Project LR1 Skyhammer: A New Shuttle [p1]

[Pipcard]

In free flight, moving the center of gravity without using thrust is straight-up warping. In reality, it's the vessel body that moves, not the center of gravity. On the ground, the body can push against the ground to force the CG to move instead. I suspect most of the center-of-gravity shifting in add-ons is actually violating physics for this reason.

So how are you supposed to hold a high AoA?

Do you have to hold a lower AoA and have a shallower re-entry?

 

[kocmolyf]

So how are you supposed to hold a high AoA?

There are two parts to the answer. The first is that moving the body instead of the center of gravity has exactly the same effect on aerodynamics, it's just that you don't violate momentum conservation.

But the real answer is that real spacecraft don't have active center-of-gravity shifting. Instead, they are designed to be aerodynamically stable at high angles of attack and maintain trim with control surfaces. This may require ballast to be placed before the mission to maintain the center of gravity within an acceptable region.

 

[Urwumpe]

So how are you supposed to hold a high AoA?

For the Space Shuttle, this is actually aerodynamics. At 39° AOA of the Space Shuttle, there is a plateau of low aerodynamic moments to rotate the spacecraft. Below 35° or so, the nose would drop with so much force, that bodyflap+elevons and RCS could not compensate it. if the Shuttle exceeds 48° (or so, would need to look at the data again), the nose would go upwards and out of control until it stabilizes flying with the heavy aft end forward. Call it a physical trick, but it works. That is why the shuttle needs to be at 39° AOA before EI already, because it is aerodynamically instable and would not automatically stabilize at this AOA, like you can do for capsules or feathercocked spaceplanes.

This would be similar for this one (similar mass distribution), though likely with some small differences in the actual lift function.

 

[Mandella]

This is going to be a fun vehicle to fly. Are there plans for a virtual cockpit?

 

[Interceptor]

This looks like it's going to be a fantastic ship,any plans on a launch pad to go with it?

 

[n0mad23]

I knew it was big, but seeing it beside the obsolete shuttle really got it across to me.

This is a HUGE craft!

I can see many uses for such a thing. Nice.

 

[kocmolyf]

Are there plans for a virtual cockpit?

Yes! It's down below ascent, landing, orbit ops, and entry, but it's there. I don't intend to do 2D panels - as soon as switches become required, I will make a VC. I assume this will happen right about the time I start modeling the internal vehicle systems or the robotic arm. The source model already has the cabin outer mold line defined (but not exported to the game).

Until then, I will stick with keyboard shortcuts or potentially a pop-out dialog if I really need to prototype something complex.

...any plans on a launch pad to go with it?

Definitely. For now I've just stacked it on the Shuttle pad, but it's pretty obvious upon close inspection that it's not a good fit. A launch pad is on my list of "polish" items that I'll do somewhere in the list when I feel like working on them (among other things like model improvements, and textures :shifty. Generally, the more fun the vehicle is to fly, the more irritated I will become if it doesn't look complete.

 

[PhantomCruiser]

I knew it was big, but seeing it beside the obsolete shuttle really got it across to me.

It isn't really fair to call the shuttle "obsolete", it was a quite capable platform (to be a prototype).

The Skyhammer here may represent an "alternate-reality" version of what could have happened if NASA had gone another way.

She looks wonderful BTW, looking forward to integrating her into my future launch operations!

 

[Scav]

The Skyhammer here may represent an "alternate-reality" version of what could have happened if NASA had gone another way.

Kinda makes me wonder what Буран may have looked like as well, given that criteria.

Ahh, enough sidestepping, and on to what's hopefully a well-thought out thought: I wouldn't be too worried about using LC-39 as a staging point for this -

1) I could see that 39A would still have many of the facilities the LR1 would need for its launch ops without having to rip the entire tower down and replumb the underside of the facility. That way NASA could still keep 39B for its SLS launches without disrupting things too terribly.

2) I can also only imagine assembly for launch would most likely mitigate all that wrangling through the high bay transfer aisles the Shuttle used to have to do through the VAB. Simply load it up with cargo, pull it into the VAB, put it on a crane, turn it on its heels, stick it on a MLP, drop the propellant tanks on it and wheel the assembly down the rock road. It may even be possible to do this without majorly remodeling the VAB (unless my projections are horribly off and the fact I don't know much about what I'm talking about really shows in this post . . . :shifty: ).

One might even get away with creating a circular/oval/(insert shape with outer and inner diameter here) spacer assembly to provide adequate structural support for the whole rig on the pad.

Anyway -- I've taken too long to weigh in on some of these thoughts that I've wanted to voice, but there you have it, and not too much backpedaling as well. :tiphat:

 

[Andy44]

Could you mate it to the tank horizontally and then erect it on the pad empty before tanking it up? With no loaded SRBs stacking this thing should be simpler than STS.

Also, remember that the Vandenberg launch pad SLC-6 had a whole different system for stacking than they had at the Cape. I wonder if that type of system would be better suited to this sort of vehicle.

 

[kocmolyf]

Could you mate it to the tank horizontally and then erect it on the pad empty before tanking it up?

That is definitely my preferred approach. I was thinking you might even be able to roll it on the gear in this configuration. In that case you could mate the tank with an overhead crane at assembly, roll it into an erector at the pad, stow the gear, and rotate to vertical.

I have to check that the loads and CG are acceptable for that first, though.

 

[kocmolyf]

The drop tank hinge works!

I drew up the free body diagram and implemented the (first-order) physics. Now instead of immediately detaching the tank (and having it slide through the orbiter), 'Alt + J' releases the forward latch and allows the tank to swing freely on the aft hinges. It's quite possible to mess it up and keep the tank pinned to the vehicle with rotational torques, especially when it's full of propellant.

