Low, but not QUITE so slow

Wildman Rocketry
Onebadhawk
madcow rocketry
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
stealth6

stealth6

insert witty tagline here
Joined
May 1, 2011
Messages
3,306
Reaction score
942
Location
Winnebago
I keep running into a design problem that I could perhaps use some discussion on. This is decidedly a very general /broad question rather than a specific rocket design problem needing a specific solution.

The situation is this. Every so often, I'm finding that I want to limit the altitude of a given rocket I might be designing - for a variety of reasons, none of which are important in and of themselves. These range from small simple A/B/C rockets to mid-power more complicated E/F/G projects involving clusters/airstarts/multi-staging/etc. But I keep running into the same problem; which is that if I tweak the parameters (weight, drag, impulse, etc.) to hit the (low) target altitude in sims, I end up needing a LONG rail or rod to get the rocket up to safe speed for stability. If I lighten the rocket, or go for a higher kick motor, I end up going higher than I'd like. If my sims are too high, and I choose a lower impulse motor, or increase the drag/weight, I can't get off the ground fast enough to fly straight.

In a nutshell, how do I go about designing a rocket that lifts off faster, but peaks lower?

Anyone else run into this? How did/do you go about dealing with it? How do you optimize lift off stability, while minimizing peak altitude? It seems to me the answer lies in choosing "just the right" motor(s), but I keep finding that that perfect one doesn't exist - so then what do I do?

thanks, s6
 
Keep the weight low and the drag high. Short fat designs with big fins or tube fins.
 
I tend to like to do the same thing with larger rockets...limit their altitude to be able to use them on small fields.

With this in mind, I find that drag tends to be variable to adjust rather than weight. The reason I say this is that weight (mass) tends to affect all parts of the flight envelope, especially acceleration. You get a low altitude, but a slow rocket off the pad as well.

Drag is more a function of speed. So at liftoff, there isn't much drag simply because there isn't much speed. You can accelerate quickly, but your top speed is limited, and you'll start decelerating quickly as soon as the thrust period ends. So you get the stability you need at liftoff, but the low altitude you need.

For the heck of it (I'm going to try this too because I am curious), I'd build several rockets in OR that are all exactly the same except for BT diameter (with the associated nose diameter and slight weight gain). I think you'll find the added frontal and form drag will have a dramatic effect on your apogee with affecting your 'off the rod' speed that much.

FC
 
Now that my interest was raised, I did some tests with a 3FNC, altering nothing but the weight and diameter of the BT (with appropriate changing of the NC and shoulder).

Using a C6-7 engine, 36 inch launch rod, zeros on everything else (wind, gust, latitude, alt, etc).

Tables from left to right: BT number peak alt, vertical speed at launch rod clearance (LRC), and weight.

First - diameter change, with appropriate weight gain:

BT-50 - 1241ft - 45mph LRC - 1.47oz
BT-55 - 1013ft - 43mph LRC - 1.90oz
BT-60 - 861 ft - 40mph LRC - 2.16oz
BT-80 - 557 ft - 35mph LRC - 3.00oz

Second - diameter change only:

BT-50 - 930 ft - 35mph LRC - 3.00oz
BT-55 - 821 ft - 35mph LRC - 3.00oz
BT-60 - 739 ft - 35mph LRC - 3.00oz
BT-80 - 557 ft - 35mph LRC - 3.00oz

Third - weight change only:

BT-50 - 1241ft - 45mph LRC - 1.47oz
BT-50 - 1207ft - 42mph LRC - 1.90oz
BT-50 - 1158ft - 40mph LRC - 2.16oz
BT-50 - 930 ft - 35mph LRC - 3.00oz

Note that in the first table, peak altitude and speed at launch rod end decrease. But which is affecting what more (ie drag/weight/alt/LRC speed)?

The second table is more revealing. Though the LRC speed doesn't change significantly (within 1mph), the peak altitude decreases by 40 percent.

The third table shows another interesting result. Though the weight of the rocket is doubled, the peak altitude only decreases by about 25 percent, with the LRC speed decreasing by 22 percent (similar to the first table).

What you can take away is that both drag and weight will affect your peak altitude...but weight is the bigger factor in your initial acceleration to flying speed.

Or what JPVegh said:

Keep the weight low and the drag high.
Click to expand...

FC
 
Sweet. Thanks guys.

What JPv said definately made intuitive sense, but it's very interesting to see the results of your little sim test FC. Thanks for doing and sharing that.

So the trick seems to be to increase drag without increasing weight (at least not by much). Looking back, each time I've been faced with this problem, whenever I tried to increase drag, I tended to increase weight along with it and so accordingly got unsatisfactory results. I'll work with this in mind from now on and see if I can do better.

s6
 
No problem. You peaked my interest because I try to have the same goal of keeping altitude low but making sure the rocket is stable off the rod. But I really hadn't ever tried to actually run several simulations to see what's better to adjust.

