Ascent Trajectory

[1987VCRProductions]

I'm going to state everything I know about ascent trajectories. I might repeat a few things that might or might not been already said in this thread. Modern launchers use a simple roll and then pitch vs. time program for the first stage of flight. After the second stage takes over, the guidance converges and the onboard computer asks, "Okay, where am I? How fast am I going? Where do I need to be and what should my cut off velocity be?"

For the Juno rocket that launched Explorer 1, an Army Redstone missile made up the first stage. The Redstone was the Von Braun team's follow up to their infamous A4/V2 rocket that terrorized Great Britain in WWII. The V2 had no smart auto pilot. It followed a set pitch profile and pitched over at a set rate until the fuel ran out. Based on how far the target was and what direction they were launching, the team would calculate with a slide rule how much fuel needed to be loaded into the V2.

I imagine that the Redstone was no different. Please correct me if I'm wrong. The Redstone first stage would have pitched over until the fuel ran out. They set the pitch rate so the rocket would be at a certain pitch at fuel depletion. The solid fuel upper stages didn't have any attitude control so they were spun up to a certain RPM before launch to keep them as stable as possible after separating from the Redstone first stage. They would have been on a timer (I'm just assuming here) so that they would fire at or around apogee, regardless of their attitude. The SRMs had a burn time of around 6 seconds I think.

I would suggest using Multistage2015. Even though it has a smart auto pilot that can be programmed, you can still use a simple pitch vs. time flight profile. I recommend it based on its cool new features and the fact that it's far more stable than the old Multistage2.

---------- Post added at 09:09 PM ---------- Previous post was at 09:03 PM ----------

I'll see if I can't play around with it tonight.

 

[Urwumpe]

I imagine that the Redstone was no different. Please correct me if I'm wrong. The Redstone first stage would have pitched over until the fuel ran out. They set the pitch rate so the rocket would be at a certain pitch at fuel depletion. The solid fuel upper stages didn't have any attitude control so they were spun up to a certain RPM before launch to keep them as stable as possible after separating from the Redstone first stage. They would have been on a timer (I'm just assuming here) so that they would fire at or around apogee, regardless of their attitude. The SRMs had a burn time of around 6 seconds I think.

Slightly wrong there. The Jupiter rocket would pitch at a constant rate to reach 40° shortly before cut-off - but long enough before cut-off to also dampen any attitude rates. Also the pitch down would not start instantly, there should be some vertical trajectory at the beginning, as you can see in the videos.

The second stage had attitude control - compressed air jets in the instrumentation section rotated the stage into a horizontal position, before the solid rocket motors are fired on apogee. determining apogee was the hardest problem to solve back then.

Each SRM has a burntime of 6 seconds, there had been 2 seconds of pause before the next stage fired for making sure that the thrust of the previous SRMs trailed off.

Also, the rotation rate of the tub increased during flight, from 550 to 750 rpm, so that the tub does not oscillated at one of two resonance frequencies of the first stage, which all moved upwards as propellant was depleted.

 

[boogabooga]

The second stage had attitude control - compressed air jets in the instrumentation section rotated the stage into a horizontal position, before the solid rocket motors are fired on apogee.

I've read that to, but how did they choreograph the attitude control on something rotating at 700 rpm without it simply precessing about. Especially with puny cold air jets.

I know that precession-based attitude control was done later (BrianJ's Giotto add-on demonstrates how it works), but don't the firings have to occur at EXACTLY the right time? The problem seems a bit complex for 1958.

 

[Urwumpe]

I've read that to, but how did they choreograph the attitude control on something rotating at 700 rpm without it simply precessing about. Especially with puny cold air jets.

I know that precession-based attitude control was done later (BrianJ's Giotto add-on demonstrates how it works), but don't the firings have to occur at EXACTLY the right time? The problem seems a bit complex for 1958.

The instrument section does not rotate, that seems to be the main trick. Also it seems to be fairly heavy in comparison to the following stages and payload. If you then rotate the stage fairly slowly, you can also do that by compressed air thrusters, since the precession is not as extreme as if the whole stage would rotate. If they did so really smart, they would maybe not even need to fight precession, since they can control the change in rotational impulse pretty easily with the unspun instrument section. Actually, controlling such spin stabilized spacecraft was no magic a few years later, it is easier than three axis control, since you need less sensors, because the spacecraft body is a gyroscope itself.

Also firing was determined by multiple sources, from what I remember, including measurements by ground stations.

 

[1987VCRProductions]

Slightly wrong there. The Jupiter rocket would pitch at a constant rate to reach 40° shortly before cut-off - but long enough before cut-off to also dampen any attitude rates. Also the pitch down would not start instantly, there should be some vertical trajectory at the beginning, as you can see in the videos.

The second stage had attitude control - compressed air jets in the instrumentation section rotated the stage into a horizontal position, before the solid rocket motors are fired on apogee. determining apogee was the hardest problem to solve back then.

Each SRM has a burntime of 6 seconds, there had been 2 seconds of pause before the next stage fired for making sure that the thrust of the previous SRMs trailed off.

Also, the rotation rate of the tub increased during flight, from 550 to 750 rpm, so that the tub does not oscillated at one of two resonance frequencies of the first stage, which all moved upwards as propellant was depleted.

I knew that the rocket didn't pitch over immediately. I knew that there's always a period of time where the rocket flies straight up to get away from the pad and pad structure. I thought that was a given.

As for the second stage having attitude control, how would that work if the stage was spinning at 750 rpm? Did the second stage not rotate?

 

[Urwumpe]

As for the second stage having attitude control, how would that work if the stage was spinning at 750 rpm? Did the second stage not rotate?

The instrumentation section on top of the first stage, with the electric motors and the guidance system remained attached to the rotating tub during the coast phase. This section also contained the cold gas thrusters.

 

[boogabooga]

The instrumentation section on top of the first stage, with the electric motors and the guidance system remained attached to the rotating tub during the coast phase. This section also contained the cold gas thrusters.

That is not modeled properly on the BigMac dd-on.

 

[Urwumpe]

That is not modeled properly on the BigMac dd-on.

Sadly. But then, if you are SSU dev, all add-ons are not properly modelled. Its a drama. :lol:

I really wish somebody would have proper meshes around for that, including the GSE.

Making something like a "ABMA missile family" add-on would be cool.

 

[thammond]

Also firing was determined by multiple sources, from what I remember, including measurements by ground stations.

I did read somewhere that the 2nd stage ignition was initiated from ground controllers.

Anyways, since I last posted on here, I have been busy learning from various tutorials and postings on the forum (with a lot of bumps and bruises along the way) on how to make an addon. And made my first simple addon (based on a tutorial from eharm website). My 1st test flight I ended up flying sideways...oops, but figured out how to fix that with the Mesh Wizzard.

Now starting to work on the Juno I rocket mesh and textures. I did a simple 1st stage without the fins and engine to test out the texturing and so far so good.