However, if you jettison it while empty like you're supposed to, it shouldn't take much at all for it to come away clean. The force on the hinges will give the orbiter a little kick that you have to take out with the gimbals, though.

Just prior to separation. RELEASE RELEASE RELEASE!

Forward latch release, tank coming up and over.

Just barely off the hinges. The tank is away!

Tank is clear of the fins.

Tank tumbling out.

Seeing this for the first time made me grin like an idiot - I don't know why it's so satisfying, but it is. I can't wait for you guys to try it.

 

[Donamy]

I know...

 

[Andy44]

Seeing this for the first time made me grin like an idiot - I don't know why it's so satisfying, but it is. I can't wait for you guys to try it.

The first time I got my Lunar Orbit Station animations to successfully deploy I felt the same way.

 

[n0mad23]

I'm a lucky one - I get that silly grin every time a new animation or even successful fairing jettison happens for me.

The other, and one I imagine all who have made any kind of addon with modeling involved, is that moment of awe with the audible, "wow, it's really beautiful...." Proud techno-parenthood moments.

It's really cool that you've gone for something so ambitious as a first addon, too. I've been kicking myself lately that I gave up C about the same time I gave up writing multiple page long dos batch files to answered the telephone for my modem driven bbs. Those strange days before there was a world wide web, and internet involved unix and a hacked university account. The project(s) I'd like to do now are all impossible with either Multistage2 or Spacecraft3, so I'm even more impressed with what you've done here.

When you're ready for alpha or beta testers, I'd be honored to give what feedback I can.

Cheers.

 

[NextBigLeap]

Any idea on the date of the first release, really want to try this thing out!

 

[Ripley]

Impressive skill/quality, and I like the overall "easy" flow of the project!
This morning I've been reading the whole thread and all I can say is...wow!!
:thumbup:

 

[kocmolyf]

Well, I've spent the last few days being reminded of how poorly I understand aerodynamics. It doesn't help that I'm using an aerodynamics characteristics report rather than an aero database, either. Lots of stability derivatives, not as many coefficients as I would like. In any case, I've got something plausible enough for ascent now. I completely bypassed Orbiter's airfoil system and just implemented it myself - it's slightly more work but means I don't don't have to trick the airfoils into doing what I want. Thank martins for AddForce().

With ascent aerodynamics out of the way, the last "major" task before the first release is done! I've accumulated around a dozen smaller cleanup tasks I want to complete first, but they should all fall quickly. Hopefully you'll have the ascent prototype to play with soon!

---------- Post added 09-16-14 at 08:44 AM ---------- Previous post was 09-15-14 at 12:46 PM ----------

I started digging into my backlog of little stuff. I started with the drop tank, which now has some rudimentary aerodynamics after separation and improved physics if it recontacts the nose while hinging. I had also forgotten one of the components of force during separation, so the dynamics changed somewhat. Now the aft end of the orbiter is "carried up" by the swinging drop tank if you separate in vacuum. If you separate at high dynamic pressure (don't do that), the aft end is forced downwards with great violence.

More importantly, I implemented anti-torque steering for the gimbals. This attempts to negate all aerodynamic torque in the pitch plane during ascent, so you can keep a constant pitch rate for those nice gravity turns rather than fighting the controls all the way up. The yaw plane is still free to stabilize aerodynamically.

Flying nearly horizontal at low altitude (don't do this). The cross indicates severe negative engine gimbal (almost -5 degrees, near the hard stop) trying to fight aero torques and keep the nose high.

Next on my list is to change out the gimbal control system. Instead of the pilot providing direct engine gimbal inputs, I'm going to treat them as commanded angular accelerations and have the engines figure out how to achieve that. That will help avoid the nose getting a bit squirrely at the end of burns when the vehicle is light and control authority is high.

 

[kocmolyf]

I am getting dangerously close to the first prototype release.

Stuff done since the last update:

1. Added aerodynamic anti-yaw steering in addition to anti-pitch steering.
2. The gimbal control system now takes desired angular acceleration as an input rather than desired engine deflection angle. This smooths out control through the whole range of masses.
3. Modified the gimbal system to take into account any unwanted pitch/yaw introduced by roll control and cancel it out.
4. Some improvements to the drop tank physics (more to come before release).

Finally, I implemented an on-HUD ascent status indicator. It uses "loft" and "orbit" instead of apoapse/periapse since the apses swap during orbital insertion (the worst possible time). The loft is your "near apsis" and the orbit is your "far apsis" during ascent. So watch the loft for your gravity turn and the orbit for your orbital insertion burn. This calculator also takes into account the thrust transient from engine shutdown.

Just after liftoff, time-to-loft spinning up.

Nearing orbital insertion, far apsis rapidly rising.

Moments after the engine cutoff command. Notice that the ORBIT altitude is higher than the ApA in the Orbit MFD.

Engines all shut down at zero thrust. Final ApA is right on the money. My sloppy demo trajectory almost left me without any fuel at the end of the burn (oops).

There are only a few items left: some clean up of the droptank physics, a better indicator for heading while vertical, a couple of small model improvements, and a scrub of the save state to ensure all the necessary values are actually getting saved to file and restored properly.

 

[Andy44]

a better indicator for heading while vertical,

Enjo's Launch MFD came up with a good solution for this; a HUD cue that shows your desired downrage direction, allowing you to put your dorsal or ventral side into it, depending on whether you ascend heads-up or heads-down (shuttle style).

Dumb question: why is the "orbit" altitude greater than the ApA? If the engines are stopped shouldn't they be the same? Or is there a difference in reference frame or something?