The trick of course is how to increase drag without increasing weight that much. It's a challenge of a different sort...the nice thing is you can end up with big demo rockets that look cool roaring off the pad but are easily recoverable.

FC
 
FastCargo said:
...you can end up with big demo rockets that look cool roaring off the pad but are easily recoverable.
Click to expand...

yep, that's what we are talking about.

One of the things I'm trying to do is design/build a mid-size rocket that launches with white or thin smoke, and then airstarts a black/smoky after a short coast. The trick is that I want that airstart to hit at between 500' and 600' or so. No more than 750'. The effect is kinda wasted if the rocket is way up high when it happens - I want it down low where I/we can really see it. And I don't really care all that much about peak altitude after that.

I'm close at the moment. My design currently sim/launches off three 24mm E18-Ws (on outboard pods), and then fires a 29mm F22-J at about 700', with apogee at 1900' or so. I'm trying to bring that airstart down a bit. Honestly if I could work it so the rocket jumps off the pad and goes to coast almost immediately, with the airstarted smoky kicking in at 200'-300', that would be awesome methinks. But I'm content to try for 500'-600'.

s6
 
Last edited:
Interesting query and answers. It makes sense....raising drag without raising the weight significantly would cut down coast time significantly - whereas having proportionally more mass would help overcome the higher drag in the coast phase (more mv).

It looks like whoever designed the Estes Fat Boy was on to something :).
 
stealth6 said:
I keep running into a design problem that I could perhaps use some discussion on. This is decidedly a very general /broad question rather than a specific rocket design problem needing a specific solution.

The situation is this. Every so often, I'm finding that I want to limit the altitude of a given rocket I might be designing - for a variety of reasons, none of which are important in and of themselves. These range from small simple A/B/C rockets to mid-power more complicated E/F/G projects involving clusters/airstarts/multi-staging/etc. But I keep running into the same problem; which is that if I tweak the parameters (weight, drag, impulse, etc.) to hit the (low) target altitude in sims, I end up needing a LONG rail or rod to get the rocket up to safe speed for stability. If I lighten the rocket, or go for a higher kick motor, I end up going higher than I'd like. If my sims are too high, and I choose a lower impulse motor, or increase the drag/weight, I can't get off the ground fast enough to fly straight.

In a nutshell, how do I go about designing a rocket that lifts off faster, but peaks lower?

Anyone else run into this? How did/do you go about dealing with it? How do you optimize lift off stability, while minimizing peak altitude? It seems to me the answer lies in choosing "just the right" motor(s), but I keep finding that that perfect one doesn't exist - so then what do I do?

thanks, s6
Click to expand...

how fast and how low ? are you looking ?

I'm new on this hobby, I did a cardstock prototype of mercury redstone, it have a high drag and cause the fins are to small i shoudl add 29g. he flew just ok A motor is to small, buy B and C motor is o.k. in small site, I got the data flight form altimeter two and i used the Estes maxis rod.

So when i read the recomendation high drag, make sense for me.

Mercury-Redstone V3.0
Rocket Weight: 40 g.
Payload Weight: 29g.
Total Weight: 69 g.
Payload: Video camera,altimeter two, buzzer and battery (I need al of this to have Stability Cal = 1 on C6 Motor)

Motor: A8-3
Apogee(m): 21
Speed(KPH): 67
Burntime:0.3
Peak Acceleration(g): 14.2
Average Acceleration(g): 6.3
Coast to Apogee: 1.6
Apogee to Eject time:0.9
Eject Altitud(m): 19
Desent Speed(KPH):17
Flight Duration: 7.2

Motor: B6-4
Apogee(m): 62
Speed(KPH): 119
Burntime:0.9
Peak Acceleration(g): 11.8
Average Acceleration(g): 3.8
Coast to Apogee: 3.0
Apogee to Eject time:0.6
Eject Altitud(m): 60
Desent Speed(KPH):17
Flight Duration: 16.9

Motor: C6-3
Apogee(m): 118
Speed(KPH): 225
Burntime:2.6
Peak Acceleration(g): 10.3
Average Acceleration(g): 2.5
Coast to Apogee: 1.8
Apogee to Eject time:0.1
Eject Altitud(m): 116
Desent Speed(KPH):9
Flight Duration: 41.8

You can watch a video here in the latest posts
https://www.rocketryforum.com/showthread.php?t=26362

Attached the Open Rocket file View attachment MR7V3.0.ork
 
Last edited:

Similar threads

M
Designing in Open Rocket
Replies
1
Views
285
prfesser
prfesser
J
Open Rocket sim issues
Replies
2
Views
427
jfcote
J
Velocity Aerospace
Risk of high g-forces?
Replies
10
Views
580
NTP2
NTP2
Superza
Road to Three-Stage
Replies
10
Views
1K
Superza
Superza
diyaerospace
ARC 2024
Replies
1
Views
790
PhlAsh
PhlAsh

Latest posts

Back
Top