burn, and for the first 110 seconds) 39200 kg of SSME fuel. Total fuel "wasted": 56000 kg, twice the payload to LEO.

Of course, I'm not criticizing the shuttle not its design , just I was curious about the numbers. Mistakes and simplifications may have caused huge errors in the figures though!

 

[Urwumpe]

You need to calculate with torque vector not just with thrust force vector - also the angle difference is much lower, AFAIR just 10.5° for the center engine.

The SSME does indeed push the Shuttle a tiny bit, that is why it launches a bit forward, instead of just up.

Also, the attitude of the shuttle is only kept straight up for the early part of flight, later it flies with a tiny bit of AOA that requires less torque of the engines to be held.

 

[evilfer]

You need to calculate with torque vector not just with thrust force vector - also the angle difference is much lower, AFAIR just 10.5° for the center engine.

The SSME does indeed push the Shuttle a tiny bit, that is why it launches a bit forward, instead of just up.

Also, the attitude of the shuttle is only kept straight up for the early part of flight, later it flies with a tiny bit of AOA that requires less torque of the engines to be held.

Ah, I hadn't considered the AOA (and wings, right?)! Are they the reason for the angle? I had thought that a different configuration (SSME total thrust, SRBs, and center of mass in the same plane) should allow using all the thrust to push the shuttle without making any torque .

Thank you for the info on the angle, with 10.5º I get 39400 kg of "wasted" (don't like this word) fuel.

 

[Urwumpe]

Ah, I hadn't considered the AOA (and wings, right?)! Are they the reason for the angle? I had thought that a different configuration (SSME total thrust, SRBs, and center of mass in the same plane) should allow using all the thrust to push the shuttle without making any torque .

The shuttle doesn't make any torque at all, unless it wants to rotate.

Your configuration would provide more total force, but would then mean that the engines have to be mounted on the ET, since the 120 tons of orbiter are not really much compared to the 2000 tons of the rest during lift-off. By these ~35,000 kg (actually much less, since the masses change and the CoG slowly approaches the CoG line without SRBs), the main engines could have been installed on the orbiter and the orbiter installed at the side of the ET and the SSMEs already ignited before lift-off for health monitoring.

Thank you for the info on the angle, with 10.5º I get 39400 kg of "wasted" (don't like this word) fuel.

Well, wasted is tolerable, the proper engineering term is "control losses" and is measured in m/s. you don't real waste the fuel for no effect, you just keep an aerodynamic configuration.

The SSMEs don't do anything meaningful at all actually during early launch, except load relief so the attitude does not have to be kept at all costs by the SRBs - the SRBs fly the shuttle, the SSMEs just mirror the motion of the SRBs for reducing the loads on the structure. The thrust vectors of the main engines don't meet in the CoG, but in a point far ahead of the CoG, which provides more stability, another factor there. the CoG is in the ideal situation just

 

[evilfer]

The shuttle doesn't make any torque at all, unless it wants to rotate.

Your configuration would provide more total force, but would then mean that the engines have to be mounted on the ET, since the 120 tons of orbiter are not really much compared to the 2000 tons of the rest during lift-off. By these ~35,000 kg (actually much less, since the masses change and the CoG slowly approaches the CoG line without SRBs), the main engines could have been installed on the orbiter and the orbiter installed at the side of the ET and the SSMEs already ignited before lift-off for health monitoring.

Yes, I knew there had to be good reasons for the shuttle's configuration . That's interesting, the shifting in the CoG. Ah, and I finally got that the fuel mass dedicated to control losses cannot be simply replaced with payload (took me a while) .

Well, wasted is tolerable, the proper engineering term is "control losses" and is measured in m/s. you don't real waste the fuel for no effect, you just keep an aerodynamic configuration.

The SSMEs don't do anything meaningful at all actually during early launch, except load relief so the attitude does not have to be kept at all costs by the SRBs - the SRBs fly the shuttle, the SSMEs just mirror the motion of the SRBs for reducing the loads on the structure. The thrust vectors of the main engines don't meet in the CoG, but in a point far ahead of the CoG, which provides more stability, another factor there. the CoG is in the ideal situation just

That makes me think, is the shuttle configuration with unaligned thrust vectors more stable than the one I proposed? Is this extra stability necessary due to the aerodynamics of the orbiter?


Out of stubborness, I calculated control losses of 900m/s for SSME fuel (if my figure for fuel mass is not wrong, and assuming that this extra fuel would be used right before MECO). Wouldn't know how to calculate control losses for SRB, with the SRB payload (orbiter + et) mass changing, and very variable thrust (according to wikipedia). If I got the total control losses, I could calculate the mass of payload that could substitute the extra fuel with a different configuration...

 

[kwan3217]

Be careful when you calculate this. To get the off-axis thrust t_y of an engine with total thrust t_t angled a certain amount theta, use t_y=t_t*sin(theta). To get on-axis thrust t_z, use t_z=t_t*cos(theta). Note that if you just add t_y and t_z you get a number greater than t_t, but this is ok, since you really have to add vectors, and when you do, you end up with a vector length equal to t_t.

I have heard this referred to as cosine loss.

For instance, the other day I was researching the boosters of an AtlasV, which are inclined 3deg from the rocket axis. The off-axis thrust is 5.2% of the thrust, but the on-axis thrust is 99.8%, so the cosine loss is only 0.2%

For the shuttle, if the engines are off by 10.5deg, the cosine loss is 2.7%.

Also note that when the shuttle has dropped its boosters and is flying above the atmosphere, it no longer has to fly pointy-end into the wind. It can pitch to an angle of attack of 10.5deg, thereby pointing the engine axis exactly where it wants to go and eliminate the cosine loss completely.

 

[evilfer]

Be careful when you calculate this. To get the off-axis thrust t_y of an engine with total thrust t_t angled a certain amount theta, use t_y=t_t*sin(theta). To get on-axis thrust t_z, use t_z=t_t*cos(theta). Note that if you just add t_y and t_z you get a number greater than t_t, but this is ok, since you really have to add vectors, and when you do, you end up with a vector length equal to t_t.

I have heard this referred to as cosine loss.

For instance, the other day I was researching the boosters of an AtlasV, which are inclined 3deg from the rocket axis. The off-axis thrust is 5.2% of the thrust, but the on-axis thrust is 99.8%, so the cosine loss is only 0.2%

For the shuttle, if the engines are off by 10.5deg, the cosine loss is 2.7%.

Ops, that's right, I had calculated SSME losses as % of off-axis thrust with respect to total SSME thrust. This doesn't work, but using % of on-axis thrust has the same problem.

Could it make more sense to compare kinetic energy? Actually (off-axis Ek) + (on-axis Ek) = (total Ek), right? In this case, I get that off-axis kinetic energy provided by SSME is 9.4% of total SSME kinetic energy (fuel mass dedicated to losses would be less than half the mass I first calculated).

Also note that when the shuttle has dropped its boosters and is flying above the atmosphere, it no longer has to fly pointy-end into the wind. It can pitch to an angle of attack of 10.5deg, thereby pointing the engine axis exactly where it wants to go and eliminate the cosine loss completely.

Yes, losses for SSME were calculated only during the first 110 seconds, not the whole burn . In any case, I knew from the beginning that these numbers were not very realistic...